Mark V El34 Tube Red Plated

[Fretman12]

Hey all! I'm hoping this is just a single tube problem but figured I'd post here for confirmation. I had and out of town job so she's been sitting for about 6 weeks or so but I fired up my Mark V 90 watt head fitted with El34s and started to smell some burning.

Initially I though maybe some excess dust burning on the tubes but I quickly looked in the back and saw a single tube red plating. I immediately shut down the amp. I waited a bit and fired her back up to see and the same tube almost immediately red plated again.

Unfortunately I didnt have an extra El34 but I did have the original set of 6L6's so I popped them in, flipped the switch and no problems. Ran fine for a solid 2 hours of play since.

So... Just a tube problem right? I will of course be getting a new set of El34's to test that there is no problem with it powering an El34 in that socket in general.

 

[mace]

Probably just a tube problem. Make sure to use variac power with the EL34s, since the normal power will shorten the EL34's useful life. Of course, the bias switch in the back needs to be set correctly (EL34 vs 6L6).

Apologize if all this is old news!

Mace

 

[Fretman12]

That's my hope. So far so good. I still have no problems with the 6L6's. I certainly had the 6l6/el34 switch in the correct position. I do believe I was using full power mode often with those El34s couldn't have had more than 20 hrs play time on them though. It does just seem to be the one tube that went bad. Thanks for the reply!

 

[conrad79]

Mesa updated the owners manual last year to say that EL 34's should be run on variac power.

 

[robertray]

+1 on using the variac power with el34s. smoked my winged c`s pretty quick (center two) before I knew about that.

 

[Fretman12]

Variac mode it is. Thanks fellows!

 

[bandit2013]

Plate voltage is too high for EL34 at full power. I found this out the hard way too but did not loose any tubes in the process.....except for one set of JJ 6CA7. Two week life span at full power. Oh well.

 

[Nicklotsaguitars]

Don't understand the problem myself with EL34's needing variac power. I use mine in variac because the manual says to do so and I like the sound a shade more.
The Mark V knocks out 475v to the plate in full power and 380v in variac. I've never seen a Marshall below 475v though all of them using EL34's. My KK knocks out 490v, my JMP 483v. Had a TSL in for major mods last night, that knocked out 475v on the nose. The main problem was the TSL leaky board. The bias was hitting 138ma, well above max. And the valves still didn't redplate (JJ's).
The bias for EL34's is cold on the Mark V.

 

[bandit2013]

It may have something to do with the extended class A pair than the outer class A/B pair of tubes. If not so, would it be related to the 10W mode? (something I rarely use).

Before I knew of this, I ran KT77 for 8 months without any issues at full power (played a few hours each day, and maybe some time off as well) and the amp never sounded better, tube still have plenty of life in them yet. Also had a set of EH EL34 (not the rebranded versions by Mesa) for a few months but did not like them, not bad but not great either.