AB763 clone - noisy first stage

[glebert]

Working on a homebrew bass amp with the preamp based on blackface Showman Vibrato channel (but without the bright cap). I'm getting about 500mVpp of white noise at the plate of the first tube stage. Replaced the plate resistor and that didn't help. Noise only comes on after the tube has started conducting. Changing tubes has not helped. Any suggestions where to look next? It seems like it should be very simple, and that I am just missing something.

TIA.
Greg

[Jon Snell]

It is probably oscillating at HF.
Check your layout and add a 22k grid stopper resistor to each grid pins 2 & 7. Mount the 22k on the solder bucket of the pin socket.
That will stop the parrasitic oscillation.

[Toolsandtones]

Forget what I said. Sorry for not reading everything.

/Max

[pdf64]

Check that the shorting contact on the main input socket is working.
And verify all resistor values with a meter, eg make sure the input mixers / stoppers are 68k not 680k.