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Old 30th Nov 2020, 10:46 pm   #1
GrimJosef
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Default Anode-grid conduction - any advice before I try violence ?

I have an amp on the bench which was working fine when I turned it off but which hummed very badly after the next switch-on, a couple of weeks later. It turned out that the 6SN7GT phase-splitter now had one of its triodes switched hard on. Measurement, as shown in the attached picture, reveals that the cold resistance between anode and grid is something over 38kohm. This number has been stable over a couple of days and doesn't seem to be affected at all by changing the valve's orientation or tapping or shaking it.

I'm not sure what the cause of this fault might be. If a grid wire had snapped and come into contact with the anode then I think I'd expect a much lower resistance, and perhaps shaking it would reveal that the contact was intermittent. Likewise if a piece of something conductive had been rattling around in the bakelite base and was now bridging the two pins, or was wedged between the leadout wires from the valve, again I might expect a lower and less stable resistance.

A few tens of kilohms feels like the resistance of, say, a thin metal film, although I'm not sure how such a thing could have appeared during the brief period of the amp's switch-off or switch-on.

It occurred to me that I might try to clear this conductance, um, violently i.e. by discharging a suitable capacitor into the 38kohm. A fraction of a microfarad would give a pulse which would all be over in a few tens of milliseconds, which feels like it ought to be quick enough for minimal thermal conduction to anything other than the very immediate surroundings. This might minimise any collateral damage. Charging the capacitor to 200-300V would store a few tens of millijoules which, looking back to my days of working with pulsed lasers, should be sufficient to blow a small amount of metal away if concentrated into a tight enough physical volume. It would also subject the a-g gap to no more than the typical voltage it would need to withstand under normal operating conditions.

Before I go ahead with this though, do you have any advice about other tests I might try first or, perhaps, about gentler solutions to this problem ? I don't actually have much to lose - the valve is useless as it is, after all. But I accept that my scheme does come with some risk, not least that I won't have any control over where debris from the exercise could end up.

Cheers,

GJ
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Old 30th Nov 2020, 10:50 pm   #2
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Default Re: Anode-grid conduction - any advice before I try violence ?

Is it possible it’s the plastic valve base that has the leak not inside the valve?
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Old 30th Nov 2020, 10:54 pm   #3
GrimJosef
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Default Re: Anode-grid conduction - any advice before I try violence ?

Yes, it is. There's nothing visible on the outside of it. I guess I could try to take it off and look at the inside, but that isn't without risk. The base would have been attached with a large blob of adhesive and I've known that blob to end up enclosing the extremely fragile glass pump-out tube. In that case it's very easy to snap the tube when trying to detach the base, which is 'game over' of course.

Cheers,

GJ
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Old 30th Nov 2020, 11:12 pm   #4
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Default Re: Anode-grid conduction - any advice before I try violence ?

Seems really weird!

I'd be inclined to do it in a darkened room, if you see a flash, at least you can hopefully tell where is is coming from.

I've got en EBC33 doing weird things. Trials finished yesterday, I'm hoping to write up in the next couple of days (a thread has been started!)
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Old 30th Nov 2020, 11:13 pm   #5
Leon Crampin
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Default Re: Anode-grid conduction - any advice before I try violence ?

Make an adapter out of the faulty valve and fit an ECC82.

Unless the amplifier is an altar piece, this would be a quicker, cheaper and a better long term solution.

I became utterly fed up with the cosmetics brigade when I once repaired an amplifier and returned it fully working with the correct 6L6 output valves - metal ones. No good at all to the brainless customer....

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Old 30th Nov 2020, 11:20 pm   #6
GrimJosef
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Default Re: Anode-grid conduction - any advice before I try violence ?

Do you take the wire wheels off E-Types Leon ? (They drive just as well with 'normal' wheels, I'm told).

It is a bit of an altar piece actually. It's one of a pair of Leak TL/10s. The 6SN7GT is an RCA one and is a matcher (could well be the original, in fact) for the one in the other amp. They're not really rare though. But they do sell for around £50, so if there was a reliable fix for this then that would be worthwhile.

New 6SN7s are no more expensive than new ECC82s.

Cheers,

GJ
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Old 30th Nov 2020, 11:34 pm   #7
Leon Crampin
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Default Re: Anode-grid conduction - any advice before I try violence ?

I don't run altar pieces on wheels or on the audio shelf.

There are loads of cheap ECC82s around "burned in"..... I guess you could always use a skirted holder and put a can over it. It's ECC83s which seem to be mopped up by the getter-conscious brigade.

More seriously, I think you risk knackering an heirloom if you attempt to clear a shorted valve.

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Old 30th Nov 2020, 11:42 pm   #8
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Default Re: Anode-grid conduction - any advice before I try violence ?

It could have absorbed some damp from the winter air.
Put it in a warm dry place for a day or two and try again once the glue in the base is fully dry.
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Old 1st Dec 2020, 9:14 am   #9
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Default Re: Anode-grid conduction - any advice before I try violence ?

Have you tried seeing what the anode grid resistance is with the heater running?

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Old 1st Dec 2020, 9:55 am   #10
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Default Re: Anode-grid conduction - any advice before I try violence ?

If you're stuck, I think I may have a couple of American 6SN7 around, but I suspect they'll be Raytheon.

For the sort of end user that needs altar-piece aesthetics, then Raytheon may favour military music over swing.

If you need one, I'll go for a rummage in the valve boxes and it's yours.

David
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Old 1st Dec 2020, 11:39 am   #11
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Default Re: Anode-grid conduction - any advice before I try violence ?

I have experienced valves with metal films deposited inside. Usually power valves of course. So I would be checking the circuit first in case something has failed that is causing problems for the valve. It could also have gone soft which might help it strip the cathode for re-deposition.

So yes, some violence to clear the fault is the way to go. I think you need to be quite brutal to clear it properly. I have to say I have not had much succes with that. The obvious reason for a sudden problem is that the something moved a bit and made better contact with the film built up over time.
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Old 1st Dec 2020, 11:48 pm   #12
GrimJosef
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Default Re: Anode-grid conduction - any advice before I try violence ?

Thanks for all the input folks - I've been out today and have only just got back to this.

Ref - the valves had been in my living room, where the central heating comes on twice a day, for more than a week before I took everything out to the workshop for a trial. . The fault appeared at switch-on which was within a few minutes of the move. It's not impossible that the cooler weather has played a part, but this amp isn't mine and I can't guarantee what its owner will do with it once I've handed it back. I can't let it go with a valve that is going to need excessive protection on rainy days .

Peter - I haven't tried that, although the valve was 'hard on' when running in the amp. It wouldn't be hard for me to hook 6.3V to the heater pins though.

David - Thanks very much for the offer. This amp belongs to a paying customer so in the end the call will be his. I shouldn't pre-judge his response, but having met him before and chatted for quite a while I suspect he will either want a valve which matches the one in the other channel pretty closely (I do know of a source of one if this valve can't be rescued) or he will settle for a modern replacement. He's not delusional about things. But one of this pair of amps is a family heirloom, so he has some attachement to it.

GMB - I did wonder whether something external could have upset the valve. In my experience sudden failure of small-signal valves is quite rare. But the amp was running fine immediately before this happened and has been running fine with a modern 6SN7 since. I can't rule out an intermittent failure in, say, the isolating cap from the previous stage. But it must have caused the valve failure very quickly indeed (a few seconds) and I haven't seen it recur since. I'm afraid I agree with your assessment of the chances of success, which is to say 'not high'. But there doesn't seem to be much to lose ...

Cheers,

GJ
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Old 2nd Dec 2020, 1:17 am   #13
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Default Re: Anode-grid conduction - any advice before I try violence ?

If it was metal deposition on a mica disc then the effective resistance may change with test voltage (I found that effect with some 5U4 diodes that showed anode-cathode resistance which changed with PIV test voltage). As a characterisation test before 'blitzing' the leakage path, you may be inclined to use a bench HV DC supply in series with a current sense resistor to check.
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