Standby switch thud...

[Helmholtz]

Have filter caps been checked for high ESR?

Reasoning:

With perfect caps the charge/discharge time constants prevent an instant change of B+ when operating the standby switch.
But this delay doesn't apply to the voltage drop across the ESR which will jump immediately.

[Bob M.]

Thanks again, everyone, for your input. I did replace the standby switch with a new one but that, in itself did not fix the thud problem. I was also having some moderate hum issues. I re-did all the grounds in the 1st (normal) channel which was humming more than the Vibrato channel and I replaced the 22uf/450Vdc electrolytic cap for the preamp with a new one and between those things the amp is quiet and flipping the standby to play mode, the thud is now more like a 'pip', about normal for most 60s Fender tube amps. While troubleshooting, playing with the tone/volume controls, I got the amp to motorboat so then I suspected the filter caps and it was easy from there.
Bob M.