JJ EL34 internal shorts

[panhead]

It appears that the JJ EL34 in a single end amp I built a while ago shorted internally between the plate and screen grid (pins 3 and 4). The base is burned between those two pins and there is luckily no damage to the rest of the amp. I was wondering about any experiences you guys may have had with JJ EL34's shorting internally. This one ran for a year with no trouble and then went poof. I always considered the JJ's to be rugged tubes, but now I'm not so sure.

[jjasilli]

I've had multiple bad experience with their kt-88's.  6L6's seem ok though. 

[punkykatt]

A few weeks ago I put in a new pair of JJ EL34`s in a Marshall JTM 60, fired it up, checked the bias, bias looked good, Started to play for about 30 seconds the amp sounded good, then started to sound dull and losing power, I smelled something burning so I shut it down. Found one of the new JJ`s had burnt the paint on the glass of the tube. I put the old tubes back in, replaced the HT fuse, the amp was fine.  I had egg on my face because I told the customer to get the JJ`s.  This was a first for me with the JJ`s.  I hope its the last.
Punky

[jerrydyer]

I bought a good size lot of jj 12ax7 and el34,,,,,, on the 12ax7  almost 50%   on the el34  about 35% failure rate 
dont like em... Id rather put a valve art in there, had great results from valve art and ruby. 

[phsyconoodler]

It's touch and go with a lot of new production tubes.I've had more JJ tubes fail than any other.

[The_Gaz]

Where are you guys buying them from? I don't mess with any new production power tubes that aren't well tested and burned in. I'm curious if you guys are still having problems even if you are buying from legit dealers.

[catnine]

 I haven't had  JJ 12ax7 or 6V6 burn out yet . I still use ruby tubes 12ax7 and 6V6 and 6L6GC and no problems so far and EH 12ay7 and 12ax7s and 6V6s. I got a matched set of JJ 12ax7s and EL 84's for a 01 Fender pro Jr they looked like sovteks from the factory  and the 12ax7s or EL84's made a rattle and hum  and had a 94 tweed pro Jr with stock probably sovteks and they never gave me a problem well a rattle and I swapped the position of the  EL84's in the 94 tweed pro jr and the rattle went away, I had that pro jr until 2004 . . Yet these tubes are from at the latest 2005 . I have not bought new tubes since then , I can't say what they are like now.

 I have to add I have only gotten tubes from Euro tubes or AES as far as JJ's go and Rubys from AES . I did have a bad JJ 6L6GC from Euro and he replaced it as it rattled and the bias would not hold steady.

[sluckey]

It appears that the JJ EL34 in a single end amp I built a while ago shorted internally between the plate and screen grid (pins 3 and 4). The base is burned between those two pins and there is luckily no damage to the rest of the amp. I was wondering about any experiences you guys may have had with JJ EL34's shorting internally. This one ran for a year with no trouble and then went poof. I always considered the JJ's to be rugged tubes, but now I'm not so sure.

I'm not convinced that a tube failure with internal short between plate and screen burned the socket. I still think it's more likely that someone was driving the amp hard without a speaker and the open circuit on the secondary reflected back a very high voltage spike that arced the socket, maybe even taking out the tube. You probably will never know unless someone 'fesses up. You need to replace that socket regardless of what caused it to arc.

[PRR]

I've been very impressed with every Sovtek that I've bought from Doug Hoffman. One 6550 biased-up spot-on, didn't drift, I beat-on it really long and hard, and it didn't even creak (my vintage Dyna/Tung-Sols creak on dummy-load). 12AX7 and EL84-pair were matched dead-close: 12AX7 164V/163V for two sides in identical circuits.

I have not happened to get EL34 from Doug; I did get EL34 from Weber and they worked perfectly fine (in an easy amp, and delivery was long). I do not remember the brand and that was a while ago.

I can't believe Doug personally tests each bottle, but he must be getting them from someone who doesn't deal in duds. 

http://www.hoffmanamps.com/MyStore/catalog/parts11.htm

[bigsbybender]

I have JJ EL34s in my pseudo-plexi and there have been no issues. I've had them in there since 2003, they haven't weakened a bit.

I have been mostly using the EH EL34 without complaints. Except one case of a key broken off the base.


j.

[panhead]

I've had a couple of JJ EL34's where the key broke off the base.

[Tone Junkie]

Ive had 2 sets of JJ el34,s in the last 2 years and 1 set 6l6 no problems

[LooseChange]

Every brand of tube has the few bad ones that slip though.
I've had every brand have some difficulty at some time.
EL34's are the most problematic in general.

There are a lot of JJ's being bought and statistically they probably have as much or as little failures as every other brand.

[FYL]

EL34's are the most problematic in general.

High slope pentodes are very fragile because of their geometry. The screen is very close to the plate, too much screen dissipation leads to overheating, a warped screen and a short with the plate.

[The_Gaz]

FYL's right. I'd be curious to know what the screen dissipation is on the OP's amp with max signal. I've measured many Marshalls with 1k screen resistors that we're dissipating way past their ratings (you could see the screens glowing with signal through the plate structure).

A lot of these failures have to do with how well the screen grids are aligned to begin with - something I do not believe can be tested for by tube dealers - or if they some how get misaligned due to some physical stress. I've tested many JJ KT77s, for example, in the same circuit where some screens glow right out of the box, and some just don't.

[plexi50]

I am glad i just saw this thread. I was going to start one on the JJ 12AX7 tubes. I have had customers drop ship them to me from a tube sales source. I have found for the past 6 months or so that many of the JJ 12AX7's that i have tested after recieving them read 100% on V1A with no emmision leakage. Then i will test V1B of the same tube and it will read 100% and then under emmision load testing leak all or most of the way down the tester scale in the bad range. I have 3 different testers and they do the same thing on all of them. I called the place where i and my customers have been buying them from and they said they have heard of no issues or problems with them. But i see other wise. Have any of you had this happening more often than naught latley?

This is common with the Sovtek,EH,Mesa,Marshall and some other brands as these will not test on some testers because of the way they are manufactured differently in specs but the JJ issue is making me wonder if they are having QC issues

[FYL]

A lot of these failures have to do with how well the screen grids are aligned to begin with - something I do not believe can be tested for by tube dealers - or if they some how get misaligned due to some physical stress.

A curve tester can help here.

[JayB]

I've been very impressed with every Sovtek that I've bought from Doug Hoffman. One 6550 biased-up spot-on, didn't drift, I beat-on it really long and hard, and it didn't even creak (my vintage Dyna/Tung-Sols creak on dummy-load). 12AX7 and EL84-pair were matched dead-close: 12AX7 164V/163V for two sides in identical circuits.

I have not happened to get EL34 from Doug; I did get EL34 from Weber and they worked perfectly fine (in an easy amp, and delivery was long). I do not remember the brand and that was a while ago.

I can't believe Doug personally tests each bottle, but he must be getting them from someone who doesn't deal in duds. 

http://www.hoffmanamps.com/MyStore/catalog/parts11.htm

I've been happy with every tube I got from Doug. I like the EL34s also. I'm a sovtek fan, they just take a beating and still keep going. I only got one dud 12ax7 sovtek from Doug. It works just fine, just the mica spacers were just a tad small so it rattles. But that would be hard to catch.

The only JJ's I have used are the 6L6GC's and they seem to just fine going full bore.

[Madison]

JJ preamps tubes have given me lots of grief.
Their 6V6s sound great but unfortunately rattle;more problems.(toss them in a head I suppose)
I have been okay with JJ 6L6s and EL34s but maybe I got lucky.

Eurotubes used to have a pretty good guarantee on JJs.......if you MUST have them.
In general, I shy away from JJs.They are way too unpredictable.

Honestly, I have learned to just buy whatever brands Hoffman sells to be safe.
Never had a problem with Sovtek preamps tubes, really like them.
No probs with EH power tubes either.

[panhead]

I replaced the socket and put in a Ruby El34 over the weekend and it sounds great. Hopefully that's the end of it. Everything measures good, and I let the amp idle for about 3 hours.

[jjasilli]

For failsafe yo can use a fuse(s) for the power; or put a 10R 1/2W resistor in ea power tube cathode > ground.  The resistor serves double duty as a fuse, and a bias sense resistor -- though it will throw your meter off by a decimal point. 

[Rev D]

 I've had one of a pair of JJ El34 die as well. I've had good luck with most of they're 6v6's, but the EL34 was the only one of they'res that has died on me. I have had about 50% of sovtek 5y3's die (2 out of 4)  but thats about it. I suspect that they're is no internal testing from the factory, maybe a quick it works or doesn't and ship it out, probably not even that, just put it in a box and deal with the dead one's later. *shrug*

Regards,

Don

[shortfuse]

The last 3 amps I retubed and or built I used all JJ's and have no issues with them.  One set of 6L6GC's were miss matched and I called Eurotubes, they sent me another set no questions asked.  But I always order mine direct from Eurotubes not a local retailer.
I have used Doug's tubes also with no issues whatsoever.