'66 super reverb, new caps - Once I turn it on, it sounds fine, but within 10 to 30 minutes I will hear a loud pop, and then the volume drops considerably. I can put it on standby and turn it back on and it seems ok, but then it will happen again. I changed out the power tubes and it still does it - someone suggested it was the power tubes. If it's a preamp tube, where would I look first? Should I try another set of power tubes? __________________
I'd check the bias draw on the output tubes as this may give a rough indication of their condition. You want to see about 30mA per tube and look at both of them. Swap a new 12AT7 in the phase inverter. If you can have the other tubes thrown on a tube tester this could save a lot of time to see if it is or is not tubes.
I don't have a tube tester but I would like to get one. Do you have a reccomedation as to where or what to look for?
A spare set of tubes will be cheaper than a decent tube tester... and far more useful. Anyone who plays a tube amp should have a stash of new or "known good" tubes for spares and for testing purposes. In this case - an amp that works, then "pops" and loses volume - clearly, a component somewhere is failing intermittently. It could be a tube or it could be something else, but the most likely problem, and the first thing to check, is definitely the tubes. Here's a little trick: I'll assume that you play through the vibrato channel... almost everyone does. Your V1 tube is the normal channel gain stage and is not used when you play through the vibrato channel, so you can treat it as a spare tube. (Hopefully, this tube is good.) Pull it out and put it into the V2 position. Now play your amp and see what happens. If the problem goes away, you've found your bad tube. If the problem persists, move on to V4 and repeat. The problem you describe would most likely be cause by V2, V4, or your power tubes (V7 and V8). Since you've already replaced your power tubes, you've (mostly) eliminated them as the culprits (sometimes even brand new tubes fail, but not very often). If new tubes don't fix your problem, you may have a failing capacitor, cold solder joint, or something more complicated. Take the amp to your local tech and have it serviced.
Doh! That's right. I forgot to mention the phase inverter (V6). If your tests with V2 and V4 don't isolate the problem, try the tube in V6. This is normally a 12AT7 but you can put the 12AX7 in there temporarily for testing purposes (AT7 and AX7 have different gain factors but are interchangeable).
A bias probe would be even more useful in situations like this and, of course, to bias your amp. WeberVST has good bias probes for sale.
Agreed that it's probably the phase inverter, but if it's not, check the ground connections from the filter caps- I've seen similar symptoms from a sfVR whose bias supply cap had a bad ground connection. The 6L6s were overheating and producing that "pop," which started out producing a big volume drop but was showing loud hum with no input load by the time it came to me. Good luck with it.
I had a somewhat similar thing happen to a '71 super reverb I had. I had recapped and it sounded great, then from time to time I would get a volume drop. I opened it up, have someone else do it if you haven't before, and found some cold solder joints on some other parts besides the caps. I touched up all of the solder joints in the amp and it never happened again.
