Brand new EH EL34 failed first time in amp.

[lovetherock]

Hi, new guy here. I have a home build that I had been running 2, 6L6's in with no trouble. It is PP, cathode biased, and cold with a 500 ohm R. I decided to try EL34's and I tied pin 1 to pin 8 then to the cathode R. B+ = 415v idle, Screens about 380v, OT: 4000ohm Primary. I have 1K screen stoppers and 1.5K grid stoppers. With 6L6's I had been getting about 33v on the cathode 500R for about 60mA total, hopefully 30mA per tube. When I fired up the 2, EL34's, the heaters went good (I have a 6amp heater winding, 35W PT). I flicked on the B+ and immediately heard hard hum from the speakers which faded in a few seconds. I noticed one EL34 glowed blue in the center and one did not, I believe. I played some chords and got heavily distorted thin blatty sound, then, after about 30sec I noticed smoke rising from my circuit board. Might have been solder smoking, or the 500R or the 25uF, 50v bypass cap--I couldn't tell. I killed the B+. Inspected things and found no obviously burnt components. Tried the B+ again and it popped loud with maybe a BAAA!!!, don't quite remember. Removed EL34's, inspected/tested all components, OT seems OK, put 6L6's back in and amp still works good. Could one or both of the EL34's have been defective right out of the box? Got em from Tubesandmore.com, EH's Russian made, one from 09 07, one from 08 10 --dates? No obvious cracks or burning evident. Don't have a tube tester. Sorry I didn't get any volt/current stats from test run. Thanks, and I'd appreciate any opinions on the problem. I concede the idea was maybe a bit ill-advised, if only rushed in too quickly.

[Reeltarded]

Yeah that happens. I like the JJ6CA7s so I bought two matched quads to use as 4 pairs. Lost one of each quad in minutes but the rest are still rockin' a couple years later.

[tubeswell]

You mean you put them in your tube tester and they failed

[pdf64]

Tried the B+ again and it popped loud with maybe a BAAA!

What exactly happened, eg what went 'pop'?
A shared cathode bias resistor requires both/all tubes to be at least conducting, hopefully at a similar current level; if one of the EL34 was dead (ie not conducting), then the other may be under biased and conduct heavily, possibly overdissipating.
My view is that it's more robust to have an independent cathode resistor for each tube (both would then need bypassing, if that's required).
A 50V rating on the bias cap is likely to be exceeded under heavy drive conditions.
Cathode bias can result in a poor overdriven tone, unless biased at / close to class A, due to there being 2 forms of signal dependent bias shift with the potential to cause crossover distortion. Power tube grid conduction forces the effective Vg-k more negative (eventually blocking distortion) and increased cathode current causes the cathode voltage to rise, causing Vg-k even further negative (Aiken's 'squish' http://www.aikenamps.com/Sag.html ).
A shared 500 ohm cathode resistor seems high to me and may exacerbate the squish effect.
However, if it sounds good to you then that's all that matters!
Pete

[sunnydaze]

A 50V rating on the bias cap is likely to be exceeded under heavy drive conditions.


+1

You may have heard the 50V cap exploding when you heard the pop. Should be able to tell pretty easily. Sniff around (literally) and you should be able to find the area the smoke came from. The EL34's may be OK.

Mike

[martin manning]

My view is that it's more robust to have an independent cathode resistor for each tube (both would then need bypassing, if that's required).
A 50V rating on the bias cap is likely to be exceeded under heavy drive conditions.

Independent Rk also means you can check the operating point for each tube separately. Based on your description, I'm betting one of the EL34's was bad or failed immediately. The 50V Ck rating wouldn't be a problem when idling, so while it is a good idea to increase the rating to ~100V, that wasn't the cause of the failure.

[Reeltarded]

Has to be. The early hum is the clue. The tube was dead when voltage hit it the first time.

[lovetherock]

Thanks for the responses everyone.

I don't think the 50v cap failed, it didn't look burnt and the amp still works about the same as before with the 6L6's. I suppose it could be damaged, but it's not shorted. Smoke was only there for a few seconds before I killed the B+. I'll definitely consider larger voltage cap/seperate resistors. The "pop" occurred when I switched the B+ back on for a second. I never heard a pop before when coming off of Stand-by.

The large 500 ohm R was a leftover from my SE cathode bias circuit. I figured it would be a safe value. I've noticed that some use about a 250-300 ohm common R. I'm trying to decide which tubes I like best, then go with appropriate values.

About the blue glow...that's normal, right? I think only one of the tubes had it before I had to shut down. I'm thinking of emailing tubesandmore to see if they'll ship me some replacement tubes...

[ampgeek]

By all means...email TubesnMore and describe the problem.

Although the details are fuzzy in my 53 yr old brain, I had some problems with some JJ brand power tubes a few years ago (6V6s IIRC) and they replaced them with no questions asked.

Good luck,
Dave O.

[lovetherock]

By all means...email TubesnMore and describe the problem.

Although the details are fuzzy in my 53 yr old brain, I had some problems with some JJ brand power tubes a few years ago (6V6s IIRC) and they replaced them with no questions asked.

Good luck,
Dave O.

Thanks Dave. Now my email is down. It never ends.

[briane]

welcome to the club.

sometimes their DOA.