Something Blew Up

[Treillw]

A few minutes ago I was just about ready to wrap up some practicing for the evening. Playing quietly through channel 1 along with a recording. I heard a humming buzzing noise and wasn't sure where it was coming from at first. I paused the music and went closer to the amp and the hum got louder. I flipped it onto standby and when I did there was a loud crack/pop sound followed by the smell of burning electronics! I turned the amp back on for a second and the hum is still present. Did a tube go, or what blew up?

Thanks.

 

[Old BF Shred]

Sounds like a tube went. Not uncommon even when fairly new...
May have shorted internally and fried a screen resistor which is likely what you smell. Usually this will cause the fuse to blow, but not always. (Do make sure you have the properly rated fuse in there.)

There are some other things that can cause these symptoms as well...but...best have it checked out!
And don't use it again with those output tubes in there until you know they're OK.

Good luck!

 

[Treillw]

Thanks. I'll take it into the shop to have it checked out. Does it make sense to swap out all of the tubes at once, or just the one that blew?

Are the screen resistors hard to fix, if that fried?

 

[Old BF Shred]

It is likely only 1 tube went...2 failing in the same amp at the same time is pretty unlikely.
You could just replace the one if your replacement matches the other three.

Screen resistors are not usually difficult or expensive to replace, but still should be left to a technician.
You probably already know this, but....there are potentially lethal voltages stored in a tube amp even after being turned off and un-plugged.

Your tech should get it fixed up in no time!

 

[Treillw]

Thanks for the help. This is my first time going through this. The Marshall combo I have has had the same tubes in it for over 15 years now and it seems to be working fine, somehow.

 

[jb's 52]

Those were probably better made tubes back then.

 

[Treillw]

Update:

I took the amp to an authorized repairer. They had it apart and checked it out and they said that nothing is wrong with it - tubes are fine, that resistor is fine, it's working at full power and sounds fine. I'm very surprised - I would have sworn something got fried. I'm going to take my cabinet and speaker cable in with me to check it out when I pick it up to make sure there is nothing wrong with them.

Any thoughts on what could have happened?

 

[Old BF Shred]

Sometimes a tube will go into "thermal runaway". Basically if the tube gets too hot, the control grid stops working as it should and the tube will draw excessive current and can "red plate".
Often just going back to standby, until the tubes cool, will correct the problem. If caught in time, usually no damage to the tube or the amp.
But if allowed to go long enough, damage will result. The output transformer can also smell if overheated from too much current.

IF that is what happened to your amp (and I can't say for sure...wasn't there....) But....
Burning smell? Not a good sign. The amp components likely survived OK, as the tech said (and Mesa's are tough), but I would not trust the suspect tube(s). Especially if let go to the point where you could smell it.
Experience has shown me that once a tube starts acting up, it does not get better and it's days are numbered!

Good luck with that.

 

[bandit2013]

The carbon composition screen resistors used in the Mark V are one of the weak links. I blew out one not too long ago, tube was fine though. However when I got the amp, it had some issues blowing out Mesa tubes early on. Amp seems to overburden one tube when using 45W mode. Also the screen resistor for that tube may have been the culprit. Also have had issues with the Rectifier tube and short life but may also have been related to the one issue. Have not had any problems with the amp ever since I replaced all of the screen resistors with Metal Oxide Flame proof type suitable for the applied voltage and power capability.

I looked in my other Mesa amps, all of them use Metal Oxide Flame proof resistors (ceramic coated, not phenolic like most carbon comp resistors found in the Mark V).

If and when it happens, let a tech do the work.