Greg_L 6/29/2018 2:30 PM
My first build - AB763 Deluxe Reverb clone - opinions please
Hello guys, my name is Greg. I've recently built my very first amp, as the title says, and overall I'm very pleased with it and all of it's functions work as they should. It's a Weber 6A20 kit - built as a head unit. I'm using a ClassicTone 40-18087 OT. Otherwise a stock type build.

My only issue is a bit of "brittle" break up right at the attack of a note, it sustains for a sec, then the brittleness goes away as the note sustains out.

I've heard this sound on other AB763 type amps, but mine seems more brittle and a little more pronounced.

It happens on either channel, normal or vibrato, reverb on or off, tremolo on or off, no matter the mic or speaker used. It's always there.

Here is a soundclip running through the amp volume, guitar volume, reverb, and tremolo. You can clearly hear the brittle breakup, it almost sounds like a faint static on the attack of the notes. Sorry for the length and size of the clip, but I wanted to provide a high-quality example with lots of variation. Please download the wav file and listen and comment on what you think this may be.

This sound clip was done using a Gibson SG with a P90 bridge pickup through a Marshall 4x12 cab with vintage Celestion G12-65s.
https://jmp.sh/KXmIVIV

Other than that, I'm very pleased with the amp and happy with the outcome for it being my first build. It sounds great overall, it's just got that hint of brittle breakup all the time.

Any pics or specs you may need to help decipher this issue will be readily available upon request. I have tons of pics and pin voltages and all that stuff. I even have a scope at the ready - I'm not very good with it, but I'll give it a shot. Haha.

Thanks for your time and I genuinely appreciate any guidance you can give me.
 
g1 6/29/2018 6:13 PM
Can you post your power tube DC voltages at idle (no signal). Do you have a means of checking idle current of power tubes?
 
Greg_L 6/29/2018 7:24 PM
Quote Originally Posted by g1 View Post
Can you post your power tube DC voltages at idle (no signal). Do you have a means of checking idle current of power tubes?
Sure thing.....

Power Tubes
Tung Sol 6V6GT


V7
1: NA
2: H
3: 435
4: 435
5: -42.0
6: 436
7: H
8: 19.5 mv - measured at 1 ohm resistor to ground

V8
1: NA
2: H
3: 435
4: 436
5: -42.2
6: 436
7: H
8: 19.5 mv - measured at 1 ohm resistor to ground
 
dstrat 6/29/2018 8:24 PM
Hi, first I will say congrats that amp sounds pretty good to me, much of what you hear IMO is those speakers.
try it with some real greenbacks you will be surprised.
 
Greg_L 6/29/2018 8:37 PM
Quote Originally Posted by dstrat View Post
Hi, first I will say congrats that amp sounds pretty good to me, much of what you hear IMO is those speakers.
try it with some real greenbacks you will be surprised.
Thank you very much. I have some Greenbacks, it's pretty much the same thing no matter what speakers or mics I use.

The sound doesn't really bother me too much because it doesn't show up in a mix or live, but it's eating at me because it just doesn't seem right. I've done everything in my power to do this build right. Everything else is great. Just that little brittleness has me scratching my head.
 
dstrat 6/29/2018 8:52 PM
um ya could be some slight oscillation which would come down to layout, grid wires (p2 - p7) on ax7 tubes can be fussy about how close they are to other higher level signal carrying wires. but I can only guess at this point. maybe a clear picture of the tube wiring etc may help.
there are lots of knowledgeable folks here that will likely have better ideas then me. only spoke about the G12-65 speakers because they always seem brittle to my ears. crank it up and play it will smooth out over some time playing it also.

yeah I do like the P90s!
 
Greg_L 6/29/2018 9:23 PM
Quote Originally Posted by dstrat View Post
um ya could be some slight oscillation which would come down to layout, grid wires (p2 - p7) on ax7 tubes can be fussy about how close they are to other higher level signal carrying wires. but I can only guess at this point. maybe a clear picture of the tube wiring etc may help.
there are lots of knowledgeable folks here that will likely have better ideas then me. only spoke about the G12-65 speakers because they always seem brittle to my ears. crank it up and play it will smooth out over some time playing it also.

yeah I do like the P90s!
Thanks. My 65s are from 1980, so they should be broken in by now. Haha. The amp is brand spanking new though. I've been beating on it, but it still has that "new amp smell". I just finished it a few weeks ago.

I suppose oscillation is a possibility. I've had it on my scope and the signal path looks normal to me from input all the way to speaker output, but I admit I'm not very well-versed with using oscilloscopes. I just got it pretty recently.

As for the grid wires, I *think* they're pretty good. My heaters are done "Marshall style" with a bit of Valve Wizard thrown in, so there's no criss-crossing with signal and heater wiring. The layout of the board in relation to the tube sockets means I can't avoid a few wires crossing here and there. But over all the amp is very very stable and quiet. It's quieter than any of my Marshalls. And original Fenders looked like pure hell inside, so I feel okay about mine. Hahahaha.

Anyway here are some pics.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180515_120715_zps9oebaplo.jpg[/IMG]

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180515_171310_zpshnboqqh0.jpg[/IMG]

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180515_120705_zpsjyf2bypt.jpg[/IMG]

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180515_184256_zpslzni4aql.jpg[/IMG]

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180527_162352_zpsmgs9l9be.jpg[/IMG]

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180527_162214_zpsk1fasqcc.jpg[/IMG]
 
dstrat 6/29/2018 10:13 PM
over all everything looks ok , I didn't look real close , this time of night my eyes are shot.
give it some time it will mellow over time if played it regularly.

are those tubes new ones or are like this real tung-sol? just curious
[ATTACH=CONFIG]49551[/ATTACH].

I have tried to cook this tube but it just keeps kicking.

yeah you did a very nice job with that amp. I would be very happy!
 
Greg_L 6/29/2018 10:26 PM
Quote Originally Posted by dstrat View Post
over all everything looks ok , I didn't look real close , this time of night my eyes are shot.
give it some time it will mellow over time if played it regularly.

are those tubes new ones or are like this real tung-sol? just curious
[ATTACH=CONFIG]49551[/ATTACH].

I have tried to cook this tube but it just keeps kicking.

yeah you did a very nice job with that amp. I would be very happy!
Thanks a lot!

Mine are new production Tung Sols. Honestly, I'm not that into NOS vintage tubes. *Ducks for cover*

I know they're nice and all, I have a few nice old 12AX7s, but my amps get ridden hard and for me new stuff seems to be fine....and cheaper. I blew up some TAD EL34s in a 50W Plexi cranked to the moon - like really blew them up. It was quite the light show. But overall I've had good luck with regular production tubes.
 
Chuck H 6/29/2018 10:29 PM
Looks and sounds right to me If you wanted a darker sounding amp you should have built a different model. DR's aren't known to be "mellow"

Here's a trick I use to kill the glassies on one model I build. Connect a 4.7pf/1000V ceramic cap from the plate to the grid on each input triode. Don't go larger or you will hear the affect on the overall EQ. Try it on one channel first (obviously) to see if it gives you what you want. It shaves off the shards without really killing the top end. Further, the effect is relative to gain. So the more you attack the more it works. Worth a try.

It's worth noting that most really good players like that glassy top end and they alter their attack to suit what they're trying to do. Removing it for such players is limiting. YMMV. And FWIW I have the 4.7pf cap on my own amp (I'm not a really good player )
 
dstrat 6/29/2018 10:29 PM
oh I just have a few old ones laying around I really like the sound of (compared to newer ones) but hey we have to use what we can get now days , I say don't worry be happy!
 
Greg_L 6/29/2018 10:38 PM
Quote Originally Posted by Chuck H View Post
Looks and sounds right to me If you wanted a darker sounding amp you should have built a different model. DR's aren't known to be "mellow"

Here's a trick I use to kill the glassies on one model I build. Connect a 4.7pf/1000V ceramic cap from the plate to the grid on each input triode. Don't go larger or you will hear the affect on the overall EQ. Try it on one channel first (obviously) to see if it gives you what you want. It shaves off the shards without really killing the top end. Further, the effect is relative to gain. So the more you attack the more it works. Worth a try.

It's worth noting that most really good players like that glassy top end and they alter their attack to suit what they're trying to do. Removing it for such players is limiting. YMMV. And FWIW I have the 4.7pf cap on my own amp (I'm not a really good player )
Hey thanks for the tip!

I like bright. That's no problem. I do not want "dark". I just wasn't sure if that little bit of brittleness in my sound is supposed to be there or not. I'm a life-long Marshall guy and this is actually my first Fender type amp. I played a lot of Fenders in stores before I decided the Deluxe Reverb is what I wanted to build.

This amp will primarily be used for semi-aggressive surf/rockabilly type tones. There will be lots of reverb and tremolo action happening with this thing!

So for those caps, I will give it a try and see what happens. I've done something similar with high-gain mods on Marshalls to kill oscillation. Do they really need to be 1000v though? There's only ~200v on the plates. Just curious.
 
Chuck H 6/29/2018 10:55 PM
500V would be fine. I suggest 1000V because you REALLY don't want that cap to ever fail short and apply HV to the input grid (which is connected to your guitar!!!).
 
Greg_L 6/29/2018 11:01 PM
Quote Originally Posted by Chuck H View Post
500V would be fine. I suggest 1000V because you REALLY don't want that cap to ever fail short and apply HV to the input grid (which is connected to your guitar!!!).
Ha right. Okay thank you.

I'll go pick up some caps tomorrow and see what happens.
 
Steve A. 6/30/2018 6:25 AM
Quote Originally Posted by Greg_L View Post
Mine are new production Tung Sols. Honestly, I'm not that into NOS vintage tubes. *Ducks for cover*
If you search around the forum here you will learn that no one still makes 6V6s anymore... the current supply are basically military tubes that have been tested and do work in Deluxe Reverbs, etc. You might see what is recommended here for DRs these days.

While NOS tubes can be very pricey there are some sellers on eBay that sell used and tested old stock tubes, usually with a 30 day exchange period.

Steve A.

P.S. The head cabinet for your DR looks really cool. I wonder if it would fit my 2006 DRRI chassis...

P.P.S. So how many hours have you run your amp? It seems to me that they mellow out a bit with time... perhaps the caps? Or maybe I just get tuned in and pick softer...

You got this covered with your original from 1980 but Warehouse Guitar Speakers makes an ET65 Celestion clone that sells for $80. GC & MF sell the 8 & 16 ohm varieties with free shipping which is really cool since WGS's shipping charges have always been a deal killer for me.

Not that I tend to go overboard (I do! I do!) but I've ordered 4 of them since discovering them last year — best.speaker.ever!
 
Greg_L 6/30/2018 8:29 AM
Quote Originally Posted by Steve A. View Post
If you search around the forum here you will learn that no one still makes 6V6s anymore... the current supply are basically military tubes that have been tested and do work in Deluxe Reverbs, etc. You might see what is recommended here for DRs these days.

While NOS tubes can be very pricey there are some sellers on eBay that sell used and tested old stock tubes, usually with a 30 day exchange period.

Steve A.
Interesting, thank you for chiming in. I'll look into that.

P.S. The head cabinet for your DR looks really cool. I wonder if it would fit my 2006 DRRI chassis...
Ha thanks. It's a Weber head cab. My wife chose the purple, I chose the Marshall grill cloth because ultimately I'm a Marshall kind of guy. I've since put a purple jewel over the pilot light and it looks pretty cool.

P.P.S. So how many hours have you run your amp? It seems to me that they mellow out a bit with time... perhaps the caps? Or maybe I just get tuned in and pick softer...
The amp is really really new. I've only put...I don't know...maybe few hours in total on it in little bursts. Did you hear my sound clip? If so, do you think that brittle sound is a normal it-just-needs-be-broken-in type of thing...or is it something I need to address? On the first start-up, I brought it up really slowly on a variac and light bulb limiter to "form the caps". Besides one of my own boneheaded mistakes right off the bat (I had the rectifier socket in backwards) the amp came alive and has worked flawlessly....except for this little brittle sound thing.

You got this covered with your original from 1980 but Warehouse Guitar Speakers makes an ET65 Celestion clone that sells for $80. GC & MF sell the 8 & 16 ohm varieties with free shipping which is really cool since WGS's shipping charges have always been a deal killer for me.

Not that I tend to go overboard (I do! I do!) but I've ordered 4 of them since discovering them last year — best.speaker.ever!
I'm a huge fan of WGS speakers. Love them. And I love their business model. What's not to love about good quality and good prices? I only have two myself - Green Berets. They compare okay with real Greenbacks. If I'm really being picky, they're a little more mid-scooped and bright than real Greenbacks, but in a 4x12, in a room, turned up loud, they sound correct.

I have lots of experience with other WGS speakers though through friends and bands that have come through my little studio. The Veteran 30s, Retro 30s, ET-65s,...all of them. Every WGS I've ever mic'd has been pretty nice.

I've never tried any of their "American" speakers though. My plan all along was to build a Fender style head to be used with Marshall cabs. I've got an empty cab out in the garage. Maybe I need to make an "American" cab for this DR clone head.
 
Steve A. 6/30/2018 11:23 AM
Quote Originally Posted by dstrat View Post
oh I just have a few old ones laying around I really like the sound of (compared to newer ones) but hey we have to use what we can get now days , I say don't worry be happy!
Always a good idea to have some tubes around that we KNOW sound good...

Steve A.
 
tubeswell 7/2/2018 5:27 AM
I think I can hear what youíre describing, but only when I listen to your soundclip with headphones.

A couple of possibilities spring to mind.
1) The OT youíre using might be the cause. Either try another 25W 6k6 OT (or at least try a couple of feedback tricks to tame the Ďice pickí - like a 100pF speed-up cap in parallel with the 820R NFB resistor, or 220pF grid to cathode on each 6V6.)
2) lead dress -try separating some of the pre amp socket leads a bit more - they look a bit bunched up with different socketsí leads overlapping/right next to each other
3) I take it youíre using a 12AT7 in the LTP?
 
Greg_L 7/2/2018 7:11 AM
Quote Originally Posted by tubeswell View Post
I think I can hear what you’re describing, but only when I listen to your soundclip with headphones.

A couple of possibilities spring to mind.
Yes, the brittle sound is subtle, and it doesn't show up in a recorded mix or live in the room. I didn't even notice it until I solo'd some studio tracks and I was like wtf is that? The amp sounds great overall IMO, but that little noise caught my attention and now I'm obsessed with it.

1) The OT you’re using might be the cause. Either try another 25W 6k6 OT (or at least try a couple of feedback tricks to tame the ‘ice pick’ - like a 100pF speed-up cap in parallel with the 820R NFB resistor, or 220pF grid to cathode on each 6V6.)
I chose this Classic Tone 40-18087 OT for the multi-taps. I want to use this head with 16 ohm Marshall cabs. It's *supposed* to be the one for this sort of thing, but you might be right. Replacing the OT will be a last-resort kind of thing for me. I've seen some of the negative feedback tricks and I do have those on my radar if the simpler stuff doesn't work first.

2) lead dress -try separating some of the pre amp socket leads a bit more - they look a bit bunched up with different sockets’ leads overlapping/right next to each other
This is certainly doable. Poking around last night with a chopstick with the amp on a scope, I found that the bridge connecting the cathodes on my PI needs to be re-flowed. It's cutting out and crackly with the chopstick test. That's something that will be addressed today. I also found that my V4 tube was buggy. The heaters in V4 went out while gently poking around and never came back. V1, V2, and V3 went dark. I replaced V4 and things came back to life. The layout doesn't really lend itself to really clean lead dress on some tube sockets, but I will definitely try to improve on it. One positive is that the amp is very quiet at idle. So I at least don't have a bunch of heater hum coming through.

3) I take it you’re using a 12AT7 in the LTP?
Yes sir. JJ 12AT7.

Thanks for your input!
 
mikepukmel 7/2/2018 10:16 AM
I did the same as you, first amp build AB763 Deluxe, started about a year ago, got sound out of it late last year. For various reasons (suggestions from friends/family), I have a Celestion Black Shadow C90 - 90 Watt - 12" speaker. When I first got it running, it was brutally 'ice pick' bright. I could not stand listening to it. I had some wiring/lead dress problems causing oscillations, cleaned that up (took two significant efforts), and it improved slightly. But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems. Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).
 
Greg_L 7/2/2018 10:48 AM
Quote Originally Posted by mikepukmel View Post
I did the same as you, first amp build AB763 Deluxe, started about a year ago, got sound out of it late last year. For various reasons (suggestions from friends/family), I have a Celestion Black Shadow C90 - 90 Watt - 12" speaker. When I first got it running, it was brutally 'ice pick' bright. I could not stand listening to it. I had some wiring/lead dress problems causing oscillations, cleaned that up (took two significant efforts), and it improved slightly. But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems. Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).
Thanks for the input. You may be right - I just haven't played it enough. I've given it some hard use, but it's been in short blasts. Basically as soon as I fired it up and verified that my voltages are in range and nothing blew up and all of the features worked, I put it to work in the studio. It definitely has not had anywhere near 100 hours of actual playing time on it.
 
Steve A. 7/2/2018 12:02 PM
Some mods for your amp...
Quote Originally Posted by Greg_L View Post
I chose this Classic Tone 40-18087 OT for the multi-taps. I want to use this head with 16 ohm Marshall cabs. It's *supposed* to be the one for this sort of thing, but you might be right. Replacing the OT will be a last-resort kind of thing for me. I've seen some of the negative feedback tricks and I do have those on my radar if the simpler stuff doesn't work first.
I don't know if this is still applicable (or would apply to the regular OT from the Weber kit) but the unofficial rule of thumb with BF/SF amps was that you could go up or down by a factor of two when matching speakers to OT impedance. So with a nominal output transformer impedance of 8 ohms using a 4 or 16 ohm cab should be "good enough for rock'n'roll". In any case it never bothered me...

I heard that there were a lot of Marshall OTs dying back in the 60s and 70s but that might have been caused by faulty impedance selector switches more than impedance mismatches.

As for "negative feedback tricks" do you mean conjunctive filters or something else? Actually for an AB763-ish amp adding a 25k pot in series with the 820R negative feedback resistor is a really slick mod for opening up the sound a bit. Adding a 25k pot in series to ground with the 6K8 "midrange" resistor in the tone stacks is another cool mod. (You did try removing the 47pf bright cap on the volume pot, right?) For a nice midrange boost adding a 500pF to 1000pF cap across the 250pF treble cap is cool (my favorite is switching a 390pF cap across a 330pF treble cap, silver mica if you have 'em.)

Adding a Vox AC-30 style Cut control after the 0.1uF PI coupling caps is another cool mod if you wire it up to a switched pot to remove it from the circuit completely when not needed. Or go hogwild with a PPI master volume control to match the output to the room...

Steve A.
 
Greg_L 7/2/2018 12:23 PM
Quote Originally Posted by Steve A. View Post
I don't know if this is still applicable (or would apply to the regular OT from the Weber kit) but the unofficial rule of thumb with BF/SF amps was that you could go up or down by a factor of two when matching speakers to OT impedance. So with a nominal output transformer impedance of 8 ohms using a 4 or 16 ohm cab should be "good enough for rock'n'roll". In any case it never bothered me...
You're probably right but I didn't want to chance it.
This is the OT I used:
http://www.classictone.net/40-18087.html
http://www.classictone.net/40-18087.pdf

"Description: This is a 4/8/16 Ohm version of the classic! This is a nice, beefy, taller upgrade version of the original Deluxe / Deluxe Reverb output transformer but still has the same mounting centers for easy, drop in installation in these great classic amps. This is also a great general purpose 4/8/16 Ohm output transformer for many 20W or "so" type new OEM amp builds and D.I.Y. amp projects as well. Furthermore, it features an attractive black finish. Like the vintage era originals, it is paper layer wound for that ultimate tone!"

I heard that there were a lot of Marshall OTs dying back in the 60s and 70s but that might have been caused by faulty impedance selector switches more than impedance mismatches.
Marshalls had that little impedance selector plug that would fall out and then boom bye bye OT. I have a 73 Super Lead that still has the original impedance plug, and it scares the hell out of me sometimes, but I very rarely use anything other than 16 ohm cabs with that particular head, so it's set there and stays there. That head also does't get taken to gigs, so it's use is pretty much always at home in the studio and it never moves.

As for "negative feedback tricks" do you mean conjunctive filters or something else? Actually for an AB763-ish amp adding a 25k pot in series with the 820R negative feedback resistor is a really slick mod for opening up the sound a bit. Adding a 25k pot in series to ground with the 6K8 "midrange" resistor in the tone stacks is another cool mod. (You did try removing the 47pf bright cap on the volume pot, right?) For a nice midrange boost adding a 500pF to 1000pF cap across the 250pF treble cap is cool (my favorite is switching a 390pF cap across a 330pF treble cap, silver mica if you have 'em.)

Steve A.
Those sound cool, thanks. More ideas. There are several little tweaks that I've thought about implementing, but I want to make sure the amp is 100% healthy and broken in first. The negative feedback mod I was referring to is simply strapping a 100pf cap across the 820R feedback resistor. It's supposed to allow super highs to bypass the resistor and get used in the negative feedback signal, thus cancelling themselves out. But first I'm going to try some different tubes and I've fixed my PI soldering. As it turns out, one leg of the little cathode jumper didn't get soldered with the cathode wire on pin 8. The wire itself got soldered, but the jumper didn't get melted in with it. Doh! Operator error. It was just laying in there making contact purely by chance. But that's fixed now and I'm going to give it a go and see what happens now.
 
Steve A. 7/2/2018 12:24 PM
Quote Originally Posted by mikepukmel View Post
But a big change came from just playing the thing. It just mellowed out after some number of hours of use. The cap mfr said "it will take something like 100 hours to .." fully break in. Im no where near that now (maybe 50 or 60), but the amp is a lot more mellow. Maybe partly due to the beating I gave the output tubes when diagnosing the oscillation problems.

Some suggested that the speaker loosening up helped as well. I wish I had good A/B recordings to show you the difference (all same tubes, same speaker, just playing and some wiring changes).
I run practically all of my amp builds into the same old speakers and noticed that some of them really smooth out the more I play them. As for breaking in new speakers I read that some boutique builders wire them up to 24vac transformers for 12 to 36 hours... after using Ohm's Law to insert a series resistor make sure that they don't exceed rated wattage.

Steve A.
 
g1 7/2/2018 12:26 PM
Re: heater issue. Probably not related to the sound issue, but you mentioned V1 thru V4 heaters went out but not the others? If so I suspect a connection at V4 socket rather than the tube itself.
A bad tube that was responsible for pulling down other pre tube heaters should have taken down the power tube heaters as well.
 
Greg_L 7/2/2018 12:27 PM
Quote Originally Posted by g1 View Post
Re: heater issue. Probably not related to the sound issue, but you mentioned V1 thru V4 heaters went out but not the others? If so I suspect a connection at V4 socket rather than the tube itself.
A bad tube that was responsible for pulling down other pre tube heaters should have taken down the power tube heaters as well.
Okay good to know. Thanks. I'll revisit the heater wiring on V4.
 
tubeswell 7/2/2018 1:39 PM
You say chopsticking some joints showed problems. Definitely give those bad solder joints a good reflow - they wonít be helping it sound good. Also, Keep the joints clean by making sure the iron tip is clean.
 
Greg_L 7/2/2018 2:21 PM
Quote Originally Posted by tubeswell View Post
You say chopsticking some joints showed problems. Definitely give those bad solder joints a good reflow - they won’t be helping it sound good. Also, Keep the joints clean by making sure the iron tip is clean.
Just did that. Reflowed a few pins, got everything nice and secure. Chopsticking shows no flutter or weirdness now. That much of it is good to go at this point.

So here's what I've done:
All new tubes - all of the chinese came-with-the-kit tubes are replaced.
Reflowed a few bad solders - all that's good.
Checked for oscillation to the best of my ability - no problems found there.
Chopsticking the lead dress showed no change.
Heated up the bias a little bit to 23ma.

Tone test - still the same. Lol.

The overall tone is indeed better. The tubes made a difference. But that brittleness is still there. Here's what I've found in addition to the previous stuff:

The brittleness I'm hearing is still on each channel, with each input. Bright switches on or off. Nothing has changed there.
With V3 reverb driver completely removed, the brittleness is still there. V3 has no effect in or out.
The brittleness is the same with the treble rolled all the way off, or all the way up. The tone gets darker and brighter as expected with the treble control, but the level and amount of that brittle sound is unchanged lurking within the base tone.
With a parametric EQ in the DAW I've kind of narrowed the noise to around the 3.2khz range. That's where the sound seems the most pronounced.

So at this point, I'm going to try the cap across the negative feedback resistor and see what happens..
 
Greg_L 7/2/2018 4:15 PM
More update:

The 100pf cap across the negative feedback resistor yielded no change.
I also tried the "copper cap" which is a solid-state rectifier tube replacement. No change.
 
tubeswell 7/2/2018 7:00 PM
You could try going to a 220pF cap.

Other than that, my hunch is itís likely to be OT or lead dress related.

If it is HF oscillation caused by inter winding capacitance in that OT, you could try one more thing which is slugging the dominant FB loop pole with 220pF grid to cathode on each 6V6.
 
Greg_L 7/2/2018 7:58 PM
Quote Originally Posted by tubeswell View Post
You could try going to a 220pF cap.

Other than that, my hunch is it’s likely to be OT or lead dress related.

If it is HF oscillation caused by inter winding capacitance in that OT, you could try one more thing which is slugging the dominant FB loop pole with 220pF grid to cathode on each 6V6.
Well check this out and tell me what you think....
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180702_200530_zpsqwzegvrr.jpg[/IMG]

That's V2b grid. Vibrato channel, vol knob around 5-6 is where this happens. I'm injecting a 1k sine about 300 mv pk to pk into input 1, and that's the weirdness I'm getting on the 2nd stage.

Here's a little video sweeping the volume knob on V2.



What do yall think of that kink in my sine wave?

That does not happen on the normal channel.

This is probably not related to my brittle noise, but it caught my attention anyway.
 
The Dude 7/2/2018 8:35 PM
I see what appears to be high frequency noise/oscillation riding on the sine wave. Some generators are not that "pure". You might scope the generator output and make sure it's clean. That said, I've got a '68 Custom Deluxe Reverb Reissue in the shop now that's doing the same thing. I'll be following along and when I get to it, I'll post anything helpful I come up with.
 
Greg_L 7/2/2018 8:48 PM
Quote Originally Posted by The Dude View Post
I see what appears to be high frequency noise/oscillation riding on the sine wave. Some generators are not that "pure". You might scope the generator output and make sure it's clean. That said, I've got a '68 Custom Deluxe Reverb Reissue in the shop now that's doing the same thing. I'll be following along and when I get to it, I'll post anything helpful I come up with.
Thanks, that would be awesome.

My "generator" is actually an app on my phone. I adjust the volume and measure the output before I put it into the amp. I don't get that kink on the normal channel though. The signal through the normal channel stays pretty pure throughout the gain stages. I only get that kink on the vibrato's first gain stages and it's only halfway through the volume sweep. At low volume, the signal stays intact. And at max volume the signal is again a smooth sine wave. It's just in the middle it gets goofy.
 
galaxiex 7/2/2018 8:56 PM
Quote Originally Posted by Greg_L View Post
Thanks, that would be awesome.

My "generator" is actually an app on my phone. I adjust the volume and measure the output before I put it into the amp. I don't get that kink on the normal channel though. The signal through the normal channel stays pretty pure throughout the gain stages. I only get that kink on the vibrato's first gain stages and it's only halfway through the volume sweep. At low volume, the signal stays intact. And at max volume the signal is again a smooth sine wave. It's just in the middle it gets goofy.
I'm just spitballin' here...

I have a 1970 Traynor YGM III that had similar looking sine wave in the middle of the vol pot sweep.
Turned out to be the reverb circuit.
If I disconnected the tank leads it cleaned up.
I don't remember from earlier in the thread if you tried that?

Edit; ahhh NVM, I see you pulled the reverb tube.

In my case it was reverb circuit oscillating.
 
Greg_L 7/2/2018 8:59 PM
Quote Originally Posted by galaxiex View Post
I'm just spitballin' here...

I have a 1970 Traynor YGM III that had similar looking sine wave in the middle of the vol pot sweep.
Turned out to be the reverb circuit.
If I disconnected the tank leads it cleaned up.
I don't remember from earlier in the thread if you tried that?
No I haven't, but I sure will. Thanks!
 
tubeswell 7/2/2018 11:22 PM
Quote Originally Posted by Greg_L View Post
Well check this out and tell me what you think....
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180702_200530_zpsqwzegvrr.jpg[/IMG]

That's V2b grid. Vibrato channel, vol knob around 5-6 is where this happens. I'm injecting a 1k sine about 300 mv pk to pk into input 1, and that's the weirdness I'm getting on the 2nd stage.[/video]


What do yall think of that kink in my sine wave?

That does not happen on the normal channel.
So if you swap V1 and V2 around, does that grid weirdness stay on the same pin/socket?
 
dstrat 7/2/2018 11:58 PM
It seems to me your seeing distortion from over driving the first triode with that 300mv.

I would try a more sane input voltage and see if its still there..
 
pdf64 7/3/2018 3:16 AM
Quote Originally Posted by Greg_L View Post
...What do yall think of that kink in my sine wave?
That does not happen on the normal channel....
Is the bright off?
Try it again with V3 reverb driver removed.

I guess that parasitic capacitances are acting to add more harmonics (from later in the circuit, eg reverb send) under certain conditions of impedance at the grid.
Being low signal level and high impedance, the preamp grid wires are very sensitive 'receivers', so lead dress is will affect this sort of thing.

300mV p-p doesnít seem an excessive input signal, equates to ~100mV rms.
 
Greg_L 7/3/2018 4:33 AM
Quote Originally Posted by tubeswell View Post
So if you swap V1 and V2 around, does that grid weirdness stay on the same pin/socket?
I'll try it. I think I got the same thing with the old tubes, but I can't remember now.

Quote Originally Posted by dstrat View Post
It seems to me your seeing distortion from over driving the first triode with that 300mv.

I would try a more sane input voltage and see if its still there..
Okay thanks. But I think my generator input is less than what a guitar pickup kicks out, no? I had a guitar on the scope just to see how hot my pickups peaked and it put out well over 500mv pk to pk with hard picking. But, as always, it's entirely possible I did that wrong. Lol.

Quote Originally Posted by pdf64 View Post
Is the bright off?
Try it again with V3 reverb driver removed.

I guess that parasitic capacitances are acting to add more harmonics (from later in the circuit, eg reverb send) under certain conditions of impedance at the grid.
Being low signal level and high impedance, the preamp grid wires are very sensitive 'receivers', so lead dress is will affect this sort of thing.

300mV p-p doesn’t seem an excessive input signal, equates to ~100mV rms.
Yes, bright switch off. Everything I'm doing is with the bright switch off. I'll give it a try, thanks. I'll try just about anything. I did play the amp with V3 removed. My initial problem was still there - the brittleness. Since that brittleness happens from both channels, I don't think my V2 sine wave kink is related. I think they're two separate issues.

So today's first round of testing will be more tube rolling, play with the input voltage, try to address lead dress better, and scope test with V3 removed.
 
Chuck H 7/3/2018 6:56 AM
I suppose it's possible to get a glimpse of what an amp may be doing at note attack by cranking up the signal generator. But the amp won't ever have to actually respond to a guitar that way. The difference is that an amp actually can manage peaks to some degree with more grace than a constant signal at that peak level. So the scope shot result can be a little misleading. That is, testing with a constant signal at pick attack levels won't show you how the amp responds to transients. It will show you how the amp responds to attempting to amplify a constant signal at the higher transient level. Not the same thing. I'm sure there are ways to test with a pulse and trigger capture the shot. I haven't worked out how to do that and it may require bench gear most average builders don't have.
 
dstrat 7/3/2018 7:23 AM
+1 ^^^ Chuck said what I was thinking but better then I could at the time.
My point was turn down the generator to the point where the sine is clean and note the voltage.
Driving it harder is no problem but does not reveal anything other then tube does distort.
imo,
the attack noise your trying to fix could be PI, power tubes, OT trfm.
 
Greg_L 7/3/2018 7:46 AM
Quote Originally Posted by Chuck H View Post
I suppose it's possible to get a glimpse of what an amp may be doing at note attack by cranking up the signal generator. But the amp won't ever have to actually respond to a guitar that way. The difference is that an amp actually can manage peaks to some degree with more grace than a constant signal at that peak level. So the scope shot result can be a little misleading. That is, testing with a constant signal at pick attack levels won't show you how the amp responds to transients. It will show you how the amp responds to attempting to amplify a constant signal at the higher transient level. Not the same thing. I'm sure there are ways to test with a pulse and trigger capture the shot. I haven't worked out how to do that and it may require bench gear most average builders don't have.
Mr Rob Robinette suggested I scope the amp while playing it. That is going way way way above my pay grade, but I suppose it's worth a shot.

Another simple variable that I just considered is my recording preamp. It's unlikely to be the problem, but I've recorded all of my test sound clips using the same channels on this preamp interface. I need to try a different preamp just to make sure that the noise isn't on the recording end of things. God, wouldn't that be a bitch if I'm chasing an amp bug that isn't actually an amp bug at all? That would be infuriating and relieving all at the same time.

Quote Originally Posted by dstrat View Post
+1 ^^^ Chuck said what I was thinking but better then I could at the time.
My point was turn down the generator to the point where the sine is clean and note the voltage.
Driving it harder is no problem but does not reveal anything other then tube does distort.
imo,
the attack noise your trying to fix could be PI, power tubes, OT trfm.
That's what I've been thinking all along. It's gotta be output stage related since the noise is on either channel and the tone stack doesn't affect it at all...
 
Greg_L 7/3/2018 10:35 AM
Okay mi amigos....more scope testing has yielded a finding.

First, I dropped my input signal to about 100 v pk to pk. That cleaned up the kink in V2.
Pulled V3 - reverb driver - and re-tested at V2b grid and speaker output - smooth clean sine wave at both locations. No more kink.
Max boosted input signal with V3 removed - still no kink. A little clipping at max volume, but the kink was not there.
Put V3 back in, disconnect reverb tank - no kink.
Reconnect reverb tank - kink is back on grid and at output.

So I'm thinking it's gotta be in that reverb driver/recovery circuit, and this.....
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180703_112440_zps71ig6aal.jpg[/IMG]

That is V3. See that little loop connecting the plates? Chopsticking that thing drastically alters the shape of the output sine wave and kink shape. Moving the grid and cathode wires near the loop did nothing. But jiggling that loop around made huge differences, even going into full-on rampant oscillation. So I'm thinking I'll need to re-route that thing somehow and re-test.

Am I on the right track here?
 
Greg_L 7/3/2018 12:21 PM
Well, I went ahead and flipped the plate-to-plate jumper wire back around behind the tube socket and my signal got MUCH better. There is only the tiniest bit of kink in the wave, I mean barely there, and it's not until I wind the volume way up to like 9-10.

Additional chopsticking and moving wires around yielded no noticeable effects at all.

Just for grins I jumper-wired a 460pf cap across the plate-to-cathode on V3b. I think that may be a Silverface thing? But what the hell, I'll give it a shot. Holy crap that totally smoothed it out and I didn't lose any amplitude. My signal seems pure....or at least kink-free and smooth like a swedish girl's hair.

So maybe I'm making progress.

Yall please keep the ideas and opinions coming. I'm taking it all in and I appreciate it very much.
 
pdf64 7/3/2018 12:21 PM
Yes, that loop will be a significant transmitter.
Shorten it right back and go over the top or around the other side.
As it is, it's awful close and parallel to V2 6-8 wires.
 
Greg_L 7/3/2018 12:30 PM
Quote Originally Posted by pdf64 View Post
Yes, that loop will be a significant transmitter.
Shorten it right back and go over the top or around the other side.
As it is, it's awful close and parallel to V2 6-8 wires.
Yeah I looped it around the other side and it got significantly better. Thanks!
 
g1 7/3/2018 1:05 PM
I'm a bit surprised at the comments about the recording preamp. I'd assumed the brittleness was something you'd noticed through the speaker?
 
Greg_L 7/3/2018 1:35 PM
Quote Originally Posted by g1 View Post
I'm a bit surprised at the comments about the recording preamp. I'd assumed the brittleness was something you'd noticed through the speaker?
No, it's subtle. I can't hear it through the speakers and I'm not going to jam my ear against it either. Lol. It doesn't show up "in the room" nor can you hear it in a mix with bass and drums and stuff. But it's there in a single isolated track. That's how I initially found it. I did some tracks with the amp using two mics on one cab. I was solo'ing each mic'd track to see which one sounded better to be the main source for the mix, and that's how I found this little sound.
 
pdf64 7/3/2018 2:19 PM
Grid stoppers on V3 are very helpful, eg 22k / grid.
 
Greg_L 7/3/2018 3:08 PM
Quote Originally Posted by pdf64 View Post
Grid stoppers on V3 are very helpful, eg 22k / grid.
Really. Hmm that's interesting. I'll put that on my checklist. Thanks.


So here's an update:

Here's a test sound clip, this time with a Strat. Please take a listen and tell me what you think....not of my ham-fisted playing...but the tone:
https://jmp.sh/JNqadSq

After all the good things I've done, the little bit of brittleness is still there. Damn. But I do feel good about the things I did actually improve, so...whatever.

Here are some scope images of the vibrato channel with a 1k-100mv pk-pk sine wave going into the high input.

No volume. Yellow input signal, blue output signal at speaker jack.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180703_145135_1530647610135_zpsscgridep.jpg[/IMG]


Volume on "5". Yellow input, blue output at jack.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/part2_zpse62nfvdk.jpg[/IMG]


Kuhhhh-ranked to 10.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/part0_zpshc3ialga.jpg[/IMG]



So, to me, in my n00b non-expert opinion of a guy just trying his luck, the signal looks pretty good. Nice and even, symmetrical, all that good stuff. So I don't know.
 
mikepukmel 7/3/2018 4:35 PM
Quote Originally Posted by Greg_L View Post
Okay mi amigos....more scope testing has yielded a finding.

First, I dropped my input signal to about 100 v pk to pk. That cleaned up the kink in V2.
Pulled V3 - reverb driver - and re-tested at V2b grid and speaker output - smooth clean sine wave at both locations. No more kink.
Max boosted input signal with V3 removed - still no kink. A little clipping at max volume, but the kink was not there.
Put V3 back in, disconnect reverb tank - no kink.
Reconnect reverb tank - kink is back on grid and at output.

So I'm thinking it's gotta be in that reverb driver/recovery circuit, and this.....
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180703_112440_zps71ig6aal.jpg[/IMG]

That is V3. See that little loop connecting the plates? Chopsticking that thing drastically alters the shape of the output sine wave and kink shape. Moving the grid and cathode wires near the loop did nothing. But jiggling that loop around made huge differences, even going into full-on rampant oscillation. So I'm thinking I'll need to re-route that thing somehow and re-test.

Am I on the right track here?
Oh brother, that is exactly the problem I had: (and your wiring is 6.02 x 10 ^23 times better than mine. Or more), reverb wiring was making the adjacent preamp wiring very angry. Very. I made the loop way shorter and put it on the other side of the tube, towards the back of teh chassis, and also, made sure that the reverb driver wiring did not run parallel to any of the adjacent preamp wiring. A feat for my 11 left thumbs, and 7 right thumbs. (significantly more difficult to diagnose without a scope as well). You also have two yellow wires, one leading to the reverb driver and one leading down someplace that are a little close and parallel. Try moving that back as well.
 
Greg_L 7/3/2018 4:42 PM
Quote Originally Posted by mikepukmel View Post
Oh brother, that is exactly the problem I had: (and your wiring is 6.02 x 10 ^23 times better than mine. Or more), reverb wiring was making the adjacent preamp wiring very angry. Very. I made the loop way shorter and put it on the other side of the tube, towards the back of teh chassis, and also, made sure that the reverb driver wiring did not run parallel to any of the adjacent preamp wiring. A feat for my 11 left thumbs, and 7 right thumbs. (significantly more difficult to diagnose without a scope as well). You also have two yellow wires, one leading to the reverb driver and one leading down someplace that are a little close and parallel. Try moving that back as well.
Ha, yes mine was angry too.

Thanks for the tips man.

Those yellow wires are both cathode wires. I will go in and start separating things a little better. The board layout in relation to the tube sockets makes it very difficult to keep things from crossing or getting to close to each other.
 
mikepukmel 7/3/2018 4:45 PM
There's a wire that runs from the cap board, through a grommet to the tube side of the eyelet board, up to an eyelet and joins with the wire runnning to the reverb transformer. How is that wire routed?
 
Greg_L 7/3/2018 4:58 PM
Quote Originally Posted by mikepukmel View Post
There's a wire that runs from the cap board, through a grommet to the tube side of the eyelet board, up to an eyelet and joins with the wire runnning to the reverb transformer. How is that wire routed?
It's a pretty direct shot, and twisted with the reverb transformer wire. I did it this way so I can flip the board if need be.

The red and orange twisted wires.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180515_120909_zps62zficy0.jpg[/IMG]
 
mikepukmel 7/3/2018 7:35 PM
Just to re-iterate, I know close to nothing. I had huge help from the gurus on this great site to get my amp playable. That said, I had problems with the wiring to the reverb transformer, the loop around the reverb driver, and also parallel wires on a few of the tubes.

[ATTACH=CONFIG]49608[/ATTACH]

Im looking for a photo of my chassis for you to laugh at.

***
The photo sites like flickr and some others seem to be getting all "pay for this now" about linking to random photos. Anyway, I spent ages hunting down what I was fairly sure were period blackface amp chassis, the ones with really good lead dress. Lots of really good examples in those older amps. Some of the later 60's and early 70's amps, I have no idea how they worked at all.
 
Greg_L 7/3/2018 8:31 PM
Quote Originally Posted by mikepukmel View Post
Just to re-iterate, I know close to nothing. I had huge help from the gurus on this great site to get my amp playable. That said, I had problems with the wiring to the reverb transformer, the loop around the reverb driver, and also parallel wires on a few of the tubes.

[ATTACH=CONFIG]49608[/ATTACH]

Im looking for a photo of my chassis for you to laugh at.

***
The photo sites like flickr and some others seem to be getting all "pay for this now" about linking to random photos. Anyway, I spent ages hunting down what I was fairly sure were period blackface amp chassis, the ones with really good lead dress. Lots of really good examples in those older amps. Some of the later 60's and early 70's amps, I have no idea how they worked at all.
Right? I too looked at tons of original examples, and I'm like, how the effin hell did these things work at all? I mean everyone screams from the rooftops about lead dress, meanwhile thousands of 50s and 60s Fender amps worked hard day and night looking like a blind 5 year old put them together. WTF?
 
Enzo 7/3/2018 8:42 PM
lead dress isn't about being pretty. There are some boutique builders who go out of their way to make all the wires in nice square corners, and lashed together, and solder joints all doped. Who cares?

What lead dress means is keeping your signal lines at 90 degree angles to wires carrying AC when they must cross. It means moving grid wires away from plate wires, and again, if they might interact, make them cross at more of an angle.

Lead dress often means routing something in as straight a path as possible. The super neat wire job where the wire goes off to one side then turns across the board and then back to a control just adds several inches to a wire that is sensitive.
 
Greg_L 7/3/2018 8:47 PM
Quote Originally Posted by Enzo View Post
lead dress isn't about being pretty. There are some boutique builders who go out of their way to make all the wires in nice square corners, and lashed together, and solder joints all doped. Who cares?

What lead dress means is keeping your signal lines at 90 degree angles to wires carrying AC when they must cross. It means moving grid wires away from plate wires, and again, if they might interact, make them cross at more of an angle.

Lead dress often means routing something in as straight a path as possible. The super neat wire job where the wire goes off to one side then turns across the board and then back to a control just adds several inches to a wire that is sensitive.
Cool, thanks.

Well one thing I've done is got the heater wires totally out of the way. I think I'm pretty good with that. The amp is very very quiet - quieter than any of my Marshalls. My heaters are crammed in the corner of the chassis and make a direct shot to and over the tube socket. My signal wires are all coming in from the sides. Some of them cross and that can't be avoided while trying to maintain a reasonably short route to the tubes.

But tomorrow I will try to separate and maybe re-route some of them better.
 
Greg_L 7/3/2018 8:50 PM
Quote Originally Posted by Enzo View Post
lead dress isn't about being pretty. There are some boutique builders who go out of their way to make all the wires in nice square corners, and lashed together, and solder joints all doped. Who cares?

What lead dress means is keeping your signal lines at 90 degree angles to wires carrying AC when they must cross. It means moving grid wires away from plate wires, and again, if they might interact, make them cross at more of an angle.

Lead dress often means routing something in as straight a path as possible. The super neat wire job where the wire goes off to one side then turns across the board and then back to a control just adds several inches to a wire that is sensitive.
Cool, thanks.

Well one thing I've done is got the heater wires totally out of the way. I think I'm pretty good with that. The amp is very very quiet - quieter than any of my Marshalls. My heaters are crammed in the corner of the chassis and make a direct shot to and over the tube socket. My signal wires are all coming in from the sides. Some of them cross and that can't be avoided while trying to maintain a reasonably short route to the tubes.

But tomorrow I will try to separate and maybe re-route some of them better.
 
Greg_L 7/5/2018 4:46 AM
So a few updates....

I think I have two things going on here - a little minor oscillation and the brittle noise that was the start of this whole thread.

First, the oscillation. I say "minor" oscillation because it doesn't freak out the scope or send the tubes into cutoff or anything, but it shows as little tiny kinks in the signal at various points in the amp. Maybe that isn't oscillation at all? I mostly notice these little sine wave deformities at the very peaks (top or bottom) of the signal and only when the amp is cranked or just shy of being cranked. And it's only in the vibrato channel.

Moving lead dress around has no effect. Besides the one plate jumper relocation on V3 I did earlier, no other wire jiggling or separating has made any difference. What does make a difference? The reverb tank. If I disconnect the reverb tank input, any abnormality in the scope display goes to perfectly normal. Any kink or hiccup in the sine wave goes smooth when I disconnect the reverb tank. What do yall think of that?

Second issue, the persistent brittle noise. Still stumped on this one. The amp otherwise sounds great and works great, it's just got that damn little sound hiding in the base tone and I want it gone if possible.

One more thing for now....is it weird that my output signal at the speaker jack is 180 out of phase with the input signal? I know the signal flips a few times as it travels throughout the amp. Am I just ending up with an odd number of signal flips and flops?
 
galaxiex 7/5/2018 6:06 AM
What reverb tank? I mean the actual numbers.
Are both tank jacks grounded?
perhaps there is a ground loop thru the tank.
 
Greg_L 7/5/2018 6:24 AM
Quote Originally Posted by galaxiex View Post
What reverb tank? I mean the actual numbers.
Are both tank jacks grounded?
perhaps there is a ground loop thru the tank.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180705_071653_zpsrrxzrqer.jpg[/IMG]


Type 4
8 ohms input impedance
2250 output impedance
Long decay
Input insulated/output grounded
No lock
Horizontal open side down mounting

My RCA jacks are grounded via their mounting contact with the chassis. Did I create a ground loop by having the RCA jacks chassis grounded?
 
Chuck H 7/5/2018 7:09 AM
Looking at the photo's it seems like your reverb transformer may be grounded at the buss near the input? It's typically grounded at the chassis with the RCA jacks. Grounding is the next thing to suspect when lead dress changes don't seem to improve stability.
 
Greg_L 7/5/2018 7:43 AM
Quote Originally Posted by Chuck H View Post
Looking at the photo's it seems like your reverb transformer may be grounded at the buss near the input? It's typically grounded at the chassis with the RCA jacks. Grounding is the next thing to suspect when lead dress changes don't seem to improve stability.
Ah, yes it is grounded with the preamp grounds near the inputs. I've got the preamps/buss bar grounded at a bolt near the inputs and the reverb transformer is grounded there as well. I can change that and see what happens.
 
tubeswell 7/5/2018 5:20 PM
Most pans have one RCA jack sleeve isolated from the pan chassis, which stops ground loop hum through the pan circuit.

If youíre still getting the HF Ďdisturbanceí, then I would still look at lead dress. Particularly keeping plate and grid leads to tube sockets physically separated by an inch, or where itís necessary to have them closer, having them criss-cross at right angles - to minimise EM induction that could otherwise generate oscillation. Also keep sensitive signal wires in shielded cable or laid close on the chassis.

Also, consider the OT. If you have used a Ďnon-standardí OT (e.g. with more interleaving than usual), you might be developing some Pr:Sec HF phase shift (as a result of increased interwinding capacitance) which could produce HF oscillation. Thatís why I suggested 220pF grid-to-cathode at each 6V6 socket. YMMV
 
Greg_L 7/5/2018 5:31 PM
Quote Originally Posted by tubeswell View Post
Most pans have one RCA jack sleeve isolated from the pan chassis, which stops ground loop hum through the pan circuit.

If you’re still getting the HF ‘disturbance’, then I would still look at lead dress. Particularly keeping plate and grid leads to tube sockets physically separated by an inch, or where it’s necessary to have them closer, having them criss-cross at right angles - to minimise EM induction that could otherwise generate oscillation. Also keep sensitive signal wires in shielded cable or laid close on the chassis.

Also, consider the OT. If you have used a ‘non-standard’ OT (e.g. with more interleaving than usual), you might be developing some Pr:Sec HF phase shift (as a result of increased interwinding capacitance) which could produce HF oscillation. That’s why I suggested 220pF grid-to-cathode at each 6V6 socket. YMMV
Oh yeah, I forgot about that. My head's been swimming.

I've moved wires back and forth, up and down, none of that has any effect on the tiny little kink in my waveform. I'm kind of starting to think maybe that's just how it is? The amp doesn't squeal, hiss, hum, or pop or anything. It's very very quiet at idle, and it sounds great cranked. The only issue was that little brittle sound that has sent me down this bottomless rabbit hole! And the little kink totally goes away with the reverb pan unplugged. The kink was much worse before I moved that plate-to-plate jumper on V3. And I'm talking a tiny little bump on the waveform. It's not the squiggly psycho oscillation.

I certainly can try little caps across the power tubes. So that's it? Just a 220pf from grid to cathode on each tube socket?
 
galaxiex 7/5/2018 5:36 PM
I don't remember if you have a shielded wire from the reverb pot to the board, (the center lug on the pot).
If not... might be something to try.
Ground the shield at one end only.
 
Greg_L 7/5/2018 5:38 PM
Quote Originally Posted by galaxiex View Post
I don't remember if you have a shielded wire from the reverb pot to the board, (the center lug on the pot).
If not... might be something to try.
Ground the shield at one end only.
I do not have that wire shielded, but I can try it. I have lots of shielded wire. Shielding ALL of the grid wires has been in the back of my mind for a few days.
 
tubeswell 7/5/2018 5:44 PM
Quote Originally Posted by Greg_L View Post
I certainly can try little caps across the power tubes. So that's it? Just a 220pf from grid to cathode on each tube socket?
Probably worth a try, but itís just a hunch - me being on the other side of the worldíníall, I canít really get up close myself to see the OT for myself with a scopeíníall etc.
 
Greg_L 7/5/2018 5:48 PM
Quote Originally Posted by tubeswell View Post
Probably worth a try, but it’s just a hunch - me being on the other side of the world’n’all, I can’t really get up close myself to see the OT for myself with a scope’n’all etc.
I think if you just start digging you might get here eventually.
 
tubeswell 7/5/2018 5:55 PM
Iíve dug myself into enough holes already. Iím staying put.

Even if itís an OT that has been carelessly scatterwound, albeit with the correct number of turns and interleaving, it could result in sound dissatisfaction. Canít really tell without at least trying something that removes the possible Ďinterferenceí pathway.

Snubbing the dominant HF NFB pole with 220pF might reveal something (or it might not)
 
Greg_L 7/5/2018 6:57 PM
Quote Originally Posted by tubeswell View Post
I’ve dug myself into enough holes already. I’m staying put.

Even if it’s an OT that has been carelessly scatterwound, albeit with the correct number of turns and interleaving, it could result in sound dissatisfaction. Can’t really tell without at least trying something that removes the possible ‘interference’ pathway.

Snubbing the dominant HF NFB pole with 220pF might reveal something (or it might not)
I'm going to try your suggestions when I get the amp back in the loud room.
 
pdf64 7/6/2018 10:55 AM
So grid stoppers on V3 didnít help with the kink?
 
Greg_L 7/6/2018 11:35 AM
Quote Originally Posted by pdf64 View Post
So grid stoppers on V3 didn’t help with the kink?
Whoa, I didn't try that. I must have missed that suggestion or it got lost in the clusterf*** that is my head right now.

The grids are linked on V3 across the socket and the actual grid wire comes off pin 7. Would I use just one grid stopper at pin 7?

Never mind, I found your suggestion. Sorry. 22k/grid.

What's the best way to implement this idea if the grids are linked?
 
Greg_L 7/6/2018 2:01 PM
Quote Originally Posted by pdf64 View Post
Grid stoppers on V3 are very helpful, eg 22k / grid.
So this might have helped a tiny bit. I put a 22k on pin 7 of V3 and cleaned up some solders. Here's my kinky kinks....



Zoomed in a little...this is taken off V2b grid - reverb tank/tube installed. Volume around 8.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180706_142040_1530905350372_zpsdx0a1tfx.jpg[/IMG]

Same signal, reverb tank disconnected.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180706_142055_1530905350831_zpsxyhix6gt.jpg[/IMG]


Backed out view - reverb in circuit
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180706_142138_1530905350696_zpsqzcuptl0.jpg[/IMG]

Reverb disconnected
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180706_142147_1530905350533_zpscs5msm3b.jpg[/IMG]



And just for reference....here's where it was a few days ago.
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180702_200530_zpsqwzegvrr.jpg[/IMG]
 
g1 7/6/2018 5:46 PM
I bet if you sweep through some freqs with a dial type sig gen, you will see the kink get more or less severe. I just consider it an artifact of the reverb circuit?
 
The Dude 7/6/2018 5:55 PM
FWIW, the amp I'm working on ('68 Custom Deluxe Reverb) exhibits the symptom more on a "D" than any other note (if that helps). Not to say that the amp in this thread has the identical symptom.

 
Greg_L 7/6/2018 6:14 PM
Quote Originally Posted by g1 View Post
I bet if you sweep through some freqs with a dial type sig gen, you will see the kink get more or less severe. I just consider it an artifact of the reverb circuit?
I'll try it. I'm really starting to think that it's just the way it is. As I've said, the amp is fine. It doesn't do anything weird at all except for the little bit of brittle that started this whole thread, and even that seems to be getting marginally better and better as I play it more and more. No weird noises, hums, pops, buzzes, nothing. It works perfectly. I've had amps go into real serious oscillation and this one does none of that. So I'm thinking I'm just gonna live with it, play the hell out of it, and go back in later and see what's up.

Quote Originally Posted by The Dude View Post
FWIW, the amp I'm working on ('68 Custom Deluxe Reverb) exhibits the symptom more on a "D" than any other note (if that helps). Not to say that the amp in this thread has the identical symptom.

Which issue are you having? The brittle sound?

I've heard my brittle sound in other Fender amps. Mine just seems a little crispier, so I wanted to try and figure it out.
 
The Dude 7/6/2018 6:34 PM
The customer described it as a "tinny distortion mostly when I play a 'D'". He is right. I hear it and it's not something typical. I've just started on the amp and also have other projects going, so it may be a bit. I haven't scoped it yet, but I'll update as to what I find.

Edit: I've also found that it's unaffected by tone controls. Even if you turn the treble down, the high frequency distortion is still present.
 
Jazz P Bass 7/6/2018 7:51 PM
Note: Reply to the Dudes post:

Cabinet vibration?

And don't forget to look for foreign metal objects stuck to the speaker magnet.
 
Greg_L 7/6/2018 7:54 PM
Quote Originally Posted by The Dude View Post
The customer described it as a "tinny distortion mostly when I play a 'D'". He is right. I hear it and it's not something typical. I've just started on the amp and also have other projects going, so it may be a bit. I haven't scoped it yet, but I'll update as to what I find.

Edit: I've also found that it's unaffected by tone controls. Even if you turn the treble down, the high frequency distortion is still present.
Yes that sounds similar to mine.

I haven't narrowed mine down to "D". When sweeping a recorded track with an EQ, it seems to me that it "pops" the most around 3200hz....which is like a G or something.
 
Greg_L 7/6/2018 7:57 PM
Quote Originally Posted by Jazz P Bass View Post
Cabinet vibration?

And don't forget to look for foreign metal objects stuck to the speaker magnet.
No, I wish, but that's not it. I've tried it through three different cabs, five speakers, six mics, several preamps....it's always the same. It's in the amp. And these cabs get loud Marshalls blasted through them regularly and they don't have this same sound issue.
 
galaxiex 7/6/2018 8:16 PM
Quote Originally Posted by The Dude View Post
FWIW, the amp I'm working on ('68 Custom Deluxe Reverb) exhibits the symptom more on a "D" than any other note (if that helps). Not to say that the amp in this thread has the identical symptom.


Hmmmm, I didn't hear the symptom on this until I turned up the bass control on my computer speakers.

(I have a Logitech THX speaker system with sub)

At first I was worried that my old guy hearing was going...
which is very worrying to me, since I have surprisingly excellent hearing for someone my age... 60.

But with the bass turned up I clearly hear a distinct "buzz".

Sorry for the slight O/T.

back to our regular program...
 
The Dude 7/6/2018 8:28 PM
FWIW: That "video" is just a generic test tone from YouTube. It's not a recording of the amp I'm working on. Sorry for the confusion. I simply linked it as a courtesy to the OP.
 
Greg_L 7/6/2018 8:46 PM
Quote Originally Posted by The Dude View Post
FWIW: That "video" is just a generic test tone from YouTube. It's not a recording of the amp I'm working on. Sorry for the confusion. I simply linked it as a courtesy to the OP.
Thanks. My phone generator app will do sweeps. I'll try that frequency and see what happens.
 
Chuck H 7/6/2018 9:36 PM
Have you grounded the reverb circuit away from the preamp yet???
 
Greg_L 7/6/2018 9:51 PM
Quote Originally Posted by Chuck H View Post
Have you grounded the reverb circuit away from the preamp yet???
Not yet. That's on my to-do list. I started to get after it this afternoon and got sidetracked.

My RCA jacks are grounded to the chassis via contact. The transformer is grounded with the preamp buss. Should I just run it to the RCA jacks? Does it matter which one?
 
Greg_L 7/6/2018 10:19 PM
Quote Originally Posted by Chuck H View Post
Have you grounded the reverb circuit away from the preamp yet???
I just thought of something that might be an issue....

As you all probably know, this circuit has four B+ "nodes".

The filter cap node (B) that powers the reverb transformer is grounded with the power supply/center taps/output section (A,B,C). But I have the transformer itself grounded with the preamp node (D) which is separate from the other grounds. My thinking on this was to keep the preamp stuff totally separated from the output/power stuff. What I didn't consider at the time was having the transformer grounding with the preamps while the reverb's power supply is grounded elsewhere.

It would be simple to alter this arrangement and have the A node isolated with the power supply and B, C, and D ground together at the buss bar, which would put my reverb power and ground on the same plane....like an original stock Deluxe Reverb.

Is this something I should explore?
 
Chuck H 7/6/2018 10:37 PM
Having B, C and D on the same ground away from the main filter is STILL less than ideal. At face value it doesn't look as though "Fender" considered grounding, but they did. A little. Their reverb circuit hums like it doesn't know the words. But this is WRT the signal path, not the power supply. Their power supply has all the filters grounded in the same place. For low gain applications this is probably acceptible, sort of. Where Fender did occasionally make concessions is with chassis grounds. I think there was clearly some alteration to what is grounded on the chassis where to achieve the lower noise option. Still not ideal, but ok as long as the amp isn't pressed into maximum output all the time (as todays amps are judged).

Each filter should be grounded with it's circuit at the chassis or on the buss as they occur in the circuit. If that makes sense.?. Short of trying to sort that out, just ground the reverb circuit at the RCA jacks for the tank and see if that doesn't help. It shouldn't matter which jack since they're only an inch apart on the chassis! There may still be a bunch of hum introduced when the reverb is turned up. We can deal with that as a different issue. Right now we just want the amp to behave.
 
tubeswell 7/6/2018 10:39 PM
Quote Originally Posted by Greg_L View Post
I just thought of something that might be an issue....

As you all probably know, this circuit has four B+ "nodes".

The filter cap node (B) that powers the reverb transformer is grounded with the power supply/center taps/output section (A,B,C). But I have the transformer itself grounded with the preamp node (D) which is separate from the other grounds. My thinking on this was to keep the preamp stuff totally separated from the output/power stuff. What I didn't consider at the time was having the transformer grounding with the preamps while the reverb's power supply is grounded elsewhere.

It would be simple to alter this arrangement and have the A node isolated with the power supply and B, C, and D ground together at the buss bar, which would put my reverb power and ground on the same plane....like an original stock Deluxe Reverb.

Is this something I should explore?
I would ground the 12AT7 reverb driver cathode, grid leak and RT secondary at the same filter cap that you are taking the reverb transformer primary from. The other ground returns for teh reverb recovery etc can be with the filter cap node that you are taking the reverb recovery stage from. YMMV
 
Greg_L 7/6/2018 11:32 PM
Quote Originally Posted by Chuck H View Post
Having B, C and D on the same ground away from the main filter is STILL less than ideal. At face value it doesn't look as though "Fender" considered grounding, but they did. A little. Their reverb circuit hums like it doesn't know the words. But this is WRT the signal path, not the power supply. Their power supply has all the filters grounded in the same place. For low gain applications this is probably acceptible, sort of. Where Fender did occasionally make concessions is with chassis grounds. I think there was clearly some alteration to what is grounded on the chassis where to achieve the lower noise option. Still not ideal, but ok as long as the amp isn't pressed into maximum output all the time (as todays amps are judged).

Each filter should be grounded with it's circuit at the chassis or on the buss as they occur in the circuit. If that makes sense.?. Short of trying to sort that out, just ground the reverb circuit at the RCA jacks for the tank and see if that doesn't help. It shouldn't matter which jack since they're only an inch apart on the chassis! There may still be a bunch of hum introduced when the reverb is turned up. We can deal with that as a different issue. Right now we just want the amp to behave.
Quote Originally Posted by tubeswell View Post
I would ground the 12AT7 reverb driver cathode, grid leak and RT secondary at the same filter cap that you are taking the reverb transformer primary from. The other ground returns for teh reverb recovery etc can be with the filter cap node that you are taking the reverb recovery stage from. YMMV
I see. Thanks. Well my amp definitely does not hum. I don't know if you heard my sound examples, but noise is not one of the issues I'm fighting. The only unwanted noise in my clips is the occasional noise of a single coil. I'm not fighting a hum issue, I'm just trying to make sure everything is working optimally.

If each filter should be grounded with it's circuit, if I'm understanding you correctly, then I should alter my filter cap grounding...putting B, C and D together at the buss bar to input jack corner ground - like Fender intended. The reverb transformer gets power from "B", and the preamp circuit, reverb circuit, and all the related pots and cathodes go to ground via the buss bar to chassis, so they'll all be grouped together and far far away from the mains/power supply/center taps ground. Moving the reverb transformer's ground to the RCA jacks (which are chassis grounded) will put it grounded in close proximity with the tank. So that will leave me with three grounding points - like a triangle - all pretty far from each other. But keep in mind I'm only trying to smooth out that kink that happens on the scope with the reverb circuit. It's not a hum issue.

Basically it would just be this:
[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/filter%20board_zpsbwgkrnyx.jpg[/IMG]

I'd remove the X'd ground wire and put one between the C and D nodes. That would leave A node grounded all by itself at the PT, and the rest grounded via buss bar to input jack location.
 
Greg_L 7/6/2018 11:33 PM
Quote Originally Posted by tubeswell View Post
I would ground the 12AT7 reverb driver cathode, grid leak and RT secondary at the same filter cap that you are taking the reverb transformer primary from. The other ground returns for teh reverb recovery etc can be with the filter cap node that you are taking the reverb recovery stage from. YMMV
If I'm understanding Chuck H correctly, you're saying the same thing, and I think I'm going to try that. It would be less intrusive and less labor intensive to move one wire on the filter cap board than it would to reroute all of those other grounds.
 
Greg_L 7/7/2018 1:04 PM
Well friends...made some changes.

Grid stopper on V3Bb - this definitely smoothed the top end a bit with the reverb. Thanks pdf64.
Grounded reverb tranny at RCA jacks - this improved the kink in my waveform to being almost not even there. It's so slight now you have to really zoom way in on it. So great tip there, thank you Chuck.
Grounded filter nodes B, C, and D together at buss bar to input jacks pretty much as Fender spec'd it out - this yielded no significant change except the amp is actually even quieter...which I didn't think could be possible, so that's great.
Did some frequency sweeping while scoping the output. It was all over the map so I don't know what I learned from that, but it was neat to do. The waveform never got too odd anywhere, so I guess that's a good thing.

Tone test...haven't mic'd it up yet, but just playing it in the room sounds great. Very loud, very punchy, reverb and tremolo work beautifully, sounds like a typical Blackface to me...so all that's good.

I think I might just chill for a little while and play the snot out of it and check again later.

[IMG]http://i29.photobucket.com/albums/c280/whodat6464/Music%20gear/Fender-Weber%20DR%20AB763-6A20%20Amp%20Kit%20Build/20180707_140750_zps4gl9aj8r.jpg[/IMG]
 
tubeswell 7/7/2018 1:18 PM
Can't remember what you said you'd done about the filter cap voltage ratings. Did you actually put 500V caps in after all?
 
Greg_L 7/7/2018 1:20 PM
Quote Originally Posted by tubeswell View Post
Can't remember what you said you'd done about the filter cap voltage ratings. Did you actually put 500V caps in after all?
Yes. The A node got 22uf/500v caps.
 
The Dude 7/10/2018 8:14 PM
Firstly, I know we're talking about 2 different amps here and my apologies for butting into your thread. I'm only posting as it may help you figure out your distortion issue as i had the same symptom. Keep in mind that even though symptoms are similar, solutions may be quite different. All that said, I'm kicking the amp I'm working on out the door and here's what I found.

Scope told me the distortion on this amp started at the PI. The '68 reissue has a 1.5K NFB resistor- not sure why they changed it from the original 820. I installed an 820 and things got better, but still a tiny bit of distortion. This also took care of a motor boating issue. I lowered the bias to about 18mV at roughly 400V. That's where the amp sounded best and the ugly distortion was even a bit less, but still there. Stubborn thing. I then stuck a slightly lower gain 12AT7 in the PI position and distortion gone.
 
Greg_L 7/11/2018 6:06 PM
Quote Originally Posted by The Dude View Post
Firstly, I know we're talking about 2 different amps here and my apologies for butting into your thread. I'm only posting as it may help you figure out your distortion issue as i had the same symptom. Keep in mind that even though symptoms are similar, solutions may be quite different. All that said, I'm kicking the amp I'm working on out the door and here's what I found.

Scope told me the distortion on this amp started at the PI. The '68 reissue has a 1.5K NFB resistor- not sure why they changed it from the original 820. I installed an 820 and things got better, but still a tiny bit of distortion. This also took care of a motor boating issue. I lowered the bias to about 18mV at roughly 400V. That's where the amp sounded best and the ugly distortion was even a bit less, but still there. Stubborn thing. I then stuck a slightly lower gain 12AT7 in the PI position and distortion gone.
Thanks for the update.

I moved my negative feedback wire to the 8 ohm tap on my OT and it seems to be better in the room, but I have yet to try it mic'd up - which is where the issue revealed itself. I had to go out of of town, but I'm back now and will test it tomorrow.
 
mikepukmel 7/11/2018 7:13 PM
Quote Originally Posted by Greg_L View Post
I just thought of something that might be an issue....

As you all probably know, this circuit has four B+ "nodes".

The filter cap node (B) that powers the reverb transformer is grounded with the power supply/center taps/output section (A,B,C). But I have the transformer itself grounded with the preamp node (D) which is separate from the other grounds. My thinking on this was to keep the preamp stuff totally separated from the output/power stuff. What I didn't consider at the time was having the transformer grounding with the preamps while the reverb's power supply is grounded elsewhere.

It would be simple to alter this arrangement and have the A node isolated with the power supply and B, C, and D ground together at the buss bar, which would put my reverb power and ground on the same plane....like an original stock Deluxe Reverb.

Is this something I should explore?
Interesting! On this blog, I posted some questions about B+ on the reverb driver (I was worried about smoking these cheap tubes made these days since it was so far over rated) and the gurus suggested that I put a dropping resistor off that main node, and another filter cap, and feed the reverb from that instead of the spot right after the choke. Ive been running my franken-deluxe like that for a while. B+ is a little lower, not near spec though. Sounds OK.
 
The Dude 7/11/2018 8:46 PM
I'm surprised you can't hear it, but the mic can. Don't discount the idea that the mic or the mic preamp might be adding the clipping sound and it has nothing to do with the amp.
 
Greg_L 7/11/2018 9:10 PM
Quote Originally Posted by The Dude View Post
I'm surprised you can't hear it, but the mic can. Don't discount the idea that the mic or the mic preamp might be adding the clipping sound and it has nothing to do with the amp.
Yeah, I've already verified that. Different mics, channels, preamps, speakers, cabs...all of it. The sound is the same.

The sound is very very subtle. You can't hear it in the room, and it hides itself in a full mix. I didn't notice it until I solo'd a recorded track during a mix and I was "wtf is that"? Ever since then I've been a on a mission. Lol.

I'm really hopeful that getting the negative feedback right has fixed it.
 
Greg_L 7/12/2018 5:32 PM
Well the negative feedback tweak didn't fix my brittle sound. Fuuuuuuudge. It's still there.

What now? I'm thinking maybe upping the power tube grid resistors?

I'm using stock value 1.5k right now. Would bumping them up to 10k...50k...help maybe?


The brittle sound goes away if I roll the vol back on the guitar, or turn the amp down. When I dig in or hit it hard, that's when the brittle sound happens. It's not the guitar - it does it with any guitar. Any speaker, any cab. I really think it's something power tube/phase inverter related. When the amp is really clean, it doesn't happen. It only happens when it's breaking up or pushed hard - either channel.
 
tubeswell 7/12/2018 5:44 PM
Did you fit 220pF grid-to-cathode on each 6V6 socket?
 
tubeswell 7/12/2018 5:47 PM
Another idea is to scope the LTP duty-cycle when the amp is under high-signal conditions. LTPís like to be centre-biased if youíre going to run the amp real hard all the time.
 
Greg_L 7/12/2018 6:06 PM
Quote Originally Posted by tubeswell View Post
Did you fit 220pF grid-to-cathode on each 6V6 socket?
No. I can try that. Do I just span a cap from pin 5 to pin 8?

Quote Originally Posted by tubeswell View Post
Another idea is to scope the LTP duty-cycle when the amp is under high-signal conditions. LTP’s like to be centre-biased if you’re going to run the amp real hard all the time.
No idea what this means.
 
tubeswell 7/12/2018 6:14 PM
Quote Originally Posted by Greg_L View Post
No. I can try that. Do I just span a cap from pin 5 to pin 8?(
Yep.

Re: the LTP, Iím in the field on my iPhone now, and Iíll give a full explanation when I get back to my work-station.
 
Greg_L 7/12/2018 6:22 PM
Quote Originally Posted by tubeswell View Post
Yep.

Re: the LTP, I’m in the field on my iPhone now, and I’ll give a full explanation when I get back to my work-station.
That would be awesome, thanks.

One thing I have not done is scoped it while playing a guitar.

The more I read about "blocking distortion" the more I think that's what I have going on. What I've read describes symptoms similar to mine.

On the plus side it seems a simple and common fix is to just up the value of the grid stoppers, and I'm currently using damn near the minimum.
 
Justin Thomas 7/12/2018 7:31 PM
Quote Originally Posted by The Dude View Post
The '68 reissue has a 1.5K NFB resistor...
Slight derailing here... the "Custom 68" series are NOT reissues. Hence all the changes. These are not intended to be as spot- on as possible Like the 65 series are. Sure, they look the part, but they're not supposed to behave like a real 68 Fender. Otherwise, they'd have that "mixed bias" that supposedly everyone hates, etc.

I know it's a small point, but I feel it's worth making...

Carry on.

Justin
 
tubeswell 7/12/2018 7:48 PM
Quote Originally Posted by Greg_L View Post
The more I read about "blocking distortion" the more I think that's what I have going on. What I've read describes symptoms similar to mine.

On the plus side it seems a simple and common fix is to just up the value of the grid stoppers, and I'm currently using damn near the minimum.
Yes you can easily go up to 10k -47k grid stoppers on an output tetrode or pentode without noticing any HF roll off, because of the low Miller capacitance enabled by the screen.
 
Greg_L 7/12/2018 8:15 PM
Quote Originally Posted by tubeswell View Post
Yes you can easily go up to 10k -47k grid stoppers on an output tetrode or pentode without noticing any HF roll off, because of the low Miller capacitance enabled by the screen.
I think I'll try that next. Seems easy enough.
 
Steve A. 7/12/2018 11:19 PM
Quote Originally Posted by Justin Thomas View Post
Quote Originally Posted by The Dude View Post
The '68 reissue has a 1.5K NFB resistor...
Slight derailing here... the "Custom 68" series are NOT reissues. Hence all the changes. These are not intended to be as spot- on as possible Like the 65 series are. Sure, they look the part, but they're not supposed to behave like a real 68 Fender. Otherwise, they'd have that "mixed bias" that supposedly everyone hates, etc.

I know it's a small point, but I feel it's worth making...

Carry on.

Justin
True enough, Justin... My own theory as to its name is that Fender decided to call it a "Custom '68" because as it was a silverface model it was not as valuable to collectors as the blackface models so it was often a candidate for mods. (As we all know for many years silverface amps were mistakenly considered to be total junk by the amp snobs... until we learned how easy it was to convert them to BF specs if so desired.)

So while not a reissue of something that Fender ever sold it might be a "reissue" (not really the best word) of an actual amp that had been customized... Probably not in the real world but it might have been, right?

Full-sized carry-on bags are not permitted on United Basic Economy flights!

Steve A.
 
tubeswell 7/13/2018 7:19 PM
With small signals in a Long Tail Pair inverter the signal is balanced, but as the signal gets bigger, the result of slight signal imbalance from the inverting triode driving the non-inverting triode gets more accentuated. If the LTP is biased on the warm side, the inverting (master) triode reaches grid-current-limiting sooner causing the input coupling cap to charge up, which in turn shifts the grid voltage more negative thus biasing the tube closer to cut-off. The signal on the master (input) triode plate gets clipped on the positive side, which shifts the whole output signal more negative to compensate. These negative signal peaks are passed into the slave (non-inverting) triode, which cause it to draw grid current, draining charge from its decoupling cap, which means the average bias on the slave triode remains more-or-less unchanged. But because the signal from the master (inverting) triode output is heavily clipped on the positive side, so it spends more time positive than negative, but the non-inverting output is doing the exact opposite. This means that one output tube gets driven harder than the other, which can also cause that side to red-plate under heavy signal conditions. Getting the LTP more centre-biased can help prevent this (along with using decent grid stoppers on the output tube grids).
 
Greg_L 7/13/2018 10:44 PM
Quote Originally Posted by tubeswell View Post
With small signals in a Long Tail Pair inverter the signal is balanced, but as the signal gets bigger, the result of slight signal imbalance from the inverting triode driving the non-inverting triode gets more accentuated. If the LTP is biased on the warm side, the inverting (master) triode reaches grid-current-limiting sooner causing the input coupling cap to charge up, which in turn shifts the grid voltage more negative thus biasing the tube closer to cut-off. The signal on the master (input) triode plate gets clipped on the positive side, which shifts the whole output signal more negative to compensate. These negative signal peaks are passed into the slave (non-inverting) triode, which cause it to draw grid current, draining charge from its decoupling cap, which means the average bias on the slave triode remains more-or-less unchanged. But because the signal from the master (inverting) triode output is heavily clipped on the positive side, so it spends more time positive than negative, but the non-inverting output is doing the exact opposite. This means that one output tube gets driven harder than the other, which can also cause that side to red-plate under heavy signal conditions. Getting the LTP more centre-biased can help prevent this (along with using decent grid stoppers on the output tube grids).
Wow, thanks for that. Makes sense.

So how do I look for something wrong there, and what do I look for?

I have not had any red plating. This amp has been pushed hard in it's short life and nothing's gone bad yet.
 
Steve A. 7/13/2018 10:44 PM
Quote Originally Posted by tubeswell View Post
With small signals in a Long Tail Pair inverter the signal is balanced, but as the signal gets bigger, the result of slight signal imbalance from the inverting triode driving the non-inverting triode gets more accentuated...
Thank you for making such an informative post! I just started a new folder for my Evernote cloud archive titled "___MEF.CLASSICS" right next to the existing "___MEF" folder... and your post was the initial entry (I saved the text of your post along with the URL. If you had uploaded any images I could have added them as attachments.)

One question: some guitarists like to replace the 12AT7 in the BF/SF phase inverter without changing the circuit. What effect would that have in regards to your explanation?

One comment/questions: the phase inverter in the Dumble ODS #124 works very well with its trim pot. Now that the circuitry of #124 has been verified and widely circulated has its PI been "officially" added to the list of cool mods for the SF/SF design? Also if connected to 4 or 16 ohm speaker tap what would you recommend to replace the 4K7/3W resistor? And finally what value fixed resistor would you suggest in lieu of the 2KB presence control?

[img]http://music-electronics-forum.com/attachment.php?attachmentid=49707&d=1531542911[/img]


Steve A.

P.S. Would anyone care to explain how to set the 5K bias trim pot?

[ATTACH=CONFIG]49706[/ATTACH][ATTACH=CONFIG]49707[/ATTACH]
 
tubeswell 7/13/2018 10:46 PM
Quote Originally Posted by Greg_L View Post
Wow, thanks for that. Makes sense.

So how do I look for something wrong there, and what do I look for?

I have not had any red plating. This amp has been pushed hard in it's short life and nothing's gone bad yet.
Then that's probably not the cause of the problem.

How did the 220pF grid-to-cathode caps work out?
 
tubeswell 7/13/2018 10:51 PM
Merlin Blencowe covers this topic in the 2nd edition designing pre-amps book.

Its more of a problem in classic Marshall amps which run EL34s at high plate and screen (~500V) voltaqes.

If its a 12AT7, 470R-560R is a reasonable (shared) bias resistor value (for a cathode-biased stage). If its a 12AX7, 820R - 1k2 is more appropriate.
 
Greg_L 7/13/2018 11:17 PM
Quote Originally Posted by tubeswell View Post
Then that's probably not the cause of the problem.

How did the 220pF grid-to-cathode caps work out?
Have not tried yet. Tomorrow is work-on-amp-day.
 
Steve A. 7/14/2018 10:34 AM
Quote Originally Posted by Greg_L View Post
Have not tried yet. Tomorrow is work-on-amp-day.
I understand how you want to fix this one last issue (it is the last one, right?) and button up your amp but as a few people have mentioned here it might resolve itself on its own over time which can be more important than amp volume alone as various parts adapt to the circuitry as they sometimes do for mysterious reasons. I attribute it to a very wise and benevolent God but the amp guys here keep telling me that it has to do with science. Go figure!

I am a "fly by the seat of my pants" DIY hobbyist and if it were me I would add a AC30 style Cut control (which is great for matching your amp to whatever speaker you will be using) and if it solves the immediate problem be done with it for now. If you don't want to add or repurpose an existing hole for the control you can use use a trim pot for now or wire up a regular pot and wrap it with Scotch 33+ electrical tape. But I am assuming that as you use the amp you will want to make fine tuning tweaks, etc., and will be pulling the chassis out quite often as I would.

Good luck! When I saw these threads popping up about building amps with reverb and tremolo I did not realize that there were kits to build them priced very reasonably these days... half the retail price of a DRRI. God bless China!

Most of the old timers here would have never thought of trying to build a BF/SF amp with reverb and vibrato from scratch - especially as a first build!

As I think you might have suggested in one of your posts it would be extremely helpful for the vendor or affiliated user group to post detailed pictures showing the exact lead dress for successful builds including the wire under the boards as it can be extremely critical especially as we push these amps with sound and voltage levels never intended by Leo back in the 60's.

BTW one point that might not yet have come up in this discussion is that you sometimes need to shield the larger signal leads later in the circuit to keep them from broadcasting crap that is picked up by the more sensitive circuitry in prior stages. I like to stock a good amount of green waxed pushback wire for that purpose... to at least temporarily wrap around wires that I suspect might be causing interference in previous stages. If you ever do that you can try connecting it to different ground points to see if it makes a difference.

And don't forget about positive and negative feedback... each gain stage inverts the signal so if there is parasitic inference with the immediately previous stage you have negative feedback which will "calm" the signal to use the technical term. It is when you go back two stages (or any even number) that you can get positive feedback which can result in those nasty oscillations the amp guys keep talking about.

Steve A.

P.S. Are the supplied eyelet boards traditional fiberboard/fishpaper/whatever-it-is-is-called-these-days or something more modern? Just wondering...
 
Leo_Gnardo 7/14/2018 11:00 AM
Quote Originally Posted by Steve A. View Post
Are the supplied eyelet boards traditional fiberboard/fishpaper/whatever-it-is-is-called-these-days or something more modern?
Depends on who is supplying the kit. Mojo AFAIK to this day still supplies Fender style kits with black cardboard - to be avoided at all costs! Ask Randall. Kit buyers should make sure they're getting fiberglass/epoxy or some other non conductive material.