JCM800 Overheating EL34s

[FiXXXeR]

hi all,

i have a JCM800 4212 and i have svetlana el34s in it they work fine but i bough some electro harmonix EL34 to replace them. i had the amp re-biased to suit the new valves and when i turn the amp past 10 o'clock on the volume after 5min my HT fuses blows and my valves are to hot to look at!!!

but i put the svets back in and no probs what so ever at all volumes, my amp seems to only like svets (which is good cuz they sound best i think) i have tried shuguang and electro harmonix EL34s and the HT fuse blows with both.

i am slightly worried because when my svets deside they dont like me anymore i will need to replace them with new ones and they might overheat like the shugs and EH.

can anyone give my some ideas why this might be happening? i have asked a tech he say it may be the transformer (if that is the case why are the svets ok and everything isnt???)

confusing post i know but any help will be great.

[Bainzy]

Hot tubes aren't a problem - it's when they're redplating that is.

[FiXXXeR]

redplating is an understatment to what happened to the poor old EH EL34

but it doesnt happen to the svets  :?

[Searcher]

Sounds to me like it wasn't rebiased properly.  Everyone makes mistakes!  You might need a second opinion on that bias job.  (An unbiased opinion?)

[Bainzy]

If it was biased properly, the EH tubes might be defective, and I'd look into returning them. It shouldn't be the transformer, unless he means the PT is putting out a high plate voltage - in which case the EH tubes might not be able to handle it.

[HJM]

EH valves - booo

Svetlanas - Yeah

Are all the valves redplating? if so the bias is off, or it's a buggered bias supply(probably not in this case)

It is possible that the valves can't take the plate voltage as Bainzy said, if so get some proper Svets (SED Winged C are the ones you want) or JJ EL34 valves, they're good with higher HT supplies. I've had trouble with EH valves before , especially 6L6s!

[FiXXXeR]

cool thanks guys,

so basically what you are saying is that they valves that i have tried cant handle the HT supply of my amp yet me svets can?

the EH were redplating yes, but svets dont

Another question have the winged C svets got bigger heat sinks? if so why is this better?

[FiXXXeR]

oh and my bias is set at (i think he said) 37ma when he set it

but dont hold me to that cuz i really cant remember

[HJM]

Winged C svets are made in the old Svetlana plant, the new Svetlanas are made in the Sovtech plant along with EH and Mullard.....confused yet?? The SED valves are just better built!


37ma doesn't mean anything without knowing the plate HT! The two are linked. It could just be that you've got a high HT supply - take it back to the tech who set the bias and tell him that the valves are going red and take it from there!

[TwilightOdyssey]

EH valves - booo

Svetlanas - Yeah

I've had trouble with EH valves before , especially 6L6s!

There are some amps that simply WON'T work with EHX valves no matter what you do.

That being said, the new Mullards are great sounding, probably the best compromise between an EL34's midrange and a 6CA7's bottom. I thought my amp was a bit too polite sounding with 4 Mullards, though, and threw in 2 EHXs to nasty it up a bit. Worked out really well!