Now a buzz problem

[DeathRex]

I decided to put the pentode back into Kenwood 1100U, but I didn't want to put in a 6AN8. So I wired it for a 6GH8/6U8/6BL8. I've never made an amp with a pentode front end, only worked on them. I've got lots of gain, and a buzz. The buzz comes on with about 20 volts PP at the speaker terminals with a 6BL8(pictured). With a 6GH8 or 6U8 I'm getting it at the bottom at about 30 volts PP.

Here's the schematic, except the 6AN8 is no more:

[DeathRex]

If I remove the feedback I only get a oscillation of 29Khz at 42VPP.

[Shannon Parks]

Since you have a HF instability, put a 100pF cap in parallel with that 20pF cap at C322. Just tack it in place for starters.

Shannon

[DeathRex]

I did actually add a 27, 18 and 200pf one at a time, and even changed the resistor, but it didn't change the buzz at all. I also have, with a 6BL8 ,a HF (29KHz) oscillation, as the tube warms up. If I increase, or decrease feedback, the oscillation is always there. Might be a bad tube, because the 6GH8 and 6U8 don't oscillate, except at higher power. I found that if I remove C321, the screen grid capacitor, the HF oscillation goes away, but the sinewave doesn't look right.
I'm tired of pentodes, they suck. I put the 12AX7 back in, but I'm going to need another tube to get enough gain. I guess a LTP will get me plenty of gain.

[Geek]

Hrm... give C326 and 327 a look over. If they failed, drifted or have bad solder joints, that can cause oscillation on peaks.

[Brik]

I like to second the possibility that C326 and C327 might be misbehaving.
I wonder what would happen to the asymmetrical waveform if you swap V306 and V307, or disconnect C326/C327.

[Impmon]

Don't see any grid or screen stoppers on that schemo. I've done projects with high gain pents (active voltage regulators) and always include grid stoppers (4.7K) and screen stoppers (1.0K) as a matter of course. So far, I haven't seen any instability issues. I don't know why they overlook this so much, but there ya go.

Mount the stoppers as close to the socket pins as possible; C-comp resistors are preferable here, but C-film may work. The latter may not be recommended, but they do seem to work.