Amp plays on standby!

[Axtman]

Sorry for the poor video and sound. I tried to capture the sounds of the amp. It was tricky tried to play the guitar strings behind my back and balance the camera in my other hand.

https://www.youtube.com/shorts/8qdmHF4uGdw

I think a capacitor is charging up then discharging. If a wait a bit before hitting the guitar strings, there is a pronounced attack then it fades.

BTW, The amp had been on standby for 5 or more minutes when I made this video.

[Axtman]

Oddly enough, I found Diode CR4 to have been reversed! And it was stock soldering from the factory! I cut it out and installed a new 1N4007 diode in the proper position.

[x-pro]

I found Diode CR4 to have been reversed!

and there's no short circuit?

[Helmholtz]

x-pro is right. Reversed CR4 would cause a short across the HV winding during each positive half-cycle.
Except if CR4 is open.