Power tube loses bias at full output.

[Gingertube]

What nickb said.
Open the feedback loop first and see if the problem goes away, If so feedback phase is wrong.
If NOT:
EL34 are high transconductance (gm) so grid stops need to be 10K minimum. 1K5 are for lower gm tubes like 6L6 but even for them this "traditional" value is a bit low.
Make sure the grid stop resistor body is hard up against the tube socket pin.

Ditto for the screen grid resistors, they have a grid stop function as well as a screen protection function.

When you suspect bias shifts always check the grid leak resistors. For cathode biased outputs it is sufficient to check that the resistance to 0V is correct.
In fixed biased amps it is necessary to check the entire chain back to the -ve bias voltage and while you are at it check that the "bypass" capacitor which establishes the signal ground for that bias supply is wired the correct polarity (-ve to the bias supply and +ve to ground).

If the impedance to ground is too high then grid current in the output tube will cause a bias shift (decreased bias) in the increased tube current direction, which increases grid current, round and round .poo. melted output tube.

If all that is OK. Then the PI might be oscillating.
His suggestion of the 100pF plate to plate is worth trying
BUT POSSIBLE LIGHT GLOBE MOMENT
Since it was mentioned above, I built a TrainWreck Clone with pretty much the circuit for the power amp which we are discussing (I used 6CA7 in lieu of EL34) and I DID have PI stability problems. I tried a heap of things (not the 100pF between anodes) but what worked in the end was fitting 10K grid stops to the phase inverter triode grids (both).

One more basic:
Have you checked for a leaky coupling cap from the Phase Inverter to the Output tube grid circuit?

The fact that the problem stays with the same output tube socket suggests a basic build problem, or component failure problem, but unfortunately does not eliminate the other problems outlined above.
So replacing the coupling caps might be worth a try too.

Cheers,
Ian

[JoeK]

Had a similar problem at idle with EL34 power amp and it ended up being the plate load resistor drifting on one side. Bias drifted and current went way up.

[loudthud]

leadfootdriver (the OP) has not been back here for over a week.

[nickb]

I think his buddy HillTopExplosion2 took up the slack. Still, it has been a while, even then.

[HilltopExplosion2]

Thanks for all the advice, and troubleshooting help! I went through a whole box of preamp tubes to replace v1 with the quietest tube I could find.. next I replaced the JJ EL34's with some EH 6CA7's that were in my other amp.. I KNOW they are good. I also replaced the rest of the pre tubes with tubes that I know are good. The hum is still there, but not as bad.

Without a guitar plugged in, I can turn everything up to 10 (Bass, treb, etc). As I increase the volume the amp starts to oscillate. Everything is fine until the volume approaches noon, then here comes the noise.

Prior to replacing the tubes, I called a friend and he helped with some troubleshooting over the phone. Poor guy was on there for almost an hour. We started with the power tubes, pulled them, then turned the amp on.. no hum. Put 'em back in, next was PI, no hum. Then V2, no hum. V1, and there it is. He had me check a few resistors, and everything was o.k. When I had the amp on, I tapped the blue 2200 pF ceramic capacitor, (on the board between V1 and V2) and it was very loud. I could also tap anything around it and it has a very glassy sound to it.. like a microphonic tube, even though that preamp tube (V1) is not microphonic in any of my other amps. I had the same result no matter what tube I put in there. He had me cut 1 leg of the cap and turn the amp back on. THAT took care of a lot of the noise. That said, my next plan of action is to replace the capacitor, and see if that helps. If not, it goes to his shop on his bench. He said it could also be a ground loop. I don't know, I'm ignorant when it comes to working on amps.

Here's a gut shot.

[nickb]

So it is oscillating. Cutting the leg of the cap breaks the signal path so yes, it will kill the noise. Ceramics are often microphonic , it doesn't necessarily mean they are bad. You could you a use polypropylene film type instead. A certain amount of hum & noise is normal - probably not real issues unless excessive. On the oscillation, it sounds like you did everything other than the advice given above in posts #30 and #31 .

[Pedro Vecino]

I see the brown wire from pin 3 of a power tube overfly the two input leads to the grids. It´s better over the chassis, and the latter elevated on it. I would lead them in parallel, not twisted and as I say, high. Can you wrap them with a spiral cable connected to ground for certain electromagnetic protection. Or directly use shielded cable.
Feedback wire is suspect. Crosses the lines of the anodes and the chassis from side to side. I would replace by a shielded cable (long) to find the best orientation (does not have to be the shortest necessarily).
Oscilations in the final stage is usually associated circumstantially with certain elements of the preamp creating confusing symptoms, but I think the problem is in the power stage.
In any case I disconnect the feedback line before anything else.

[HilltopExplosion2]

Thanks for the advice! I replaced the 2200pf cap with a JNC JN222M, and the "glassy" sound went away. If you tap the cap, it's nowhere as noisy as it was. Now for the ground loop. I can understand a layout, and basic components, but unless I read what does what in print, I'm lost. Would you mind pointing out where and what to disconnect to open the feedback loop? Thanks!

Here is a better pic, and layout.

[loudthud]

The feedback is the wire that goes from the impedance selector to the resistor on the Presence control. Disconnect it at the impedance selector end.

[tubeswell]

If it is HF instability from NFB phase lag at HF, then a 100pF to 250pF from grid to cathode on each power tube can effectively 'slug the dominant pole' (in Merlin's parlance) without compromising the guitar audio bandwidth.

[HilltopExplosion2]

Thanks!

[HilltopExplosion2]

So far what I have accomplished, thanks to you all!

1) Replaced the 2200 pf cap with a new one
2) I removed the ground loop. It made the amp very noisy, so that's not the culprit.
3) I untwisted the red and yellow leads per Pedro's suggestion, but have no shield or shielded wire yet.
4) I also made sure the Brown wire to pin 3 of the output tube is up against the chassis.

These suggestions have helped some, and I will make more changes as I get some parts. I appreciate all the help!

I made a clip of how the amp sounds. Right now it pops when I flip the bright switch. I start in the middle position, then go up, then cycle through all positions. I next put all knobs too noon and the bright switch in the middle position. I used my guitar's volume knob to adjust gain/volume. The amps is very clean, but as the gain kicks in you can hear the Peeyowwn sound I'm talking about. If anybody has experience with, or has come across this problem, any help would be appreciated!

https://soundcloud.com/user-43772858...swoosh/s-Xo4nd

[Chuck H]

So far what I have accomplished, thanks to you all!

1) Replaced the 2200 pf cap with a new one

Why?

2) I removed the ground loop. It made the amp very noisy, so that's not the culprit.

I think you mean feedback loop. Removing ground loops is always a good thing It made the amp noisy because it raises the gain. The point of removing it was to see if the current balance between power tubes improves. I don't think it was ever the intention of any of the posters to leave it disconnected. You should repeat the current test with the feedback loop lifted and report the results.

3) I untwisted the red and yellow leads per Pedro's suggestion, but have no shield or shielded wire yet.

Untwisting the leads can't improve anything (and may actually make an oscillation worse) without adding the shielded leads. Simply untwisting them is a half measure that doesn't achieve the intent of the suggestion.

4) I also made sure the Brown wire to pin 3 of the output tube is up against the chassis.

Yup, good. It's actually best to twist the OT secondary leads together (the blue and brown) for as much distance as practical as well. Just making sure they're on the chassis is ok.

I'm not being a hard ass. It's just that we can't do any testing from our remote involvement. If you are in doubt about any test condition, intention, procedure or intended outcome I hope you will ask. Because your post makes me think you don't understand the intention of the troubleshooting procedures being suggested so far. We genuinely want to get you to a solution.

[HilltopExplosion2]

, Not taking you as a hard ass at all Chuck.. I appreciate the help! I'm trying to make sure I didn't buy a P.O.S, and this is a simple fix that I can do, OR It's something that I need to take to a professional for repair. I understand the basics of amps and components. It's the "why" you run a capacitor in parallel with a resistor I get lost at. UNLESS it is in writing. There is a HUGE learning curve here.

I replaced the cap because of noise. The amp had a glassy sound. Similar to mechanical noise from an output tube. When I cut 1 leg, it went away. I replaced it and all is better. No glassy/rattle sound.

As far as removing the feed back loop, I was going by post #31 and also the last tip in post #21. I disconnected the wire to see if feedback phase was wrong. My understanding is, after removing the wire, if the amp's hum went away, feedback phase was wrong. It made the amp noisy so I reconnected it.. What was I supposed to do after I disconnected the wire at the impedance selector end? My amp is still apart and it wouldn't take 20 minutes to disconnect, test, (whatever measurements I need to take), then reconnect.

Also from post #21 he suggested to run the leads parallel, so I did, just to see if it made a change. It didn't really. I just haven't wrapped them with a grounded wire yet. BUT.. it's on the list of things to try.

Hum isn't a problem now... I think the combination of changing the capacitor and tubes helped. The problem now is the response of the amp at high volume. If you listen to the clip I made, It seems to go out and back in phase at high volume.... What I just said is not a technical phrase, It's just the only way I can describe it.

I Really appreciate all the help and advice! Thank you all!

[Chuck H]

I disconnected the wire to see if feedback phase was wrong. My understanding is, after removing the wire, if the amp's hum went away, feedback phase was wrong. It made the amp noisy so I reconnected it.. What was I supposed to do after I disconnected the wire at the impedance selector end? My amp is still apart and it wouldn't take 20 minutes to disconnect, test, (whatever measurements I need to take), then reconnect.

It was suspected that the feedback loop may be positive (it's supposed to be negative) and causing an oscillation that might be exacerbating the imbalanced current draw in your power tubes. I wouldn't expect it to affect hum at all really. But I also don't suspect the feedback loop is positive. In THAT amp design I would expect howling feedback if it were. Further, if the loop were positive I don't think disconnecting it would have increased gain as you experienced. So leave it connected for now I suppose.

Did you check the screen grid resistors? Voltage at the screen grids?

If removing a cap reduced the glassy sound it's possible the ceramic cap just sounded bad in the circuit. They can be a little "dirty" compared to film caps. Most TW's used the single film cap but some mojo has been attributed to the parallel cap thing. I think it's a little silly and I don't like ceramics for that circuits function. More likely Ken (Fischer) just had that cap on hand for a build or two and his particular ceramic caps didn't sound bad when ear tested. That doesn't mean choosing to parallel a ceramic intentionally is the best idea and I don't think Ken would disagree.

any hum you have/had may be related to the current imbalance in the power tubes. And that imbalance is the bigger issue anyway.

You didn't buy a POS. It looks like a reasonable build and the design is known to sound good. So it's just a matter of getting it to work "right".