I have a B52 AT100 head. The power amp tubes were replaced with JJ 6L6's less than 4yrs ago and I haven't played it a lot since then. Last night I swapped out the four 12AX7 pre amp tubes for new JJ's. I was extra careful not to touch the tubes. When I turned it back on to play it there was hardly any volume at all. What could my problem be? Should I have changed out the other three 12AX7's as well for the effects loop and reverb and the phase inverter? any help would be appreciated. I believe the phase inverter was a 12AT7
If I had to guess I'd say one of the preamp tubes (probably V1/V2) is not seated all the way down.

Get some Dioxit contact cleaner for the tube pins.

You can touch tubes all you want dude. Use your fingers and make sure they are pushed all the way in.

Swap some of the tubes around - maybe you got a dud, or put the old ones back in to validate.

Why did you change the tubes to begin with, were you just being proactive or wanted better tone?
I was wanting better tone. I've heard the B52 factory tubes weren't the best and I figured new pre amp tubes would be a cheap easy fix to start with
You did the right thing. JJ's should rock in that amp.

I wouldn't worry about the FX loop and reverb driver tubes right now. The Phase Inverter is important though. Do all the tubes light up? Turn off the lights or put somewhere dark so you can see real well. I would systematically start swapping tubes around and get a fresh tube in the PI. The only thing to be careful here is to take your time and not bend a pin. Use your fingers, the oil on your skin is not going to hurt your tubes at all (that is an old myth).
Thanks for the help, it was V3 that wasn't completely seated. The JJ's make a world of difference. Sounds like a new amp. Should I go back a 12AT7 for the phase inverter or would a JJ ECC83 like I replaced the pre amp with work? what exactly does the phase inverter do? And as far as reverb and effects loop, I don't use reverb, and the only pedal I have is an MXR noise clamp
I would definitely try one of your JJs in the Phase Inverter. It should turn your amp into a fire breathing monster. I'm guessing that 12AT7 in there now is holding back the amp a bit. You are not going to hurt anything. The Phase Inverter feeds the Power Tubes and is technically considered part of the power section.
That depends on the brand of the tube too.

If the 12AT7 is some crap chinese tube then yes. I don't remember the layout exactly on a B52AT but if V1 drives your clean channel then try the AT7 in there and a JJ in the PI.

Just experiment and go with what sounds best.