Questions from a Luddite

Hi, excuse my ignorance but hopefully you can answer 2 questions for me regarding my 1978 Silverface Deluxe Reverb.

1. I am going to change the valves. If I just go ahead and change all of the valves and fire it up will it do the amp any harm? ie if I don't get the bias looked at, will I trash my amp?

2. On said amp, I was looking inside the back and noticed that hidden behind what appeared to be a little transformer type 'doobry' (technical term!) which is itself situated behind the two power amp valves (6V6GTs) is what appears to be a 5mm jack socket. Can anybody enlighten me as to what it is?

thanks in advance guys, appreciate your help,

francis.

a) normally, the preamp tubes rarely go bad. If any, it would be the 12AT7 (V3) that drives the reverb tank.

b) You can swap out output tubes without checking the bias. It is, however, a good idea to.

c) that thing you are wondering about is the chassis mounted bias adjust pot for the output tubes.