50 Caliber + head......12ax7 or 12at7??

[joeyb2112]

Hey guys!!!
Just picked up a 50 Caliber + head in great shape all around except that after it heats up the volume cuts in and out when using the front panel input. (about 25 mins.) I installed new 6L6gc's and tried it again.....no fix. (Also I noticed that the reverb sounds real bad when a reverb is input throught the front panel even if the onboard reverb is off.)
I then hooked an RP1000 pedal board up through the fx loop and no cut out. So I've narrowed it to the pre-amp section. The guy that I got it from said that the last tech who serviced it pulled out the 12AT7 in V4 and installed a 12AX7. When asked about this the tech said that the 12AT7 was wrong. Here's my question:

The manual specifically states that the 12AT7 is the correct tube for V4, however there is a diagram (sticker) on the back of the amp showing 12AX7's in all 4 positions. Which is correct? Is the V4 a reverb tube?

 

[MrMarkIII]

The info I have says V4 is the reverb tube and should be a 12AT7.
But I don't think that would cause your problem.
Generally a 12AT7 is used for it's lower noise, and is often the reverb tube in black face Fenders.
The effect loop jacks could cause the fade outs, but don't when using the loop jacks, as you found when hooking up the RP. The effect loop jacks are not the usual jack, but are closed when nothing is plugged into them, thus hard-wiring the connection when unused. If they get loose or old, the result could be a poor connection when nothing is plugged into them.
Some decent contact cleaner like De-Oxit should fix it.

 

[topalchohol]

I'm not familiar with this amp, but on some amps, just running a patch cable from send to return can clear up an issue like this. Worth a try anyway.