I just repaired a 80's JCM800 (2205),..one of the tube had shorted and melted the tube base.
I just repaired a Traynor Bass Mate it had one power tube socket shorting to chassis. I had an NOS old black socket to keep the amps somewhat original looking.
I am guessing you have a 2205 - with these knobs :
presence
master
reverb
bass
middle
treble
volume
gain
-red LED -
bass
treble
volume
This is a 2-chnnel JCM-800 2205 with the famous diode gain. It is the first two-channel JCM800. I have one and I like it, but I like the tone of my 2204 better. But your 2205 has an effects loop which is a GREAT place to put your EQ (after the pre-amp). Only put chorus and maybe a wahwah in front of the amp - you need chorus, delay and some kind of a boost for leads going into the effects loop.
This is how I got mine se up after years of experimentation however.
Getting just the right sound dialed in is also a real challenge - but it can be done. The right tubes & speakers make a big difference. I used not-so-bright speakers EVM12L and a g12-65. I also have an NOS Mullard.
I had an NOS old black socket to keep the amps somewhat original looking.