|lowell||6/4/2017 6:21 PM|
|Gibson Explorer GA-15RVT|
So I got this amp in and the interstage audio transformer had an open primary. I got a 124a from hammond, threw it in, next thing I know one side of the secondary is open now. ACK!! How could this happen? I turned the amp on for literally 1 minute, and had the bulb limiter in place too. While it was on I noticed one EL84 was getting hot. I powered down and checked some things to find the open secondary. I'm shocked, frustrated, and oblivious as to how this could happen, and so quickly. Both EL84 tubes are brand new JJ.
|lowell||6/15/2017 1:28 PM|
|Actually after ordering a 2nd replacement PI driver transformer I realized this amp is NOT at all like the schematic. It has a reverb transformer. Also, the PI trans driver tube is V3 not V2. V3 is still a 12au7 like the schematic says. And V1/2 are 6EU7.|
Anyone have the correct schematic for this amp? I've searched but didn't hit. All I can find is the schematic with the tube-driven capacitively coupled reverb circuit.
Oh and it has a choke as well...inside the chassis.
|DrGonz78||6/15/2017 2:05 PM|
|You might have seen this one already but here it is anyway...|
God those Gibson schematics are always a headache.
|lowell||6/15/2017 2:28 PM|
|That's it! Thanks a bunch!|
|Enzo||6/15/2017 2:38 PM|
|Lowell, welcome to the world of Gibson/Epiphone amps. The schematic OFTEN is different from the model number on the amp.|
In my collection, somewhere in the GA15-19 models I have one, might be the 15 in fact, where I have three different Gibson published schematics and THREE more I have hand drawn from the same model numbered chassis. That is six different circuits with the same model number.
Not only that, you will find sometimes a chassis that originally had 6EU7s has been rewired in one or more of those sockets to take 12AX7. The tubes are similar but the pinout is different. So in those cases we have to decide whether this is a different circuit or a rewire.
And sometimes you find a GAxxx model is really under the skin an EPiphoe EAxxx. Or vice versa.
This is one case I prefer the paper manual over stored computer files. I can flip through page after page of the book a lot faster than I can open 150 pdf files. If you ever see a copy of the Gibson MAster Book, grab it.
I have many times leafed through teh Gibson Master Guide book, and I find the tube chart to be most helpful. Most of their schematics include a small drawing of the chassis with the tube named. SO I can flip through pages looking for an amp with say two 6L6, a 12AU7, a couple 6EU7, and a 7025. or whatever. When I find a drawing with the same tubes inthe same order as my chassis, I look to see if it in fact agrees with my chassis.
|lowell||6/16/2017 4:59 AM|
|Right on. Sounds like that book sure does come in handy. Think I found it, but in pdf CD format.|
Is this it? CD Gibson Tube Amplifier Master Service Book - Manuals, Schematics and Parts pdf | eBay
|Tom Phillips||6/16/2017 7:13 AM|
Edit: A Google search netted several hits such as https://elektrotanya.com/gibson_mast.../download.html
It's the same 177MB PDF file that is on the ebay CD.
|Enzo||6/16/2017 10:16 AM|
|Yes, the digital scan file can be found for free in several places. It is the same. But as I point out, flipping pages can happen way faster than opening 100 pdfs. Having said that, yes please do add a copy to your stored files. I like skimming to book for that one function - solving mystery circuits. But I do much prefer a schematic on my screen over that big clumsy book open on my bench.|
|lowell||6/16/2017 11:01 AM|
|I can have the local printer make it into a book too. I'll see if I can find the free file online somewhere.|
Back to the amp: There is an insane amount of HUM. The dreaded hum. Some 60hz and 120hz. I've done quite a few things that I thought might fix it, but they haven't. One of 'em actually makes it MUCH worse. Here they are:
First I replaced all filter caps as they were original, not sure if this hum was there before because the driver transformer was bad
Checked that all grounds have continuity and all tube voltages look good and correct
Disconnect filament ground, added simulated center tap with 2 100ohms - no help
Pulling the 12au7 driver tube kills the hum, so it's not a PA (power tubes) issue, and the bias is good at 12watts each
Pulling only V1/2 doesn't help, actually makes it a bit worse
Grounding V1b reverb return grid makes it WAY WORSE
Cut out the death cap - no improvement
The high voltage secondary center tap is good and grounded
Ground the 12au7 driver tube grid doesn't help at all, so I'm thinking this must be the driver transformer or power supply cap or ground - but still not convinced because of the following:
I've been using my scope a bit to see if it's 60 or 120hz, and in doing so, it seems as though there is 44v p-p of 60hz riding on the chassis itself, and this is with the power switch OFF. If I unplug the power cord this goes away.
The hum seems to be there at the input of the amp as well on the scope, albeit much less, but again pulling V1/2 doesn't help the hum, makes it a bit worse, therefore I don't believe the source of this hum is from the input stage/s
I've tried to use my meter to measure AC voltage and it doesn't show anything... I think it's not sensitive enough to read lower AC voltage
At this point I'm a little confused - help needed!
|lowell||6/16/2017 11:13 AM|
|Something tells me the chassis has voltage on it. If I measure AC voltage from earth ground (in my shop outlet) to the chassis, there is 6.6vac. Assuming this is because I added the 100ohm filament center tap resistors. Meter doesn't read the 44vac p-p like the scope did. What's baffling is that the power switch isn't on, the amp is simply plugged in.|
I'm missing something obvious... I've thought multiple times about adding a 3-prong and seeing if earthing the chassis would fix it, but don't think so as grounding my scope at the chassis (therefore earthing the chassis) didn't help the hum level.
|Enzo||6/16/2017 1:52 PM|
|Please do put a three wire power cord on it. Most Gibsons have two ON positions on the power switch. As in off/standby/on/on. Is that your case? If so, do both on positions hum equally? Also, in the old days of two prong plugs, we routinely had to turn plugs over to kill hum and shocks.. In other words pull the plug from the wall, and flip it over to reverse the polarity. Leakage to chassis is quite common, especially in amps with a "death cap". That cap after all is an AC connection from chassis to one side or the other of the power line.|
When you have multiple sources of hum, they are often out of phase with each other and can cancel. When you pull a tube and hum INCREASES, first thing to my mind is you just removed one of the hums that was cancelling the other. Absent that cancellation, you hear more hum. So that input stage hum you think there isn't is probably there and was what was cancelling the hum from other sources.
Every source of hum has its own cure. heater leakage hum will not be helped by more filter caps. And power supply ripple will not be affected by a grounded heater center tap. Rather than just do everything you have heard reduces hum, find the sources of it and cure each one.
|lowell||6/16/2017 1:59 PM|
|Forgot to mention that YES I did swap the 2-prong plug the other way, same result. Yes it hums in both ON positions.|
I'm a bit at a loss as to what and where to attack. I feel like I don't have a good method for troubleshooting hum and it gets me almost every time. I've learned quite a bit about grounding schemes, and the heater center tap trick, and those have helped a lot. But in this case, I find this very odd: 12au7 grid grounded hum exists, pull the 12au7 hum is gone. And I mean LOUD, it's akin to the hum before a filter cap blows... that increasing intensely loud hum then BOOM...only, no boom.
And yes, tried 2 different brand new JJ 12au7 (ecc82)
|Enzo||6/16/2017 2:04 PM|
|When you ground a grid, try tacking a very short wire from the grid pin on the socket right to the grounded end of the cathode resistor. Don't ground it to chassis.|
|nosaj||6/16/2017 7:47 PM|
Maybe grounding one or the other will put you in a direction?
P.S. I read over thread several times after looking at the schematic and found no mention of the footswitch being available or not.( may not even be important though).
What about input jack washers? I don't remember how to tell by the schematic what they are supposed to be. My traynor has plastic washers(or a fibre washer) The junk box ones I've built toothed washer to get a good bite.
|Enzo||6/16/2017 7:53 PM|
|"Driver" works for me.|
|lowell||6/17/2017 1:35 PM|
|Any ideas on really zooming in on the source of the hum? Using a scope or meter? I tried looking for it on filter cap nodes but..nothing. I also notice the driver tube has a Vk of +9v. I thought that HAS to be it, the heater hum is being injected....so I raised the filament CT to +9v...no change. |
I cant imagine the input washers have any part in the hum because both V1/2 pulled doesn't affect the hum.
|DrGonz78||6/17/2017 2:19 PM|
|Just sending this as I believe the Epiphone EA-32 RVT Comet matched up to the schematic of GA15RVT-1965 that is mentioned...|
Then there is this video by the Guitologist where he fixes hum and it ends up being related to the reverb tank.
Not saying that is what is going on with your amp, but since your amp relates well to the video I figured to post it here. Perhaps watching what he did to quiet down the amp might inspire a solution for you. If not it is at least a good video to watch.
|nosaj||6/17/2017 2:27 PM|
What about C19 is it still there or disconnected?
And the footswitch? There or not?
|lowell||6/17/2017 2:36 PM|
|Ground V3b grid does nothing. Even grounding grid of V3a does nothing...and V3b goes into V3a.|
Footswitch is disabled I removed all wiring to it.
I cut out C19 earlier on.
Curious about the driver transformer. I dont see how phase would matter in regards to hum.... but maybe I'm missing something. I should try reversing the secondary grid wiring.
|nosaj||6/17/2017 3:16 PM|
If that's not it, I also couldn't find anywhere in the thread if any of the controls change the sound or if there is any DC on the pots.
Not to be too much of a pest but how is X1 disabled or is it just unplugged?
|lowell||6/18/2017 7:35 AM|
|All tube socket voltages look good per schematic.|
I removed all wires from the footswitch because the owner doesn't have the footswitch anymore and so as to remove it from the equation for troubleshooting. I removed all wires TO the footswitch so I didn't have any antennas.
No DC on the pots. The hum is there with all controls at zero. It's lessened only with level pot at 50%, which I believe is because the hum is being somewhat phase cancelled. Which maybe this points to a ground loop issue combined with some other source. Then the hum is back 100% with level at full volume.
I've used my BLUE ESR meter on all filter caps and everything was in spec.
I should mention again, if I haven't before, that the amp sounds really good despite this hum issue.
I thought also that perhaps one of the 2 rectifier diodes was open, they both test good.
Again - it's confusing because grounding the driver grid doesn't remove the hum, but removing the tube does. So it'd make ya think the tube is the issue, but 3 new tubes in total haven't had any positive effect.
EDIT: Enzo, thanks for the grid ground suggestion. Ground the driver grid to its Rk ground works much better. The hum greatly reduced when grounding the grid there, however it's still there to the point of being a concern. This result at least points me in a direction - the filter cap for the driver tube and/or it's ground.
|nosaj||6/18/2017 8:17 AM|
|Is there still AC on the chassis? What about Lead Dress? I had one I'd built and had the signal wire too close to a HV wire induced hum there.|
How many different ground points do you have? I believe signal grounds and chassis grounds should be separated. (I once tried a star ground with signal and chassis grounds together it was also an issue with hum.
Dirty tube sockets?
Did reversing the driver secondarys effect any change?
Also some amps I've worked , not all of them will pick up the flourescent light I use.
how about unplugging the reverb tank and hows the hum then? I know I can pick up the christian radio tower a mile away through the reverb coils(freaked me out the first time fire and brimstone sermons coming from the amp )
Does it still hum if you put it back in the cabinet? (I've had some that display that. inside cabinet there was a foil tape that with chassis in completed a faraday cage.)
Got any pics?
|DrGonz78||6/18/2017 12:20 PM|
|Enzo||6/18/2017 12:36 PM|
|And I still think you need to think in terms of hums rather than hum.|
Grounding the driver grid doesn't help, but removing the tube does? OK, so the hum you hear is not coming into the grid, that is what I get from that clue.
|lowell||6/18/2017 4:53 PM|
|Yes that's right. Your suggestion to ground the grid at the cathode resistor pointed me in the right direction. It was the ground scheme of the filter cap for that tube. And probably the others too. I had them all grounded in what i thought was acceptable for a low gain amp. And especially with all the level controls at zero. So I redid everything concerning grounds...and did it JUST the way I did on a recent scratch build that initially had tons of hum. |
B+ filter and screen are grounded at power tube cathodes.
The remaining caps are grounded right at the cathode resistors of their related stages. That and the filament center tap quieted things right up.
I have to admit, the hum was INSANELY loud. Very surprising to me that grounding things incorrectly could cause such an insane hum....and at the driver stage too!! Its not as if it was at the input and being amplified through subsequent stages. Wild stuff. Thanks to everyone for the help!
I replaced the speaker with an Eminence Private Jack, one of my fave speakers, this amp sounds really good.
|lowell||6/20/2017 9:03 AM|
|Oh and I forgot to include that nothing is grounded to chassis until it reaches the input jack. So it's a buss ground scheme. This has proved to be an exceptional ground scheme twice now. Works REALLY well especially for single ended amps that lack the benefit of push-pull noise rejection.|
I still havent even tried the verb or vibrato. So, onward.
|g1||6/20/2017 11:41 AM|
|Just an aside for future reference (seeing as you have good result as is with this one). One of the prime directives is that the main filter cap ground should be the same point as the PT HV center tap.|
|lowell||6/20/2017 3:58 PM|
|Agreed, it wasn't before, but now is.|