Did my valve redplate?

PaulRR

I just fitted a matched pair of new TAD EL-84 STRs to my Jim Root Terror. After about an hour of playing, one of the valves started sparking purple, and the volume died. I switched the amp off immediately, and then tried it again a few hours later. The amp was making sound again, but it seemed to sound rougher, and the valve that had gone purple now wasn't glowing at all. I'm guessing the valve needs replacing, but I'm no expert...

If anyone has any advice, I'd be very grateful.

Cheers,

Paul.

ICBM

It didn't redplate, but it is most definitely deceased... the purple fireworks of death are pretty, but conclusive.

Don't run a cathode-biased amp like this one with only one functional power valve, it's very bad for the other one. You may have got away with it, but replace both of them anyway and keep the surviving valve as a spare if the seller doesn't accept returns.

jpfamps

It's almost certainly arced over and died.

The amp will sound bad because only one of the 2 power valves isn't working.

You shouldn't run the amp with only one valve in it.

It will need replacing.

PaulRR

Thank you both - I'll pop the original valves back in and hopefully get a replacement set. 60 minutes seems a little on the light side for valve life...!

This forum is a great resource, and the advice is much appreciated.

Cheers,

Paul.

ecc83

I had a weird problem with batches of TAD EL84s.

They worked fine but when brand new would produce a low level "crackly/burbly" noise. Low level, never notice it on a stage but definitely in a quiet workshop. The noise subsided over period of a few hours soak testing, especially if blasted into a load box.

Not all samples crackled but those that didn't could often be induced to do so if given a flick from finger and thumb. This led me to suspect the cause was a microscopic piece of debris in the valve that eventually got "gettered"?

I suspect Paul you had a TAD with a rather larger than microscopic piece of ***t!

The issue seemed resolve itself after a couple of months.

Dave.

Pete24v

I've had the same thing with TAD EL84's in my old Dr Z Maz 38. Put a fresh quad in and 1 blew after a few seconds, took the amp to hotrox where I'd purchased the valves as Mark couldn't believe it, he'd personally tested those valves. So he put another in, and that blew as well.. He checked the amp over, it was fine. Changed brand of valve to JJ and they lasted fine.

PaulRR

Hmmm. Can anyone recommend a better brand of replacement EL84? The JR Terror has quite a lot of background hiss and hum with its stock tubes - the TADs tamed that quite well before they died, so I'm after something else that will do the same...

P.

ICBM

JJs are the most reliable modern EL84s in my experience.

Karltone sells them.

Cirrus

Which is apt, because Karltone is the most reliable seller of modern EL84s in my experience!

PaulRR

Thank you again everyone for the advice! 

P.

PaulRR

The JJ EL84s worked a treat, and Karltone gave great service, as advised. Despite the TAD hiccup, it was definitely worth revalving the amp - there's a noticeable improvement from stock.

Cheers,

Paul.