Help! My Mark IV cut out on me!

[placeboeffect81]

I recently got a mark iv combo, went to my first practice (first time playing at volumes above practice volume), and about an hour into practice the volume cut out severly. I finished practice through a solid state fender, now that I'm home and can take a look at it, the clean channel is just barely audible at any volume and the two distortion channels are much quieter than before, also, the gain is greatly reduced (they sound like the clean channel with the gain turned up.

Does this sound like a bad tube? How can I troubleshoot it and find out which tube is bad? I have a spare new sovtek preamp tube for my marshall that I can use, and the guy I bought it from gave me a spare set of 6L6 power tubes. Should I just change one at a time until I find the bad one?

Or does this sound like something more serious? I really hope not, as this is my first nice 'expensive' amp, I saved up a while to buy it and I don't really want to have to take it to a tech. I'm handy with a soldering iron and a multimeter, but I don't have a lot of experience repairing amps.

Any help would be much appreciated, thanks!

 

[GD_NC]

Most likely a power tube. The power tubes need to be changed in matched pairs. The tubes pairs are the inside and outside tubes. (Like this: A-B-B-A ) First put you amp in Triode mode which uses only the outside (A) pair and see if that helps. If that takes care of it change the inside pair first. If not try the outside pair first.

Either way it's probably better to change all 4 if you can.

 

[primeholy]

If changing your tubes doesnt help, clean your effects loop. I was getting volume drop with a .22 and all I had to do was clean my loop. Although with mine it was dropping in and out. Easy to do though.

 

[babow2]

The power tubes need to be changed in matched pairs. The tubes pairs are the inside and outside tubes. (Like this: A-B-B-A ) First put you amp in Triode mode which uses only the outside (A) pair and see if that helps.

WHOA WHOA WHOA!! I don't think that information is accurate. I just checked the manual on the MB website. Be very careful.

Putting the amp in Triode mode does not shut off the inside pair!!! According to the manual, the triode/pentode switch only AFFECTS/CHANGES the outside pair. All four tubes are still running, but you can switch the class A outside tubes between triode and pentode. It is the Simulclass/Class A switch that chooses between all four tubes and the outside pair. Please check the manual. If you have it in Simulclass and pull out the two inner tubes you are risking damage to the amp.

I think what he was trying to suggest is to switch it to class A and see how it sounds just with the two outside tubes. Then replace those two tubes with the two inner tubes and see if there is a difference in operation. If there is, you know which pair is bad, and have narrowed the problem down from any of the 4 tubes to just two tubes.

You then can find the bad tube by taking one of the tubes from the good pair (assuming they are all 6L6's) and putting it in one outside tube socket. then try each of the tubes from the bad pair in the other socket to see which one isn't working right.

to shut off the inside pair, switch to Class A. now only the outside tube pair are active.