600 Volts JJ EL84 survives 6V6 fries

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stevesuk
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600 Volts JJ EL84 survives 6V6 fries

Post by stevesuk »

FWIW.
I didn't do this on purpose, but I was experimenting with various layouts and wiring and accidentally gave a new JJ El84 and a NOS Sylvania 6V6 GT about 600V B+ for a couple of minutes, (Didn't hang around to measure it).

Plenty of redplates and burnt logos very quickly.

After restoring things to a sensible level the 6V6 is now officially deceased.

The JJ EL84 sounds as good as ever.

Steve UK

www.valvepower.co.uk
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msormune
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Post by msormune »

How much did you put on the screens?
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Merlinb
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Post by Merlinb »

msormune wrote:How much did you put on the screens?
Yeah, the EL84 has a higher screen dissipation rating doesn't it?
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msormune
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Post by msormune »

The reason I'm asking is that the way I understand it's the screen element that gets toasted fastest than the rest of the tube when giving really too much voltage. It's actually pretty "flimsy" when compared to plate structures. JJ EL84 actually may be more durable because it has a more durable screen element.

This maybe because a lot of classic tube amps run with plate voltages up there in 350's and the screen voltage just a few volts below that. *cough*ac30*cough*. This is just speculation, though.

Don't know much about 6v6's myself. It does seem to have a bit lower maximum screen voltage, but not much. Max screen dissipation seems pretty much the same.
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zaphod_phil
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Re: 600 Volts JJ EL84 survives 6V6 fries

Post by zaphod_phil »

stevesuk wrote:After restoring things to a sensible level the 6V6 is now officially deceased. The JJ EL84 sounds as good as ever.
I'm pretty sure if it had been a JJ 6V6, it would have still been OK.
msormune wrote:The reason I'm asking is that the way I understand it's the screen element that gets toasted fastest than the rest of the tube when giving really too much voltage. It's actually pretty "flimsy" when compared to plate structures.
Yes, the screen grid is indeed very fragile. However, what really fries the screen grid is excess power dissipation - ie the combination of voltage and screen current. Being far more thin and delicate than the corresponding anode, it takes much less power to melt a screen grid. So what usually happens during an over-voltage condition, is that both the screen voltage and current increase, so that the screen grid gets fried. Tubes can generally withstand quite a lot of voltage above what their spec says, provide they don't also draw excess current at the same time, like what most probably happened with the NOS 6V6 in this case. A pure over-voltage condition will cause arc-over inside the tube, typically with a crackling sound. If the grids are involved in the arc-over, then they can also get severely damaged. Sorry for being long winded....
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stevesuk
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Post by stevesuk »

ZP,
You are absolutely right, there was a crackling sound inside the 6V6.
Being made of blacked out glass, the redplating and arcing isn't visible until it is too late.
Just as well I have a bunch of 6V6's.

Steve UK

www.valvepower.co.uk
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CurtissRobin
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Post by CurtissRobin »

Wow!! That must've been some experiment, Dr. Frankenstein. 600V is more than Traynor puts on the EL34 plates of my YBA-1A (measured last year at 534VDC).

KennyO
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