Mark III "pop"

I'm brand new to Boogies, and just bought a Mark III that arrived yesterday.
After it's been running for a while (45 minutes or so), it makes a loud pop, similar to what you might hear if there was a short in the guitar (it's not that, or the cord). It's occasional, and did not happen when the amp was just on, but not being played.
The previous owner said that he did not have the problem during a long gig two weeks ago.
My suspicion is that a tube didn't survive the trip.
The power tubes are new JJ 6L6s, but no telling how old the others are.
Does anyone follow the manual's instruction to use only Mesa tubes, or is the potential for improper bias a minor issue?
I'd welcome any comments or suggestions (other than sell the amp cheap).
Thanks.
-Mark

In the absence of advice, I replaced the JJ tubes with a pair of SED re-issue 6L6s and a pair of Sovtek el34s. The sound is much improved even at low volumes, and there were no untoward noises during four hours of use at a gig this weekend.

Well, the pop continues to occur intermittently.

I don't know how old the preamp tubes are, and the power tubes also have some use on them, so I'm thinking the next step might be to replace all of the tubes and at least rule those out as possible culprits. Anyone have any other ideas about what this might be or how to track it down?

Thanks.

Start by isolating different parts of the amp/circuit.

That is, does it happen with effects loop in or not? With reverb or without? Which mode/channel or all of them?

Unplug guitar to input cord....still there?
Move the input cord around to see if you can make it happen.
Same with speaker cable....
Pound on top of amp with fist (not too hard, of course) to see if you can cause it; if so it's a bad tube or loose connection somewhere. Try shaking the whole amp.....
Check cables to reverb tank.

You can also try to (gently) tap on each preamp tube to see if it happens; tubes can start to go microphonic with age.

Try swapping tubes with new or at least known good one...one at a time until problem ceases.
Start with v1, working on down.....

Hopefully this will get you pointed in the right direction.
Good luck

Thanks for the suggestions. Wiggling the input or speaker plugs didn't cause the pop. I haven't had time to do much checking on it lately. It's difficult to identify the situations that lead to the noise, since it is infrequent. I'm going to try tapping the tubes and if that doesn't identify the source, will try some judicious chopstick-prodding of internal parts. Still intend to change out all of the tubes, but that's sort of slid to the back burner for now.

Does it do it at different locations, thermostats i.e. refrigerators/freezers or even light switches etc can make a pop sound through an amp if in the same mains circuit ??

Well, the popping stopped.....along with all other sound from the amp. It wasn't sudden, just faded out like turning the volume down. No burning smell, and the fuse does not look blown.

I'm thinking of shipping it to Boogie for an checkup/freshening-up. Have any of you done this?

Now for another question from the seriously clueless: where is the serial number on these amps?

Also, after reading all the redstripe/greenstripe/bluestripe (is there a dr seuss in the house?) conversation, I tried to find a stripe on mine. There is no stripe in the area of the power cord, only 'RCS' in marker below the cord. Is this a pre-stripe?

Thanks for any help or advice.

-Mark

I would bet your filter caps went. Its an old amp and probably about time for a new set.

BTW: RCS is the initials of the CEO of Mesa/Boogie. Nice!

Thanks, lerxst.

I appreciate the help. The info on the initials is nice to know, too.

-Mark