big trouble in home brew

[daz]

This el34 jcm style build (similar, but not exact) has been my obsession for over a year now, and i finally got it to a place tonally where i felt there wasn't one detail that wasn't good to perfect. Then i made a big mistake. After all this time it was a slop fest from infinite soldering and unsoldering of parts, so i decided to clean it up. Lifted the board, cleaned up all old solder and made it tight and clean, tho still looked like WW2. It sounded fine till i swapped out a couple parts that i realized should be a different value. Then it started sounding horrible. It sounds like there is a lot more gain but only in the very top of the high end, and the harder you hit it the worse it gets. It's a nasty high end distortion that sound like you recorded to tape and set the channel 3 times too high. A spitting staticy hi end distortion thats hardly noticable at all on the low strings. I swapped every single cap including PSU caps, (except main can) most resistors, checked all voltages and bias,checked OT and choke, all solder joints of course, replaced all tubes, replaced or checked every pot, disconnected NFB, and other things i can't think of right now i'm sure.

i realize this isn't something anyone will likely be able to help me with just from this description considering how much i've already checked. But at this point i'm desperate and don't know what else i can do. I have thought to build a new one, but this one already cost me $600 in parts and i'm not thrilled with the prospect of making spending another 6 bills. So if anyone has any suggestions as to what i might look at next i'm willing to try anything at this point. Thanks.

[Bruce / Mission Amps]

I don't know what to say but.... it could be as simple as some very slightly different wire layout from or in the NFB loop, speaker jack, presence pot wiring, phase inverter... etc.... now creating an ultrasonic oscillation that you can not hear the fundamental of but can hear the consequences of it.
This is when owning and using a decent signal generator and O'scope, plus knowing what you are looking at, is really a great tool set and skill indeed.

[daz]

creating an ultrasonic oscillation that you can not hear the fundamental of but can hear the consequences of it.

That sounds exactly like whats happening. But the layout is the same as it was, and i have chopsticked it ad nasium so i don't think thats the cause.

[defaced]

This has me confused:

It sounded fine till i swapped out a couple parts that i realized should be a different value. Then it started sounding horrible.

So you changed something while you were in there, and now it doesn't work, yes? If so, have you changed it back?

[daz]

Of course. The first thing i did after it started this was put two parts back in that i swapped out....the slope resistor and a 470k that was 1/2 of a voltage divider. And i have since swapped out most every part of the amp aside from the trannys. It seemed to start right after i took out the slope resistor and put in a 47k in place of the 33k i had just tried to see if it worked better now with the new circuit i was digging. Naturally i suspected a cap in the stack went south when i was soldering the slope, but i have since replaced every component in the stack.

[daz]

heres a clue, or maybe not. But if i send the output of the gain pot which is right after the first stage to the 3rd stage bypassing V1B, the tone is fine. Very little gain of course, but none of that issue. So i replaced all the components between the gain pot and 3rd stage grid yet again, and theres that nastiness still. The tube has been swapped so it can't be that. Has me wondering about the tube socket, but i doubt it's that.

swapped sockets...not that. So apparently it's probably something that is oscillating as bruce said, and as you know that happens when the gain is high so it makes sense it was clean with one stage out of the circuit. So it's not that stage it's a lack of enough gain to make it happen. That doesn't he;p much tho because i've already swapped most everything out. After all this time i just may have to trash this thing. $#%@