can anyone identify this broken amp sound?

[krash]

apologies for the extreme poor quality recording, just did it on my phone real quick.

Here's the story: this is an amp of my own design; I have built several of them and never had a problem like this. It has two channels, one a TMB channel with some gain and the other is a 6DJ8 cascode. This is the tone of the 6DJ8 cascode channel. The TMB channel sounds fine.

The two channels feed into the LTPI through 220K mixing resistors. So each channel has a .022uF coupling cap, into a 220K resistor, and that attaches straight to the grid on the high side of the LTPI. Other end of the PI is .1uF to ground. 1M grid leaks on the LTPI, 750K cathode, 56K tail. Pretty standard, and like I say, I've built about four of these.

The cascode channel has a 15K plate resistor on the same B+3 rail as the last stage of the TMB, which has a 100K plate resistor.

Pulling the tubes in the TMB channel has no effect on this bad sound.

If I disconnect the TMB channel's coupling cap from the PI (unhook it altogether) then the cascode channel sounds fine.

Any ideas what this could be?

[krash]

BTW, I got the bad sound to go away, in exchange for tolerating more gain than I wanted in the amp. I had to change the 6DJ8 top plate resistor to 100K from 15K, and that fixed it.

I think the 6DJ8 was arcing.

Can't figure why it was doing this only when the TMB channel was hooked to the PI.

And I can't figure why it was doing this only on this amp and not on the other identical amps like this I have built.

Again, I am eager to hear anyone's assessment of what this problem could be.