input impedance and potentiometer

[tonescout]

Hi - sorry for delay, back from Christmas break!

I know the chassis is connected to ground to the earth pin on the mains cable, and I know somewhere there is a 100R between chassis earth and circuit ground, BUT I don't know where this is.

Some images attached of the underside and top of the amp.

I don't have a schematic sadly :-(

I think the power supply uses a voltage doubler and the B+ for the driver valves are taken off this also (so all rectified through the 2x 5u4GB)

Thanks 🙂

 

[tonescout]

I also tried a different pre-amplifier from my brother and that had the same outcome // zzzz when using the pot on the 211