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So I finally finished building my guitar amp. It sounds great, except for one thing that occurs when gain is too high. What happens is that when I turn one of the preamp volume pots high enough strange ringing occurs. The funny thing is that on the other channel (which is identical, because it is a stereo amp with switchable guitar add-on, so you have two totally independent and identical guitar channels), no such thing happens.

So I made a quick analysis with sine wave as input. enter image description here Isn't I a beautiful slightly overdriven sine wave?
And now look what happens when I turn my preamp volume pot just slightly more: enter image description here And even more: enter image description here

Switching tube doesn't help, so I would rule out bad tube.

Here's the part of the circuit that most likely causes the problem (deduced by checking what pots change this weird effect):

enter image description here (yes I use power tube in preamp, because I like power tube distortion and I don't like playing very loud for most of the time).
I am pretty sure that oscillation must come from this part, because the 'preamp volume' pot has certain position, beyond which the oscillation occurs regardless of position of other pots (actually even regardless of the volume from previous preamp stage), and increasing master volume doesn't change anything, it just makes the singnal louder.

At first I thought that amp is osciallating in 200khz region or something like that (and I'm unfortunately unable to check for sure because I don't have oscilloscope), so I added capacitor in negative feedback loop to fix phase shift for large frequencies, but nothing changed. And I think if it was RF oscillation, master volume would change somethinhg and it doesn't.

Anyone has any idea what could it be? Perhaps it is a known issue?
Please tell me if you need more information about the amp.

Here are some photos: enter image description here enter image description here enter image description here enter image description here

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  • \$\begingroup\$ That channel appears to be going slightly unstable. Next suspect would be the electrolytic capacitors. C1 for a start. (And maybe the film capacitors if they are old enough to be oiled paper (1960s or earlier). (EL84 in a preamp is weird : does the original valve show the same instability? \$\endgroup\$ – user_1818839 Mar 28 at 15:19
  • \$\begingroup\$ @BrianDrummond There is no original valve, it is my own design from scratch. As I said in question, I have second identical channel and it's just fine. C1 is polipropylene capacitor, brand new (made by mundorf). I am aware that E84 in preamp is weird since it is a waste of power, but I heard that people don't like amps with master volume pots because power tube distortion is better, so here I have power tube distiortion at any volume I want. \$\endgroup\$ – J K Mar 28 at 15:29
  • \$\begingroup\$ Ground loop is my guess and it is just a guess. \$\endgroup\$ – Andy aka Mar 28 at 15:32
  • \$\begingroup\$ Amp had some minor hum problems in the past and I've checked for ground loops several times, but I just checked once again, no ground loop. (or perhaps I should get new pair of glasses). \$\endgroup\$ – J K Mar 28 at 15:41
  • \$\begingroup\$ It might be caused by induction from one wire on an output to a sensitive input wire. \$\endgroup\$ – Andy aka Mar 28 at 15:46
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You have some feedback route. EL84 and the preceding stage can amplify substantially at megahertzes. Impossible to say anything sure without seeing the exact construction. Try to reduce the hf response in the preceding stages and filter more their operating voltages with RC lowpass filters. Be sure the first stage signal ground wiring is totally free from the supply current of the later stages.

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  • \$\begingroup\$ I put a 720pf cap from EL84 control grid to ground (creates low pass filter together with R3, cutoff freq is 32khz). And now something really strange is going on. Now it depends on the master volume. On low volumes amp oscillates when no signal is applied (I can hear some very high sound coming from the speaker). Then there is a sweet spot that seems to work just fine, but it is very small. And at high volumes, there are random noises, at perfectly audible frequencies. They sound a little bit like car engine. \$\endgroup\$ – J K Mar 28 at 16:41
  • \$\begingroup\$ Thank you for your suggestions, well now I see that there is indeed positive feedback somewhere. Construction of the amp is a little bit messy (which maybe could also be the reason), so I think I will see what I can figure out myself now and if I end up with nothing, I'll post more information. \$\endgroup\$ – J K Mar 28 at 16:45

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