Ampeg superbreverb Variable Bias Trem/(vibrato) thump all the time, footswitch good

[Smitty02]

Hi guys I got a 1964 Ampeg Superbreverb r15r in, closet classic. Needs some love and caps... first issue is the trem thumps all the time the amp is on the footswitch is good , it does make a minor difference when switched you can hear it switch yet thumps away. This is a variable bias type trem I know the caps can be issue with this but never seen one thump all the time board is in real clean shape I found one blown cap on the power tubes pin 5 paralleled with the 140ohm 5 watt resistor, replaced that one, a 25uf 25v made no difference I have worked on a couple of these old rockets very similar but not variable bias trem. Is this something a complete cap job will fix. so far i tested all the wire wound cement resistors and they are all good, 7591 power tubes and a lot of the resistors all seem good. Also can or does anyone know how to check the bias on these i believe they are cathode bias? here is a schematic . I planed on a full cap job as he wants it to be dependable and plans to play it a lot So i think a full recap would be smart..but would like some info on the constant thumping what is causing it and why so I can learn about them