Mark V 25 - Intermittent no sound and hum

[eightsixboy]

Hi all,

Only had my Mark V 25 for about 3-4 months. I first noticed an issue about a week after I bought it when it lost all sound. After turning it off and coming back in a little while it worked fine.

The amp has had very minimal use and has been fine up until yesterday when as I turned up the master while playing (had already been playing for about 1/2 hr) the sound went really quite, almost non existent, and then this hum was the only audible thing coming from the amp. I turned it on/off but hum still there, I ended up just turning of and coming back again later and it worked fine.

Is this just a bad power amp tube or is there some other potential issue with the amp? Hoping its not a transformer or something major.

Since the first time it happened I have changed the preamp tubes so I wouldn't think it would be that.

Thanks.

 

[Monstercastle]

More often than not it's a tube issue in my experience. I would swap the power tubes and see what happens. I have the Mark V:25 too, and had an issue with one of the EL84s that shipped with the head. Volume just dropped out one day while playing the clean channel. Popped in my spare set and it's been good for a couple months now. Mesa took care of the tubes under warranty. I now have a couple sets of spares just in case. They are cheap enough.
If that doesn't do it, you may need to get it to an authorized tech.

 

[elvis]

All channels and modes? Probably PI or power tubes.

If you can get ANY channel to work, then it's probably a preamp tube.

Turning off and on suggests that you are getting some expansion from heating that causes an open-circuit when hot.

 

[pokerrules47]

As others have stated, get some tubes, a set or two of el84's and an assortment of 12ax7's. First replace the power tubes, you'll likely find the culprit there.

Don't be afraid to pop the chassis out to start swapping 12ax7's until you get a sound that you like the most. The chassis is held in by 4 long bolts. Unplug the amp, disconnect the reverb (remember how the reverb is connected because it's difficult to change once you have the amp back in) and carefully remove the head and put it upside down on a bench as you test.

It takes time, but you'll find it. Consult the "Tube Task Chart" in the manual. Understand what each position does. Also the closer you are to the start of the chain, the more sensitive to noise you'll be. V1 and V2 should have very good, and quiet tubes in there.

I really like Mesa's SPAX7's and JJ 12ax7 golds.

Keep spare power tubes on hand. Preamp tubes tend to last a long time and go bad slowly so you will have some "grace" period but it's always handy to have a few of those on hand too.

Have fun!

M

 

[jmkratt]

By chance did you discover a solution to this issue?

Thanks!