Rewired a build, changed it to a switchable Rocket/Liverpool type amp, two or three stages into cathode follower, marshall tone stack, LTPI, 2xEL84.
When I connect more than one stage together, I get anywhere from -10 to -20 volts on the 12AX7 grid. This has to be oscillation right? The waveform shows hard clipping on one side of the sine, when the tube goes into cutoff.
Paralleling plate resistor with small cap doesn't fix. Added more filtering to final node, didn't fix...
Any ideas? I moved around wires to no avail...
Oscillation leading to negative grid voltage on preamp?
Moderators: pompeiisneaks, Colossal
Re: Oscillation leading to negative grid voltage on preamp?
Try putting a 10K gridstop resistor in series with the lead to the 12AX7 grid. Mount the resistor right at the tube socket.
I've seen 1MHz oscillations on 12AX7s but it did not cause the bias to go that far negative. It did make for some nasty tone.
A complete schematic would be very helpful.
I've seen 1MHz oscillations on 12AX7s but it did not cause the bias to go that far negative. It did make for some nasty tone.
A complete schematic would be very helpful.