I swear I’ve encountered this problem before and I can’t remember if it was the plate resistor or a cap that was causing this pop. Amps been serviced not to long ago and I haven’t played it much but last week I noticed when I turn is off standby, there was these quiet little succession of pops. Then today after I cleaned the tube sockets I’m getting this pop but a few seconds after I switch it off standby. Like something is building up and then releasing. Only happens once then all is normal. Amp is my 69 deluxe reverb. Any ideas where to start ? Can’t figure out how to post a video but in it You would hear me flip the switch and then probably 8 seconds later you will hear the pop. It’s not the switch! Haha like I said, I sounds like something is discharging but which something.