Mesa Mark V Problem

Hey guys...I'm a long-time lurker who is experiencing a problem that I hope you guys can help track down.

Backstory:

I purchased a used Mark V head from a trusted dealer a couple months ago. The amp was made in 2011 and cosmetically, it looks perfect. The amp has worked beautifully (aside from a shorted footswitch) until last Sunday. I flipped the power on and nothing happened. I checked the fuse and it was blown so I began looking up possible culprits. I narrowed down the issue to a bad rectifier tube and replaced it (electro-harmonix) with a Mesa brand Rectifier tube. I replaced the fuse with a 250v 4 amp slow-blow and fired it back up. It powered up nicely but I immediately noticed a new issue. The amp barely had any output until I bypassed the fx loop. When the loop is bypassed everything works as it should but as soon as I enable the loop the output is diminished to a barely audible signal through my 2x12 rectifier cab. Also, if I am playing on the clean channel and I set the loop assign switch to "ch all" everything works as it should. When I switch to the other channels, the output is diminished drastically again despite the loop assign settings until I bypass the loop again. I am pretty new to Mesa amps but I believe I may have a bad pre-amp tube? Is this a common occurrence when other tubes go bad or do you guys think something more serious may be going on? I was recording with the amp Saturday night and everything was well. I put it on stand-by like I always do and then powered it off a couple of minutes later. I'm not sure if my house got a power surge at some point throughout the evening and I have bigger problems or if its something simple that I have overlooked.

Thanks for the help, guys. Hopefully we can get this sorted as I have loved every minute I have had with this monster so far.

Tommy

The fuse blowing means something unusually catastrophic happened. The rectifier tube (or whatever else started the problem) may have taken other components with it. The preamp tube may have gone first, or simultaneously, or after. You may have had a surge on the filament supply that affected other tubes. Lots of possibilities.

I recommend having it looked at. The amp may have been in perfect condition when you bought it, but an energetic tube failure can damage it to the point where other repairs may be necessary. Probably nothing expensive to fix, just a component or two, but probably important to be sure.

Check the loop output signal control on the back of the amp. You may have set it to a low setting by accident when you changed the rectifier tube. If you are using the effects loop jack with pedals, verify that you have the send going to the input of the pedal and the output going to the return (in the event you unplugged them).

If the problem is still occurring, I would agree with elvis on a preamp tube issue.

If you have a spare, replace the V6 preamp tube.

For a temporary means, swap V2 and V6. The V2 circuit will not use the same part of the tube that is used by V6 responsible for the effects loop circuit. In other words, there are two identical triode circuits in a 12ax7, typically referred to as "A" and "B". V2 only uses circuit A, and not B. V6 uses circuit B for the effects loop return, and circuit A for CH3 master.