mikepukmel 7/17/2018 4:54 AM
Debugging a nice 60/120 hum in a reverb circuit
Down to the last 'bug' in this Deluxe Reverb AB763. With the reverb knob at minimum, amp is quiet even at high vol except for normal hiss when the vol is way up. As I turn up the reverb, hum increases with the knob setting.

Hum is a very clean 60/120 (I can't tell), not a squeal, so I think its got to be something leaking in from the PS somehow. will grab some photos.

Wishful thinking that it was a tank placement issue, I turned everything down, and moved the tank outside the cabinet, pointing in various directions far side from the transformers, no reduction in hum.

The wiring is fairly close to stock, as close as I could get it with my skills. I do have an extra node off the main power supply: one dropping resistor and a 16uf cap, that feeds the reverb driver. Will remove that and see if the hum goes away.

What should I try next, chopstick test? Ive enjoyed playing this amp so much, I hate to take it apart for a few days, but the hum is at an unacceptable level, since I play mostly very low volume, and with reverb.

I also have an old juker 'scope that does provide a trace. Not confident enough to go probing yet. (no isolation xformer yet, will buy one this week).

Thanks, All.
 
pdf64 7/17/2018 6:17 AM
If the hum doesn't reduce with the reverb 'send' cable removed from the back of the amp, I wouldn't bother to bother messing with removing that extra HT node (I suspect it won't).

The hum is most likely a 0V loop, due to a minuscule difference in Vac between the reverb return's HT 0V and signal 0V; have you got your head around Merlin's document on this? http://www.valvewizard.co.uk/Grounding.html
An isolated RCA socket for the return may be helpful, even if real Fenders managed without it; then you can play with exactly where to reference the return 0V connection.
 
Chuck H 7/17/2018 7:55 AM
THAT^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

IMHE all vintage Fender reverbs hum. The cure is usually to isolate one of the tank jacks from the tank chassis so the cable shield is only connected to 0V at the amp chassis. Usually done at the tank input jack.
 
Enzo 7/17/2018 3:03 PM
Think about it, if the power supply was hummy, it would still hum with the reverb all the way down.
 
mikepukmel 7/18/2018 5:59 AM
Thanks everyone. I did read (many times) Merlin's grounding doc. I think I have a pretty good understanding, even if not exactly how to 'see' the issues in an actual amp.

Its more than a little hum, its really annoying. Hard to remember whether I put both socks on this morning, but I don't remember our old, beat up, war torn tube amps back in the 70's having that lound a hum. I think this is out side normal.

I didn't measure it, but one of the blog threads here talked about the Accutronics tanks big plastic isolators on the jacks, and someone said that they did not 'ground' both jacks, only one side has the shield part of the jack connected to the chassis of the tank. I will verify that.

I will try isolating one side, easy enough to do.

As for removing cabling, the reverb driver is an amplifier, can I damage something if I pull the tank cables off with the amp running, do I need some kind of dummy load?

Will try first to isolate one side, and hope it reduces.

As an aside, if I scope the signal, should I see 60/120? Would this be in the several volt range p-to-p (3v 5v?) its after the preamp? I haven't scoped inside anything with high voltage before, heard that its a must to use an isolation xformer on the scope.
 
mikepukmel 7/18/2018 6:05 AM
GOing to go in to work a few minutes late anyway, so: popped the tank out. Accutronics 9AB2C1B. put the ohm meter on the shield portion of the input RCA jacks. Red jack shield to case: 0.3ohms. Red jack shield to white jack shield 0 ohms. white jack shield to case 0 ohms. So, the output jack shield on the tank is connected to the case of the tank, but not the input.

So, any chance that instead of a loop, there is no ground and that is the problem? I will check the amp, shield side of both jacks to the amp chassis case when I get home.
 
mikepukmel 7/18/2018 6:08 AM
maybe my 'extra node on the power supply' wiring is no good. Will jumper that, run a wire the same way the original Fenders's did, and cross my fingers.

What was that saying again .... "If you don't have any idea what you're doing, then ... build it stock..."
 
mikepukmel 7/18/2018 6:17 AM
Quote Originally Posted by Enzo View Post
Think about it, if the power supply was hummy, it would still hum with the reverb all the way down.
I think I don't know enough to ask the right questions. So, if a power supply is hummy, the noise goes everywhere. But you could have a quiet power supply and still get 120 leaking in someplace, right? Maybe a badly run heater wire?

And, a related question: if you have a ground loop, is there a specific frequency to look for, or could it vary?
 
Tom Phillips 7/18/2018 9:31 AM
Quote Originally Posted by mikepukmel View Post
GOing to go in to work a few minutes late anyway, so: popped the tank out. Accutronics 9AB2C1B. I put the ohm meter on the shield portion of the input RCA jacks. Red jack shield to case: 0.3ohms. Red jack shield to white jack shield 0 ohms. white jack shield to case 0 ohms. So, the output jack shield on the tank is connected to the case of the tank, but not the input...
Hi Mike,
You are misinterpreting the results and misunderstanding the design goal. In the stock Fender design the reverb tank input (Drive) connector ground is isolated from the metal case. Only the the output transducer connector ground should be connected to the metal case. THis is done to prevent a ground loop.

You measured "Red jack shield to case: 0.3ohms" That is essentially a connection. Mayne not a perfect one but the resistance is so low the 0.3 Ohm reading could be just a imperfect probe connection.
You measured "Red jack shield to white jack shield 0 ohms" This indicates that both shields are connected together. In this case that path will be through the metal tank case.
You measured "white jack shield to case 0 ohms" More evidence that both connector shields/grounds are connected to the case.

Quote Originally Posted by mikepukmel View Post
...So, the output jack shield on the tank is connected to the case of the tank, but not the input...
That's an incorrect conclusion as explained above.

Therefore, disconnect the reverb tank input connector from the case. That will break the ground loop and, I suspect, solve the hum problem.

Edit: You will notice that I refer to the connections as "Input/Drive" and "output" rather than "red" or "white." I have seen the colored connectors incorrectly installed by the manufacturer so I don't trust them unless I have the tank in front of me to verify. There are also lots of reverb tanks in use that don't use the colored connectors at all.

Cheers,
Tom
 
mikepukmel 7/18/2018 4:59 PM
Hi Tom, Sorry, Oh BOY do I feel stoopid (and I looked in the mirror and look stupid too). I should have written OL not 0 Ohms, in 3 places.

Output jack shield to case: 0.3 ohm.
Output jack shield to input jack shield: OL (no connection)
Input jack shield to case: OL (no connection)

Output pin to shield: 258 ohms
Input pin to shield: 2 ohms

So, anyway, I think its not a ground loop through the tank, maybe in my internal wiring.
 
Enzo 7/18/2018 5:56 PM
Mike, if we report the reading as "open" it eliminates such confusion. OL is the way your particular meter indicates open, it is not an industry term.
 
Chuck H 7/18/2018 7:25 PM
I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.

I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?
 
Leo_Gnardo 7/18/2018 8:28 PM
Quote Originally Posted by Chuck H View Post
I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?
The last few years of USA made Accutronics tanks, they were no picnic either. Failing left and right, never mind the hum follies. I bought a dozen at a time from Magic Parts and there were typically two maybe three that sounded above average, most of the rest OK, one or two that were awful. But even the awful ones found amps they miraculously sounded good in. It was about the same time they started using the two pin slip on connectors to the transducers when things started to suck big time. Beware "improvements," sometimes they are the opposite of improvements. I'm having better luck with the current tanks labeled Ruby (thru Magic) and CE (thru Antique/CE/tubesandmore.) I think they're the same tanks from the same production line, labeled for the outfit that ordered them.
 
mikepukmel 7/19/2018 4:50 AM
Quote Originally Posted by Leo_Gnardo View Post
The last few years of USA made Accutronics tanks, they were no picnic either. Failing left and right, never mind the hum follies. I bought a dozen at a time from Magic Parts and there were typically two maybe three that sounded above average, most of the rest OK, one or two that were awful. But even the awful ones found amps they miraculously sounded good in. It was about the same time they started using the two pin slip on connectors to the transducers when things started to suck big time. Beware "improvements," sometimes they are the opposite of improvements. I'm having better luck with the current tanks labeled Ruby (thru Magic) and CE (thru Antique/CE/tubesandmore.) I think they're the same tanks from the same production line, labeled for the outfit that ordered them.
Thanks Leo. This one says "Accutronics & Belton ... manufactured in Seol Korea". I did get it from Antique Electronic Supply. it doesn't sound very good. That is being generous. I might break down and try a different tank, one of the ones you list. Thanks
 
mikepukmel 7/19/2018 4:53 AM
Quote Originally Posted by Chuck H View Post
I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.

I don't get what's happening to reverb tanks anymore at all. I have to reinvent the game every time I use one and one occasion I had to buy three tanks to get one that sounded good and wasn't horribly sensitive acoustically. Then on the last one I found that transducer orientation in the standard mounting plane hums so I can't mount the tank as it's intended. How is this crap getting to the parts vendors without someone at the design level picking up on it?
Thanks Chuck. I tried taking the tank out of the combo cabinet, and sliding it around on the floor. The last trial, it was outside the cabinet, pointing away from the left side of the amp, and as far from the amp as possible. BUT I did not orient the tank as you suggested, jacks up. THe jacks were facing front or back. Anyway, no matter how far I got the tank, the hum character didn't change, so I think I screwed something else up inside the amp.

If the tank has a problem, like Leo mentioned, transducer mounts, etc, can this cause hum?
 
pdf64 7/19/2018 5:19 AM
As a sanity check, if you've got a footswitch plugged in for the trem/reverb, unplug it; the reverb switch cable requires very good screening, and routing away from hum sources

The DrZ EZG50 (think of a single channel no trem SR AB763 with a type 4 master) has a excellent reverb but only has a short tank (accutronics 8AB3A1B). I'll never buy a long tank again!
The relevance here being that it's much easier to find a happy place for a short tank.
 
Leo_Gnardo 7/19/2018 6:08 AM
Quote Originally Posted by pdf64 View Post
The DrZ EZG50 (think of a single channel no trem SR AB763 with a type 4 master) has a excellent reverb but only has a short tank (accutronics 8AB3A1B). I'll never buy a long tank again!
The relevance here being that it's much easier to find a happy place for a short tank.
"If it sounds good, it IS good" applies here. In some cases I've had to try every tank I have in stock, and it shouldn't be surprising that sometimes it's a short tank that wins the prize. Wish there was a way to predict which would work best in an application but - there isn't.

One nice thing about the CE long tanks intended as direct replacements for old Fender amps, tubesandmore has them available with short, medium and long decay times. I find short decay is the best choice, the reverb doesn't hang in there forever and create the sonic illusion you're playing in a huge arena, otoh it goes on long enough to sound authentic. Now please don't buy 'em all up and leave none available for me.
 
Helmholtz 7/19/2018 6:54 AM
I can't remember which brand I ended up with in my last reverb build, but I had to do something pretty unconventional to mitigate power transformer induced hum. And it's not as if I was in tight quarters either. This was a full size 2X12 combo cab. Anyway, I tried positioning the pan, jacks facing front AND back, all over the floor of the amp with no joy. The only orientation that would silence the hum was with the tank bottom facing the inside of the lower rear baffle, jacks up. So that's how I mounted it. I had to take some additional measures against acoustic vibration because that panel is prone to do that. The amp is a little more tender than average to reverb crash when moving it while it's on. NBD though because at least it doesn't hum.
Hum is induced in the output transducer (receiver) coil. The better versions use a 3-limb closed core like the one shown at the bottom of the article below:
https://www.amplifiedparts.com/tech-...d-and-compared

Closed cores are far less sensitive to magnetic AC field pick-up.

The steel can shields against outer magnetic and electric fields but leaves the bottom side open/ unshielded. I always wondered why nobody uses or offers a steel bottom plate to close the can/tank and keep interference fields completely out. This should make it mostly independent of positioning/orientation.
 
Chuck H 7/19/2018 7:40 AM
Thank you for that piece of info. I don't have the amp here to see what sort of core structure the tanks output transducer uses, but I'll surely check that out if/when I get it on the bench again. It would be interesting (in a 'ah ha, you're a cheap bastard' sort of way) if I found that the simpler two leg, open core were used on that tank.
 
Justin Thomas 7/19/2018 8:06 AM
Quote Originally Posted by Chuck H View Post
I don't get what's happening to reverb tanks anymore at all... How is this crap getting to the parts vendors without someone at the design level picking up on it?
WHY THE HELL ARE YOU STILL USING THAT TECHNOLOGY FROM THE 1940s WHEN OUR DIGITAL SILLYCOOTIE WHIZMOTRONS WILL DO THE EXACT SAME THING? And doesn't waste all that extra metal, and doesn't crash when you kick it... See? Good for the environment AND solves the noise pollution problems! Sucks to your spring reverb, because our potato chip centipede reverb does exactly the same thing! And who wants Spring Reverb anyway, when you can have hall and plate? Damn Luddites, ah tell ya what...

(Sarcasm)

Jusrin
 
J M Fahey 7/19/2018 9:06 AM
Please post here the schematic you are using.

If something you did does not match the original to a t, please state so.

You do NOT need an isolation transformer for your scope in this case, because your amp is transformer powered.

Please scope the output and show us both the "no hum, only hiss reverb on 0" image, of course with no signal input, volume and tone controls set to 0, reverb on 0 (it should look like "grass") and with same scope settings, rising reverb to half ("5") and full ("10").

Thanks.

PS: it would be great if scope setting/scale can be shown, so we can compare , say, "20mVpp Hiss" to, say, "150mVpp Hum/Buzz"

So we also have a before/after reference point to know how much we "improve" results doing different things.
 
Chuck H 7/19/2018 9:13 AM
Quote Originally Posted by Justin Thomas View Post
WHY THE HELL ARE YOU STILL USING THAT TECHNOLOGY FROM THE 1940s WHEN OUR DIGITAL SILLYCOOTIE WHIZMOTRONS WILL DO THE EXACT SAME THING? And doesn't waste all that extra metal, and doesn't crash when you kick it... See? Good for the environment AND solves the noise pollution problems! Sucks to your spring reverb, because our potato chip centipede reverb does exactly the same thing! And who wants Spring Reverb anyway, when you can have hall and plate? Damn Luddites, ah tell ya what...

(Sarcasm)

Jusrin
Well... Not entirely sarcastic. I've had enough frustration with spring reverbs that I've considered going to the "brick" type products. In fact the ONLY reason I don't is that most people interested in tube amps (which is what "I" do) don't like silichicken reverbs on principal (though what principal is never well defined).
 
Justin Thomas 7/19/2018 9:44 AM
To be a bit more honest...
In a modern high-gain or otherwise "not-vintage-Fender-or-Ampeg" a Fender or Ampeg reverb is pretty overkill. The Ampeg especially; never turned that thing above 5. Generally if I'm playing a rock or hard rock amp, I'd rather have fine control over a narrower range than full-on surf at my fingertips.

But if you're talking vintage stuff, there is no substitute. And I appreciate the individual charm that comes with each amp's particular circuit. Parts drift, etc. I know they all have the same "circuit," but dang that Dual Showman Reverb had the best reverb I ever heard. So if you're looking for the exact same tone all the time from every single amp, sure, use a modeler, a pedal, whatever newfangled contraptions they use nowadays. Myself? I'd rather ave the mystery, charm, and individuality of each particular amp... But I'm a dinosaur.

Justin
 
J M Fahey 7/19/2018 1:14 PM
FWIW the Belton Brick sounds very good.
I was *amazed* to learn they don´t use a DSP with some complex algorithm but a trio of very cheesy "karaoke approved" PT2399.
Yet they are clever and use them very well.
 
Tom Phillips 7/19/2018 7:16 PM
Mike,

After reviewing the complete thread again I suggest that you try a few troubleshooting tests to isolate the potential problem. Following are the suggested steps. Please do them in the order listed and report the results for each test.


  1. As pdf64 asked in post #16, tell us if you have a footswitch plugged in. If so then unplug it and tell us if there is any change in the hum issue.
    Next...
  2. Unplug the reverb tank output cable at the tank end. Leave the cable attached at the chassis end. Any change? If so what?
    Next...
  3. Unplug the dangling tank output cable from the chassis. Any Change?


The purpose of this exercise is to determine if the excessive hum is originating before or in the reverb recovery gain stage. What you discover will determine the next troubleshooting tests.

It’s also helpful to know if the hum is 60Hz or 120 Hz. You can determine that with your scope or you can use your guitar. The B flat on the 5th string at the 1st fret is 116.5Hz. The B on the 5th string at the 2nd fret is 123.5Hz. If the hum note lies between those notes then you know the hum is predominantly 120 Hz. You should be able to hear the beat note. If the hum is 120 Hz then we will assume it is power supply related. If it is 60 Hz then we will assume it is heater circuit, line circuit or power transformer field related. I said “predominantly” because, if you measure with sensitive instrumentation there is always a mix of the two frequencies present. (or 50Hz/100Hz in countries that use a 50Hz grid) 60 Hz hum is not always caused by poor wiring or strong stray fields. It can originate inside a tube if the construction of the tube is faulty. Therefore, an easy test is to try a different tube. In your situation the most likely culprit would be the reverb recovery circuit tube which is the fourth tube from the pre-amp end in a Deluxe Reverb AB763. I’d try that if the hum remains after you have done 1,2 & 3 above.

Cheers,
Tom
 
mikepukmel 7/19/2018 8:55 PM
Ok some info: If I take the tank side of the output cable off the tank (cable from tank to reverb recovery) gets noisier, sounds like a guitar cable plugged into the amp but not into the guitar. But fundamental freq hum about the same. If I unplug the cable completely (cable from reverb tank out to amp reverb recovery), the hum is about the same. Increases when reverb pot is increased.

No footswitch.

I tuned my open E string down about 4 fret steps to match the tones fundamental. Its pretty low.

To Juan: my hand mod schematic below. I will scope the output, but I have to dig out the scope and hook it up, need to clear out space on my desktop.

...

[ATTACH=CONFIG]49805[/ATTACH]


A little more info: I did the cable in/out tests with the tank sitting outside the amp, farthest from the transformers.
 
mikepukmel 7/19/2018 9:08 PM
The additional cap is as close to the turret board as I could get. Its mounted to 2 3 lug terminal strips. The resistor is closer to the board. The + side of the cap faces the output tubes. I ran a wire from there, close to the chassis under the board to where the original junction was located, that feeds the reverb driver tube.

[ATTACH=CONFIG]49806[/ATTACH]

When I did this, the capacitor 'can' was full, has 2 x 22uf 500v and 3 x 16 uf 450 caps. I recently bought a 30uf 500v to replce the 2 x 22uf (a little closer to stock specs re capacitance), so I can move the cap that is inside the chassis next to the transformer inside the 'cap can', if that might help.
 
mikepukmel 7/19/2018 9:17 PM
Just looked it up; the low E is 82hz? No kidding? I would never have thougt the fundamental was that low. Then, the hum fundamental is probably 60.
 
Leo_Gnardo 7/19/2018 9:31 PM
Quote Originally Posted by mikepukmel View Post
Just looked it up; the low E is 82hz? No kidding? I would never have thought the fundamental was that low. Then, the hum fundamental is probably 60.
Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.
 
mikepukmel 7/19/2018 9:41 PM
Quote Originally Posted by Leo_Gnardo View Post
Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.
Thanks Leo. Then it can't be 60. Oh boy, where is this hum coming from? I'll dig out the scope (the high school football team is coming over tomorrow to help me carry it from the storage shelf to my desk), I'll try to probe and get a shot of the trace.
 
Tom Phillips 7/19/2018 11:24 PM
Quote Originally Posted by mikepukmel View Post
...If I unplug the cable completely (cable from reverb tank out to amp reverb recovery), the hum is about the same. Increases when reverb pot is increased.
No footswitch.
I tuned my open E string down about 4 fret steps to match the tones fundamental. Its pretty low...Just looked it up; the low E is 82hz? No kidding? I would never have thougt the fundamental was that low. Then, the hum fundamental is probably 60.
It appears that the problem is in the reverb recovery circuit and that the hum is 60 Hz. You can leave the reverb tank disconnected while you continue troubleshooting your hum issue since we seem to have ruled out the tank as the cause of the problem.


Quote Originally Posted by mikepukmel View Post
...Then it can't be 60. Oh boy, where is this hum coming from?...
Yes it could be 60 Hz. Leo just said "Ordinarily the hum fundamental is double the line frequency due to rectification." Not always. I suggest that you re-read the last paragraph in my post #25. Try changing the reverb recovery tube.
 
J M Fahey 7/20/2018 2:46 AM
Quote Originally Posted by Leo_Gnardo View Post
Ordinarily the hum fundamental is double the line frequency due to rectification. 120 Hz in US/Canada & other 60 Hz countries, 100 Hz just about everywhere else where 50 Hz line is standard.
Sorry, the Hum FUNDAMENTAL is 60Hz (in USA) 120Hz is its first harmonic, and rather than HUM, is RIPPLE, not the same by a Country mile.

Besides, 60Hz Hum is basically senoidal, may be somewhat buzzy if mains voltage is dirty (usually because of noisy electric motors or gas discharge loads, think Fluorescent lamps or Neon tubes) while Ripple is basically a Sawtooth.

We are talking different frequencies, although one is an exact multiple of the other, very different waveform (which we won´t know unless we scope them) and very different "sonic signature", which we can tell by ear .

For a given amplitude, ripple (what we call power supply hum) is way more audible than 60 Hz hum, which is both deeper and darker, which our ears don´t pick that well

Real hum comes from poor AC returns, is picked inductively "from the air" , capacitively by electrostatic induction in high impedance circuits, while Ripple-Hum usually gets injected by poorly filtered rails *and* again by poor grounding paths.

That´s why I asked for plain Hiss and still mystery Hum scope captures, which in this case will tell a lot.
 
mikepukmel 7/20/2018 4:42 AM
Great, thanks. My todo list for this weekend:
1) pirate a new 12Ax7 I have earmarked for another amp project (actually have 2 to try) on the recovery side.
2) drag out the scope and probe a) the grid of the reverb recovery, b) just after the 0.003 dc blocking capacitor that is after the reverb recovery tube plate and before the reverb pot.

So, the hum is getting into the circuit in this area? [ATTACH=CONFIG]49814[/ATTACH]
 
pdf64 7/20/2018 6:34 AM
Don't scope the grid, the scope probe will likely cause more hum etc; just the output across the speaker / bench load will be fine.
Yes, the hum is likely to be contaminating things around that stage, possibly due to resistance between its signal 0V and the HT 0V.
 
Tom Phillips 7/20/2018 8:44 AM
Mike,

One additional easy test you can do is ground the grid of the reverb recovery triode and determine if that affects the hum. You could do that with a clip lead or just use the reverb foot switch. This test will tell us if the hum signal is entering through the grid or not. You have made some good progress. The advantage of the troubleshooting approach we are following is that we are closing in on the trouble area by test and measurement rather than jumping right into parts changing. (Except for the tube which won't require soldering work)

Another tip is that you can "borrow" another 12AX7 from the same amp. In this case you could pull the tube from the normal channel pre-amp to try in the reverb recovery stage. That technique is very useful if you need to do emergency repairs at a gig.

Always mark the tubes or otherwise keep track of which is which. We always think we will remember but it's easy to loose track of the original tube positions or the order of the troubleshooting steps performed thus obscuring the clues to the root problem.

Cheers,
Tom
 
Chuck H 7/20/2018 7:13 PM
Ah! Been at work all day and got home to see Tom asking the question I had. That is, what happens when you ground the grid of the recovery stage. Knowing that will narrow the possible sources further.

So...
The hum is 60Hz
Even with the tank cable unplugged from the amp the hum still manifests analogous to reverb knob setting.

That's actually a lot of info right there. But I'm stumped as to how 60Hz can get into that circuit isolated from the tank and transformer. I can't actually think of anything but the filament circuit. Since the rest of the amp isn't humming that seems odd. Might just be a bad tube.
 
mikepukmel 7/22/2018 7:01 PM
Chuck, yep, you and Tom both suggested the same. Before I swap tubes I wanted to see if I could "fire" up the old scope and get some readings. Managed to get the "Compact and Light Weight Tek 7613 off the shelf and I think Im close to getting it to do something./
 
mikepukmel 7/22/2018 7:35 PM
OK Swapped out V1 into reverb recovery spot. Same hum. (left the tube that was there out of the amp, channel 1 is dead quiet). After some back and forth with the old scope, between probing the amp across the speaker, and probing my computer speakers that has a 60hz test tone running, fairly sure its 60hz in the reverb circuit.

Don't have an easy way to get a picture of the trace (would have to drag my camera out), but its not a nice sine wave, its repeatable, sharper peaks on the top than bottom. Really odd looking thing. I will drag the camera out tomorrow and get a photo of the scope trace.

I think the noise is about 200mv p-to-p. I can watch the wave increase and decrease with reverb knob setting, so fairly sure that's what Im looking at. Based on the trace I get with the 0.4v cal square wave.
 
mikepukmel 7/22/2018 9:12 PM
Don't know if it makes any difference but the speaker is out of phase. The signal is on the - side and ground +.
 
mikepukmel 7/22/2018 9:13 PM
Chuck: re
Quote Originally Posted by Chuck H View Post
.... That's actually a lot of info right there. But I'm stumped as to how 60Hz can get into that circuit isolated from the tank and transformer. I can't actually think of anything but the filament circuit. Since the rest of the amp isn't humming that seems odd. Might just be a bad tube. ...
You haven't seen my wiring!!
 
Tom Phillips 7/22/2018 9:14 PM
Quote Originally Posted by mikepukmel View Post
...someone over the past 22 years sitting in a closet gave the "Cal" knobs a twist...
Just turn the Cal knob all the way clockwise until it "clicks." That is the calibrated setting for the scope.


Quote Originally Posted by mikepukmel View Post
...Vertical Amplifier is set at 20mv/div. There are 9 horizontal lines that span the display. Between each horizontal line there are 5 tick marks. Is the tick mark 20mv?...
No. Each major horizontal line will berepresent 20 mV with that setting if you are using a 1X probe. It will be 200mV if you use a 10X probe.


Quote Originally Posted by mikepukmel View Post
...The probe says "10 Meg ohm" and 13pf....
That tells us it is a 10X probe.


Quote Originally Posted by mikepukmel View Post
...There's a little button on the probe. if I push it the trace flattens out, so this must turn on the cap...
No. It shorts the probe tip to the shield. That is so you can determine the ground reference point on the display. The flat line will be the 0V point. You can adjust where that is on the display with the scope's vertical position adjustment.


Quote Originally Posted by mikepukmel View Post
...So, is this a fixed 10x probe? Read a little, so I think this is a fixed 10x probe. So, all the vertical amp readings read 10x the value...
Yes. They show 10X the value set on the vertical amplifier.


Quote Originally Posted by mikepukmel View Post
...Set 5mv, then each tick is 50mv? 8 vertical ticks is 8 x 60 = 200mv? Or I could be completely wrong...
To be clear, each major division on the scope face will be 50mV. There are 8 major divisions. Each major division is divided into 5 minor divisions. I am guessing those are what you are calling tick marks. For the settings you describe each minor division will represent 10mV.


The 7313 scope has three plug in bays. The usual configuration is to have one horizontal sweep (Time base) plug in and two vertical plug ins. Don't know what is in your scope.
 
Tom Phillips 7/22/2018 9:35 PM
Quote Originally Posted by mikepukmel View Post
Don't know if it makes any difference but the speaker is out of phase. The signal is on the - side and ground +.
Makes no difference regarding your 60Hz hum issue.

Please do the grounded grid test we discussed and report the results.


Quote Originally Posted by mikepukmel View Post
Chuck: re You haven't seen my wiring!! [IMG]http://music-electronics-forum.com/images/smilies/smile.png[/IMG]
It would be helpful if you showed us photos of the wiring.
 
mikepukmel 7/22/2018 9:42 PM
Quote Originally Posted by Tom Phillips View Post
Makes no difference regarding your 60Hz hum issue.

Please do the grounded grid test we discussed and report the results.


It would be helpful if you showed us photos of the wiring.
Got it, and thanks for all the scope info. I think Its setup pretty close. I will do the grounded grid test tomorrow, out of steam tonight. And, sure, I will get photos of my wiring job and post tomorrow as well. Thanks!
 
mikepukmel 7/22/2018 9:47 PM
Sorry for one more question, Ithink I had it right: the probe is 10x, so reading 20mv per division the input is actually 200mv, is that correct? At the scope, its 20mv. but back on the on he side ofthe probe, its 200mv. So, I think the added noise is something line 200mv. p to p. (How easy is it to clean the controls, switches and pots, without killing them?)
 
Enzo 7/22/2018 9:58 PM
A X10 probe shrinks a signal to 1/10 its size. SO a signal that originally filled one grid at 20mv, now needs 200mv to fill that grid.
 
mikepukmel 7/23/2018 7:12 PM
Hi All, I put a jumper on the grid of the recovery tube, but it picked up more noise. I think I need to make a really short cable and solder one end. I will do that later. I didn't think it reduced the hum but with the other stuff it was picking up, hard to tell. I chop stick moved what wires I could move, but didn't hear any improvement. Some photos:

[ATTACH=CONFIG]49856[/ATTACH]

[ATTACH=CONFIG]49857[/ATTACH]

[ATTACH=CONFIG]49858[/ATTACH]

[ATTACH=CONFIG]49859[/ATTACH]

Noticed that if I turn the volume on 10, I hear a similar hum, but much lower level than the reverb knob. I never had the volume on that high while practicing, so probably didn't notice. The hum in the reverb is worse. I will hook up the scope again and get a photo of the trace.
 
mikepukmel 7/23/2018 7:19 PM
I re-measured the voltages on the two reverb tubes. Didn't see anything odd, but .. probably would not know.

reverb recovery reverb driver
pin1 200 392

pin2 0 0

pin3 1.73 7.74

pin4 3.25ac

pin5

pin6 198 389

pin7 0 0

pin8 1.74 7.71



Voltage on either side of the added dropping resistor for the reverb driver.

426 391
 
mikepukmel 7/24/2018 8:42 PM
I soldered a very short jumper from pin2 on the reverb recovery tube to the ground lug on the nearest RCA plug, turned off all fluorescent lights, brought in a LED lamp from another room. Turning up the reverb knob is about the same: hum increases with increasing knob setting.
 
mikepukmel 7/24/2018 9:23 PM
Heck of a time getting a photo of the scope trace. scoped hooked up to speaker output inside the chassis. Kinda stinky but this is it:

[ATTACH=CONFIG]49868[/ATTACH]


Peaks are about 17ms across, so about 59hz (within the calibration state of the old scope). Reverb knob on 10, about 150mv. Reverb knob on 1, just some noise.

This was taken with the short jumper soldered from pin 2 of the reverb recovery socket to nearby ground.

Maybe I screwed up the cap board wiring?

Could this be a leaking filter cap but the reverb recovery just picked it up and maybe its there on the other channels at a lower level?
 
mikepukmel 7/25/2018 5:36 AM
Quote Originally Posted by Chuck H View Post
Ah! Been at work all day and got home to see Tom asking the question I had. That is, what happens when you ground the grid of the recovery stage. Knowing that will narrow the possible sources further.

So...
The hum is 60Hz
Even with the tank cable unplugged from the amp the hum still manifests analogous to reverb knob setting.

That's actually a lot of info right there. But I'm stumped as to how 60Hz can get into that circuit isolated from the tank and transformer. I can't actually think of anything but the filament circuit. Since the rest of the amp isn't humming that seems odd. Might just be a bad tube.
Hi Chuck, maybe the amp is humming, but I didn't notice it. With the scope hooked up this time, after ther reverb knob test, I turned up the channel volume to 10 (never did that before, usually on 2 or 3). I did hear a similar hum, and the scope picked it up (didn't get an image, will do tonight.
 
mikepukmel 7/27/2018 4:36 AM
I searched old blog posts, and found one that sounded similar but it was a vintage repair. The owner said he found a leaking fiberboard. Any chance that something could be leaking across this board? Is there some way to measure that?
 
Chuck H 7/27/2018 9:14 PM
Quote Originally Posted by mikepukmel View Post
I searched old blog posts, and found one that sounded similar but it was a vintage repair. The owner said he found a leaking fiberboard. Any chance that something could be leaking across this board? Is there some way to measure that?
Did you use a crappy, black paper and resin circuit board? IMHE with older amps the problem isn't always moisture. Some here believe that old black boards that leak are ALWAYS fixable by drying them out, but in several cases with Fender amps and one Traynor there was no joy in attempting to dry out the boards. I've come to think that with repeated exposure to high heat and voltages that the carbon used to pigment those boards (most black pigments used are just "lamp black" = carbon) aligns to form conductive paths that can no longer be broken. Tell me you're not building on a black paper, carbon soaked board. With what we know today there's no reason other than the "authentic look" to do that. With cheaper practices for manufactured goods causing problems all the time I'd trust a newly made black paper board about as much as a politician. But...

I don't think it's the board. I've seen a lot of problems due to leaky boards but 60Hz hum hasn't been one of them. There's no 60Hz connections on the board! Even in the odd event that there might be it would be on the power supply end where it's as far from the preamp as possible. Further, the problems with conductive boards tend to be leaky B+ related. That is, high voltage DC. 60Hz could only be present at less than half the B+ so I would expect other leaky board symptoms to show first, or at least as well.
 
mikepukmel 7/28/2018 6:58 AM
Hi Chuck, Top of the morning to you! (Wow fi you're in the North West, you're up very early!) Nope, Didn't use the paper/cardboard/resin eyelet board. This version of the amp, Im using a fiberglass double turret board (at least what they say on the site) from Watts: https://tubeamplifierparts.com/turre...tegory=9635856

Im not so much interested in all authentic look, would be nice to get the amp working .

I ordered the board in 'black', should have gotten natural. The reason for asking about leaking board, is that this board, the surface is shiny, like as if it was painted. The other few fiberglass boards I have do not have a sheen.

The scope trace, to me, looks really weird. Like a superposition of a partially rectified sine, but with those sharp peaks. I done did something stupid.

(On another note: owe you all an apology for too-many-edits. I peeked at my post history and eek, like 25 edits on 3 posts, which of course generated one email for each edit! Sorry)
 
Chuck H 7/28/2018 7:58 AM
Shiny happens. I wouldn't expect it to be a "finish" per se, but rather a thin layer of the same epoxy the glass is bonded with. And if it's a clear finish there wouldn't likely be anything in it that's conductive. Unless the manufacturer did something really stupid. An easy tell for a leaky board would be to put your black probe on the chassis and firmly press the tip of the red probe into the board near a HV turret.
 
mikepukmel 7/28/2018 8:21 AM
Thanks Chuck, will try that leaky board test this afternoon. (Have to run out and paint some pieces of siding on the saw horses in the back yard before it rains. Have a roofing company coming soon, they'll need to replace a few pieces around the roof line.) Does the scope trace look like anything you'd expect?
 
Chuck H 7/28/2018 8:24 AM
Ok...

The hum is coming from the reverb recovery.

The hum is still present with the grid grounded. (this eliminates the grid and anything before)

The hum is dependent on reverb knob setting. (this eliminates the ground for the recovery as the source)

The normal channel is fine. (this eliminates the D HV supply node as the source of the hum)

A tube swap with a tube that was hum free in the normal channel did not stop the hum. (this eliminates the tube)

So that only leaves the 100k recovery stage plate resistor and the .003 coupling cap. Nothing about either of those components or their proper, or even improper implementation should cause 60Hz hum.

There is nowhere left to go except to assume that there is a mistake in the wiring, most likely having something to do with the filament circuit on the recovery triode OR there have been errors in your reporting.

Please measure AC voltage on top of the reverb pot. Not the grounded lug or wiper. If you find AC you should look for a wiring error on the socket or possibly a stray wire strand coupling the filament supply where it doesn't belong.
 
mikepukmel 7/28/2018 4:30 PM
Chuck, just to double check, when you wrote "Please measure AC voltage on top of the reverb pot. Not the grounded lug or wiper." you mean the metal case on the back of the pot? I tried all 3 lugs, and the back of the pot, didn't see any AC. I get about 0.4vac with one probe grounded on the chassis, and one probe in the air.

The only place I can find AC, is about 2.1vac at the turret where the wire comes from the rectifier tube, and either side of the choke, and either side of the standby switch.
 
mikepukmel 7/28/2018 4:59 PM
Double checked, no AC on any part of the reverb pot.

I did also turn up the volume for the channel to 10 (never do that) and I do get a hum, louder than the reverb hum. It gets loud at 8 or so. Nothing plugged into the input jacks. Something big wrong.
 
Chuck H 7/28/2018 5:49 PM
Quote Originally Posted by mikepukmel View Post
Chuck, just to double check, when you wrote "Please measure AC voltage on top of the reverb pot. Not the grounded lug or wiper." you mean the metal case on the back of the pot? I tried all 3 lugs, and the back of the pot, didn't see any AC. I get about 0.4vac with one probe grounded on the chassis, and one probe in the air.

The only place I can find AC, is about 2.1vac at the turret where the wire comes from the rectifier tube, and either side of the choke, and either side of the standby switch.
Well that eliminates any stray strand to or from the filament supply. There is literally nothing else I can think of to check. Unless the lead dress and wire routing is so incredibly bad, like intentionally bad, like if you bundled the lead between the .003 cap to the reverb pot with the untwisted filament leads for most of it's length bad.

I actually have a problem with no readable AC. For starters you wouldn't use the open air meter reading as a null. The null would be with the probes touching each other. And since you have excessive audible hum with the reverb pot turned up, there HAS to be AC on it. No AC = no hum.

This would be my next move...

Ground the recovery stage grid and turn the reverb pot up full. Confirm that in this state the amp is, indeed, humming. Now turn the reverb pot all the way down and confirm that the amp is, indeed, not humming. Set meter for AC (appropriate range if applicable) and touch the probes together and scribble down any null other than zero. Turn the reverb pot back up full and NOW measure AC across the two outer lugs of the reverb pot. What happened?

EDIT: Even a few mV of AC will be greatly amplified by the mixing stage because there is very little padding between the reverb recovery stage and the mixer. So something like 4mVAC ends up being something like 150mVAC going into the PI grid. This is why a very controlled reading is critical here.
 
mikepukmel 7/28/2018 9:23 PM
OK ran through your instructions carefully. I found the meter has a mv knob setting. Shorted, reads 0. Shorted the recovery input with a small piece of bus wire. Looking at the back of the pot, left lug is ground, center wire goes to a 470k resistor, right lug wire goes to a 0.003 uf cap.

Pot at 1, no hum. Left lug ground reads 0, center lug reads 0, right lug reads about 20mv (fluctuates 15mv to 20mv ac). Turn the pot on max (10). Left lug reads 0, center lug reads about 15mv right lug reads 15mv.

So, its that yellow wire! Dang I knew I shouldn't have used yellow. Bad tube socket maybe? I have a couple more tubes, will try them.

I removed the extra 16uf cap and resistor and put the reverb driver wiring back to the factory AB763 routing, through the grommet up to the cap board to point marked B on the schematic. Hum seems a little louder.
 
mikepukmel 7/28/2018 9:41 PM
When I put the reverb in, I switched two 16uf 475 filter caps for 500v caps but they are 22uf. I took the 'cap can' cover off, checked the wiring, didn't see anything awful.

I also tried turning up the volume on either channel, with no input. I do hear a hum similar to the reverb put hum. Will check ac mv readings on each vol pot.
 
mikepukmel 7/28/2018 9:57 PM
So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)
 
Leo_Gnardo 7/28/2018 10:17 PM
Quote Originally Posted by mikepukmel View Post
So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)
Should be perfectly safe. One tenth of 200V is 20V hardly a hazard. But with 10x probes the signal you're trying to see will be diminished by 90%, more difficult to see.

All in all I'm wondering, is this hum you're trying to chase down really so bothersome? Scarcely anyone runs their amp with volume controls & reverb cranked all the way up. And we expect to hear some hum when they are cranked up. There's practicality ... and there's unnecessary perfectionism. Where are we going with this? To be honest, I have similar problems here. My home & workshop ambient is so quiet that a normal amount of hum or other noise sometimes initiates a long snark hunt - eventually after excessive time is spent I have to stop myself from going kookoo. They're guitar amps after all. Onstage, one rap on the snare drum and all that noise immediately becomes irrelevant, lost well below the signal/noise ratio.
 
Chuck H 7/28/2018 10:37 PM
Quote Originally Posted by Leo_Gnardo View Post
All in all I'm wondering, is this hum you're trying to chase down really so bothersome?
Well... 15mVAC from the reverb recovery passes through the 470k/220k divider. So about 10mV now.?. That's coming out of the mix triode at something like half a volt of noise without a guitar signal. That doesn't even include any noise from the reverb because the recovery triode is grounded.
 
Chuck H 7/28/2018 10:41 PM
Keep in mind that some of that AC is Johnson noise, or hiss. But 15mV of hash on the plate side of the reverb recovery seems like a lot considering the grid of that stage is grounded.

You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot.

The frequency of 60Hz is still confusing me. The last test tells us nothing about frequency though. And like I mentioned, hiss will be part of that 15mV. The test doesn't tell us how much.
 
mikepukmel 7/29/2018 7:17 AM
Quote Originally Posted by Leo_Gnardo View Post
Should be perfectly safe. One tenth of 200V is 20V hardly a hazard. But with 10x probes the signal you're trying to see will be diminished by 90%, more difficult to see.

All in all I'm wondering, is this hum you're trying to chase down really so bothersome? Scarcely anyone runs their amp with volume controls & reverb cranked all the way up. And we expect to hear some hum when they are cranked up. There's practicality ... and there's unnecessary perfectionism. Where are we going with this? To be honest, I have similar problems here. My home & workshop ambient is so quiet that a normal amount of hum or other noise sometimes initiates a long snark hunt - eventually after excessive time is spent I have to stop myself from going kookoo. They're guitar amps after all. Onstage, one rap on the snare drum and all that noise immediately becomes irrelevant, lost well below the signal/noise ratio.
Hi Leo,
thanks, Yeah, this hum is just above the annoying range. I never crank the amp. Although I do have a very very small office/practice/music room, the amp is pointing towards my feet. The hum is annoying at about 3 on the reverb knob, similar on the volume. I never noticed the hum that comes from turning the volume way up, never did that before. But even at low volume, the reverb hum is blaah.
 
mikepukmel 7/29/2018 7:18 AM
Quote Originally Posted by Chuck H View Post
Well... 15mVAC from the reverb recovery passes through the 470k/220k divider. So about 10mV now.?. That's coming out of the mix triode at something like half a volt of noise without a guitar signal. That doesn't even include any noise from the reverb because the recovery triode is grounded.
Thanks Chuck. Is 15 to 20mv at this stage in the amp to be expected?
 
mikepukmel 7/29/2018 7:29 AM
Quote Originally Posted by Chuck H View Post
Keep in mind that some of that AC is Johnson noise, or hiss. But 15mV of hash on the plate side of the reverb recovery seems like a lot considering the grid of that stage is grounded.

You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot.

The frequency of 60Hz is still confusing me. The last test tells us nothing about frequency though. And like I mentioned, hiss will be part of that 15mV. The test doesn't tell us how much.
Yes, thanks. The johnson noise is tolerable with the channel volume from 1 (min) to about 6 or 7. Above that, the 'hiss' gets to the intolerable point.

Re
"You're reporting similar readings on the normal channel. Where did you set the volume control and where were you taking that measurement? Obviously the normal channel doesn't have a reverb pot. "

Yeah, sorry for my convoluted writing style:
- Original issue I reported in the thread: Volume channel 1 = 1, and no instrument plugged in. Volume on channel 2 = below 3 or 4, with my guitar plugged in, turn the reverb up to about 3, no more than 4, hear an annoying hum. Turn reverb back down, don't notice the hum.

- Later on (after going through the tests you suggested) I put the reverb on 1, then turned the volume pot to 10. Also, did the same with channel 1 (channel 2 vol = 1, reverb = 1), turned vol pot to 10. On both cases, I could hear a louder hum than the reverb knob only test with volume at 1.

Going through the workflows you suggested, I found that the amp does have hum, just that it comes up later on the volume pot setting, that I don't ever use.

Chuck, did you see the scope trace above? I could not quite understand what it means. Its got sharp peaks that are 60hz, but it looks like some kind of odd partially rectified thing. Peaks, then a smoother bump in the middle.

Could I have hooked up the Immortal Mod 'solid state rectifiers on the input of the GZ34' wrong, or could this be causing some of the problem?

Should I get a scope trace on the + side of the main PS capacitors, with AC coupling on the scope? Any chance the AC is laeking in from there, some problem with the capacitors? Or maybe a bad GZ34?
 
mikepukmel 7/29/2018 7:33 AM
If I have AC coupling turned on on the scope, what is the safest DC voltage I can probe with this old 7613 scope? For the AC coupling setting, they must have some kind of capacitor switched in to filter out the DC, but the switch, the cap and other components have some voltage rating. I tried looking in the manual could not find it right away.
 
mikepukmel 7/29/2018 7:45 AM
oOoch. Maybe bad GZ34 out of the box?

https://www.thegearpage.net/board/in...e-mod.1529042/

See post #3 from Mr Robbins:
"If a valve diode does go bad, then hopefully you hear some added hum, and then get the amp serviced, otherwise you may end blowing the first filter cap from higher ripple in the long term."
 
Chuck H 7/29/2018 8:36 AM
Well to be perfectly honest Mike, I'm having some trouble following along at this point. I don't have an interpretation of the scope trace you posted. I'm afraid no one else does either because the context is a little confusing and there's probably not a precedent reference. That is, what would a scope trace look like for that measurement on another amp? I'm trying to digest info about the extra power supply node and now the added diodes. There have been a couple of things that MAY be part of the issue, but haven't been fully defined and I'm starting to think there may yet be other deviations not disclosed. Not to mention that you are now reporting hum in operating conditions other than the original condition that we worked hard to isolate for. Basically tossing every premise we had out the window.

60Hz could be a harmonic artifact of 120Hz caused by improper implementation of the diodes on the rectifier, I dunno. Otherwise 60Hz has to be inducted (into???) via the power transformer EMF or coming through the filament circuit.

You can always remove the extra diodes to rule that out. The amp doesn't need them so it's safe. If they aren't the problem then put them back in if you like.

I actually don't use a scope much for chasing hum or buzz issues. I rely on my ears to tell me the frequency and start with those possible noise sources, tracing back through the associated circuits examining lead dress, grounding scheme and any filtering/decoupling. So if you do indeed have 60Hz hum there are limited possibilities for where it's coming from.
 
Leo_Gnardo 7/29/2018 11:28 AM
Quote Originally Posted by mikepukmel View Post
Maybe bad GZ34 out of the box?
It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.
 
nosaj 7/29/2018 12:21 PM
Quote Originally Posted by mikepukmel View Post
If I have AC coupling turned on on the scope, what is the safest DC voltage I can probe with this old 7613 scope? For the AC coupling setting, they must have some kind of capacitor switched in to filter out the DC, but the switch, the cap and other components have some voltage rating. I tried looking in the manual could not find it right away.
I have used a very low value 600v capacitor attached to my scope probe to probe higher than 300v. Film cap with one end fishhooked and the other end done in a wrap fashion so probe clips on it. Then using the fishhook on whatever I want to probe.

nosaj
 
nosaj 7/29/2018 12:22 PM
Quote Originally Posted by Leo_Gnardo View Post
It's been known to happen. Also a common misteak to wire the filter bank to pin 2 instead of 8, imposing a 5V AC wave on top of the rectified voltage. Who knows, that kind of error might have been made inside the tube but I can't say I've ever seen it. How about subbing in another GZ34, or for that matter tube rectifier of any variety, or even a solid state replacement just to see if the hum level drops. FWIW our man Stan - not heard anything of late but he was a valuable MEFster - recommended Ruby's 5AR4C as top of the pops for currently made GZ34/5AR4 types.
Have to put that one in my bank of common mistake to not overlook.
nosaj
 
Enzo 7/29/2018 2:13 PM
How exactly would that work? The 5v is AC, but has no reference to ground. The B+ is referenced to ground, so how would the 5vAC add and subtract from B+?
 
Tom Phillips 7/29/2018 2:30 PM
Also remember that some rectifiers, such as the 5U4, have directly heated cathodes. Therefore, the output is always connected to the 5V AC heater supply.
 
mikepukmel 7/29/2018 5:48 PM
Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.

Thanks Leo, I will go back again and check the socket wiring. I went through the main board and all wires from preamp through to the reverb, think everything looks right, but will check again. I have another GZ34 for the next amp build, will pop that in and see what happens. I read a whole bunch before buying this rectifier tube and arrived at Ruby as well. Maybe I screwed it up somehow.

THanks for the tip, Nosaj, working up the nerve to try looking at some of the HV traces. Not sure it will lead anywhere, at this point.

Thanks Tom, I think that is what this amp has. One side is one leg of the heater + HV output, pin8, what Leo is talking about mis-wiring?
 
Chuck H 7/29/2018 7:04 PM
Quote Originally Posted by mikepukmel View Post
Sorry, Chuck, really sorry, didn't mean to be a pest. First amp build, thought I remembered everything in there, but had a few rewiring rounds and such. I though it was pretty much stock except for the extra cap+resistor for teh reverb but forgot about the 'immortal mod' diodes. I don't think anything else in there.
NBD. Really If it were I probably wouldn't even have posted. As it is now I'm probably just going to lurk unless a symptom that offers a clue (to me?) is given.
 
mikepukmel 7/29/2018 7:37 PM
Thanks Chuck, Id hate to wear out my welcome.
 
Chuck H 7/29/2018 10:07 PM
Quote Originally Posted by mikepukmel View Post
Thanks Chuck, Id hate to wear out my welcome.
You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.
 
nickb 7/30/2018 1:01 AM
Quote Originally Posted by mikepukmel View Post
Heck of a time getting a photo of the scope trace. scoped hooked up to speaker output inside the chassis. Kinda stinky but this is it:

[ATTACH=CONFIG]49868[/ATTACH]


Peaks are about 17ms across, so about 59hz (within the calibration state of the old scope). Reverb knob on 10, about 150mv. Reverb knob on 1, just some noise.

This was taken with the short jumper soldered from pin 2 of the reverb recovery socket to nearby ground.

Maybe I screwed up the cap board wiring?

Could this be a leaking filter cap but the reverb recovery just picked it up and maybe its there on the other channels at a lower level?
That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?
 
mikepukmel 7/30/2018 4:45 AM
Quote Originally Posted by Chuck H View Post
You? Not a chance! There have been a few. I was probably one once upon a time You're easy comrade More important, you're clearly enjoying this stuff rather than being angry about it.
Chuck, the most enjoyable thing I've ever done!
 
mikepukmel 7/30/2018 5:05 AM
Quote Originally Posted by nickb View Post
That looks a bit like the current that flows to the primary filter caps. Here's a couple of suggestions. First move your wire to between the grid and cathode bypass cap ground of the reverb recovery. Keep your wire short. If this eliminates the hum then it coming from the ground. In that case a gut shot might help.

Les likely but should be checked if that didn't help, try disconnecting the wire to the plate of the reverb recovery tube. The objective being to leave the 100K connected to the supply and the coupling cap in place. This will tell us if it's coming from the supply. Have you checked that the 100K really does connect to node "D" of the power supply?
Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?

Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.

Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

Todo:
- different routing of shorting jumper on reverb recovery grid.
- disconnect reverb recovery plate lead
- try a different rectifier tube
 
mikepukmel 7/30/2018 5:14 AM
In the photos, post #46 on the previous page, you can see my "jungle gym" attempt at a star ground. I took 4 lugs, sanded and put nokorode, bolted them together and soldered them all together so they would not lose contact. Then I bolted them to the chassis and put some more solder on so that they are soldered together and to the chassis.

It turned out much more ugly than I thought it would.

Two things that bothered me about the way it turned out were
1) the long wires that need to be routed from/to the ground lugs, to various points in the circuit
2) not sure if it mattered which wires should be soldered to the same lug, e.g. should power tube cathode and power transformer center tap be on the same lug? Its not really a 'star' if various pairs of wires are soldered to different lugs, so they are still separated.
 
mikepukmel 7/30/2018 5:24 AM
One more variable: I didn't solder grounds to the back of the pots. (many threads on MEF about why we should not do that). Instead, I ran a wire from the pot lug to the ground bus wire. I twisted all of the wires coming from pots to the main board. Should I have ran the ground wire separately, i.e. not twisted it in the bundle?
 
ric 7/30/2018 6:27 AM
The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

(Nokorode on surfaces not letting solder bond)
 
mikepukmel 7/30/2018 7:04 PM
Quote Originally Posted by ric View Post
The corrosion inhibitors used in house wiring have silicone in them I believe from the way they feel. Kind of a silicone caulk that doesn't set up. If that's what you're talking about using before you soldered your ground connections, that could be a concern.

(Nokorode on surfaces not letting solder bond)
Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.
 
loudthud 7/30/2018 7:09 PM
Quote Originally Posted by mikepukmel View Post
So I have this old throw away TEK 7613 scope. It came with one set of 10x probes. If I use Ac coupling, can I probe the plates in the preamp? that would be 200v range dc, but curious what AC might be there, and what the trace would read (if it won't do something terrible like sparks or fire)
The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.
 
mikepukmel 7/30/2018 8:05 PM
Quote Originally Posted by loudthud View Post
The TEK 7613 can use several different input "amplifiers" that plug into the scope below the screen. The 7A18 is a common dual trace plug-in. The AC coupling cap in that plug-in is rated at 600V.
Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches? The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?
 
loudthud 7/30/2018 9:14 PM
Quote Originally Posted by mikepukmel View Post
Thanks! Yes this one has a 7A26 and a 7A18. So, with 10x probe, the cap would 'see' 1/10 of whatever the probe touches?
No, the cap sees the full DC Voltage at the probe tip.

Quote Originally Posted by mikepukmel View Post
The rectified output of the GZ34, with about 760vac rms input, (330 - 0 - 330) or about 330 *1.414 less 40 volts or so lost to the tube rect, something like 420v. So the scope end of the probe sees about 42v dc? So, I could probe that without blowing up the scope, or causing a fire, or other horrible thing? These are really old probes, should I clean off the wires with something maybe alcohol?
I don't recommend cleaning any part of the probe unless you know what you are doing. Parts of the probe might not be able to stand the liquid. Some plastics might absorb the liquid and change their high frequency characteristics.

All the circuitry that changes the Volts/Division (called the Deflection Factor) is accomplished by passive circuits before any active circuitry on the 7A18, slightly different on the 7A26. At the 5mV/Div position, the signal goes straight thru to the input JFET. The active circuitry is protected against excessive Voltage, but, like anything that might be subject to abuse by a customer, you can't protect against everything. One of the toughest things to protect against is the charging and discharging of that input coupling capacitor. It's no accident that the AC/DC switch has Ground as the center position. To protect the input, Switch that switch to GND, connect the probe to the energized circuit, then move the switch to the AC position. To remove the probe, switch to the GND position, disconnect the probe from the circuit and touch the tip of the probe to ground to discharge the cap. Then switch to DC and you can use probe as usual.
 
mikepukmel 7/31/2018 6:02 AM
Thanks!
 
nickb 7/31/2018 6:11 AM
Quote Originally Posted by mikepukmel View Post
Thanks Nick. When you write "move your wire" do you mean the shorting jumper? So, short the grid to the ground side of the bypass cap on that tube?
Correct

Ah, ok and second test, just turn off the tube. but this also keeps the supply out of the equation at that point in the circuit.
The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.


Re node D Yes, I checked a couple of times, think its OK. Will check again. I put 4 photos covering the amp guts viewed from the top, post #46, should be fairly high res, on the previous page. I didn't get shots of the cap board, though, will do that when I get home tonight. The photos show the extra cap and dropping resistor that I have removed while going through the suggested tests.

Todo:
- different routing of shorting jumper on reverb recovery grid.
- disconnect reverb recovery plate lead
- try a different rectifier tube
 
ric 7/31/2018 6:37 AM
Quote Originally Posted by mikepukmel View Post
Thanks Ric! I checked with the gurus on MEF, and this is what they suggested for soldering to the steel chassis, like fender and clones. With a big soldering iron, works great. It's really a flux, rather than a corrosion inhibitor. I think the name means that it is not acid core, like some plumbing solder fluxes.
Gotcha, the flux in a little tin. I actually have some in with my welding equipment box.

I was thinking of the stuff I've been using lately to rewire the house, a completely different thing using a similar name.

Ah, the joys of aging.

Hope you get the hum tracked down. You can always figure the first build is what it is and use imperfections as a reason/ excuse to build version 2.0...
 
mikepukmel 7/31/2018 5:14 PM
Quote Originally Posted by nickb View Post
Correct



The point is to disconnect the tube but leave the supply in place. I don't really suspect that supply but never say never.
Progress!! I got home, dead tired as usual, but threw some water on my face and got enough steam to try some of the changes out (didn't want to wait all the way till the weekend).

1) Move ground short wire from input pin2-nearby ground, to input pin2 to ground side of cathode cap/resistor. Check connections, power up. Turn the reverb knob to 10, no hum at all. The only thing that increased was the 'johnson noise, a little white/pink noise. Turned off, drained filter caps. Triple checked, looks like this is "it".

So, the nasty signal is making its way into the tube through my nightmarish grounding wiring? I did something dreadfully wrong. Maybe several things.

Now the ground bar is wired to the lugs soldered to the chassis. So, what I don't understand is how this signal could get in that way.
 
mikepukmel 7/31/2018 5:17 PM
My all time favorite quote from the old TV show "Mash", maybe my all time favorite quote ever. By Colonel Potter:
"You're askin' a man who graduated 246 out of a class of 248 to do his BEST?"
 
mikepukmel 7/31/2018 5:32 PM
So, in the original amps (the ones that work ) there are 3 wires leading from the - side of the cathode cap/resistors to the brass plate. Then there is one black ground wire coming from the cap board, through the grommet and over to the brass plate. But its inbetween the rightmost cathode ground and the next one over.

There is one more ground wire coming from the cap board that is grounded soldered to the chassis, over by the power transformer. The main difference I can see in my ultra-humluxe is that instead of running a ground wire from the bus bar right down to the chassis, I ran a wire from the middle of the bus bar over to where the PT CT, reservoir caps (-) and bias are all 'grounded' to the chassis. Is the problem that this 'star' ground is badly constructed?

[ATTACH=CONFIG]49916[/ATTACH]
 
mikepukmel 8/3/2018 8:23 PM
Wiring all redone. I used the hybrid 7 grounding scheme.

[ATTACH=CONFIG]49982[/ATTACH]
 
mikepukmel 8/3/2018 8:24 PM
Wiring all redone. I used the hybrid 7 grounding scheme.

[ATTACH=CONFIG]49982[/ATTACH]
 
Tom Phillips 8/4/2018 11:04 AM
Quote Originally Posted by mikepukmel View Post
... Is the problem that this 'star' ground is badly constructed?...
Yes. You have used the star ground point you created with multiple solder lugs at the PT end of the chassis for too many connections. If you are going to follow the approach used in the old Fenders that point should include only the PT center tap and reservoir (First stage filter cap) connection. That is a very "dirty" & noisy circuit point and it is best not to connect other circuit grounds directly to that point. There is much to say about the merits and pitfalls of various grounding schemes. Sorry I don't have time to add more details about your situation at the moment. Maybe later. Meanwhile, there are lots of good existing MEF threads on the subject. Here is one suggested thread to read http://music-electronics-forum.com/showthread.php?t=33322
 
mikepukmel 8/4/2018 6:05 PM
Thanks for the info Tom, and thanks for the link. (understood regarding time, and this thread is getting pretty long ) I've been reading myself blue re grounding schemes. I got it pretty wrong. This morning, backed out most of the mistakes. Starting to understand more bits and pieces re Fender multiple ground points, what they put at each point. I'll get it right sooner or later!

The only thing I could not "fix" was Fender fed the reverb driver tube with the dirtiest node in the power supply, but tied the cathode cap negative side and cathode resistor to the second preamp cathode and grounded those at the same point on the chassis. I ordered a turret staking tool, will add one more turret and break those two ground points. ...
 
mikepukmel 8/4/2018 8:49 PM
Hopefully bringing some closure to this rather long thread, that someone might get some use out of it at some point.

- Took apart the failed star ground.
- Put the power transformer center tap, and negative lead coming back from the 2 reservoir caps onto one lug.
- Found that I had mistakenly put 2 wires running from the ground bar to the failed star ground. They were separated by quite a bit of distance on the bar. Removed both wires from the star lugs, removed one wire completely. Ran a ground wire from about 2/3rds of the way down the ground bar to the chassis. Would have been about where Fender would have ran a wire from the cathode cap/resistor to the brass plate.
- Removed the 2 wires leading from the power tube cathode resistors to the star lugs. Shortened the wires, and soldered to the chassis right between the power tubes.
- Ran a wire from the first filter cap (the one right off the downstream side of the choke) to where the power tube cathode wires were soldered to the chassis.
- Ran a wire from the negative lead of the second filter cap to the ground end of the negative feedback resistor that grouds the phase inverter, then ran a wire from there to the chassis about where Fender would have put one.
- Found 3 or 4 tube socket wires that were too long (still after 3 re-wirings!!!!), shortened, and resoldered.
- Cut the end off the 'ground bar' that connected it to the phase inverter resistor that goes to ground.
- Ran a wire from the last filter cap, the D node, to the ground bar about where it is grounded to the chassis (about where Fender would have run a bus wire to the chassis).

Checked connections, powered up. Much less hum in the reverb! If I put the reverb on 10, its there, and noticeable, but at 3 or 4 (way more reverb than I usually play with), not noticeable. Amp does seem quieter overall as well.

As mentioned elsewhere, what I can't fix:
- The reverb driver tube is fed from the dirty end of the power supply, first filter cap node. But the reverb driver tube's bypass cap and resistor are grounded to the same point where the second preamp tube is grounded. This can't be good. I ordered a turret staking tool, will try to put one more turret in and move the reverb driver's bypass to a separate wire and run that over to where the first filter cap negative lead is attached to the chassis.
- Maybe last thing to do, move the ground for the bias board away from the "dirty signal" ground lug.

Thanks everyone for all the input, suggestions and help.