"New" bias problem!

[thomasdj]

Ok..

I am building the dual rectifier 3ch version. The three channels works (I've got sound) but when I adjust the bias, the amp oscillate.

I use 4xEL34 and adjust the bias with two pots (1 for each section).

The amp is stable when the bias is around 130ma (65ma for each tube) but starts to oscillate if I turn the pot further..the current jumps up or down and the tubes begins to glow red = I hit the standby quickly

I measured with only the power tubes and the phase inverter inserted and the negative feedback disconnected.

I've tried another compensating cap (150pF instead of the 75pF cap shown in the schematic).

I've also tried with other PI tubes.

I've got 30v lower voltage than shown in the schematic (guess it's because I don't have the rectifier tubes).

I have attached the schematic and my PCB-layout
(from left to right V1-V5)

btw..the preamp tubes is underneath the PCB and wired to the PCB with about 2" wire.

Thanks

Thomas

[stokes]

Assuming you have about 440vdc on your plates,you're biased way too hot.You EL34's should be about 38-40ma's per tube.The fact that you have 30v less has nothing to do with the rectifier,a tube rectifier will drop more volts than a diode.