POwer loss

[hebaton]

I sudenly realised at practice, my JTM45 had lost significant volume. SO I checked tubes to realise one of them ( KT66 output ) was cold.
I swaped them ( changing position ) to see if the issue would follow the tube or the socket. But after resart , now both tubes get hot and volume is back...

Now, where the hell do I look ?

[johnnyreece]

If the tube were cold, to me that would suggest a filament not lighting. If swapping fixed it, maybe you just had a dirty tube socket, or an intermittent bad connection somewhere in the filament string.

[hebaton]

THank you ! I will check those filament connections for sure.

[Stevem]

Original Marshall output tube sockets due to there design grip the tube pins very very as opposed to Fender style for example.

It takes a lot of crap in the Marshall style socket or tarnish on the tube socket pins to have a poor connection, but it is possible.

It’s not unheard of to have poor solder connections in the tube pins themselves, as I have had this issue numerous times over the years!
In light of this I would reflow solder into pins 2 and 7 on both of your output tubes if they look the least bit questionable.