VOX AC10 - Power Tubes oscillating

[saint gillis]

I have a Vox AC10C1 making a low frequency oscillation
The oscillation comes when the amp is hot, after about 40min, when I slightly hit the board, the oscillation stops or starts, so I thought it was a cold solder, but it wasn't.
Then I've realized the oscillation stops when the EL84 power tubes are took far away from the speaker, it's like a kind of feedback between the power tubes and the speaker.
I've put another set of EL84 and I'm waiting to see after 1h or so what's going on
Has anyone ever experienced such thing?

 

[GussyLoveridge]

I have had similar problems with microphonic tubes, but usually this would present itself almost as soon as the amp would turn on. How did you make out with simply swapping for a known good set of tubes?

Have you tried moving some of the hook up wires around that are going to the board for the power tubes? Low freq oscillation sounds like some kind of positive feed back loop - some of your output leaking back into the input? It's the part about it starting after 40 mins that seems strange to me.

Anyway, I'm certainly no expert but I'd love to hear how you make out with it.

Best of luck.

 

[saint gillis]

Hi, the new set of tubes did the trick, I didn't do further experiments..