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Old 14th Sep 2021, 7:59 am   #1
GrimJosef
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Default What happens if a valve's cathode becomes electrically detached ?

I recently took delivery of a matched quad of EL34s for the output stages of a stereo push-pull amp. When the amp was switched on all four heaters lit up and I expected, once the valves were passing current, that it would start working. However, before it did there was a short burst of noise - clicks and pops followed by a couple of seconds of squealing - from one channel. After this everything was fine.

It took me a while to track the fault down to one of the new valves (as I shifted the valves between channels the fault followed this one, and when I substituted it with a spare the fault went away). I wondered what might actually be causing it.

So I set up voltmeters on anode, screen grid (the output transformer feeds this through an ultralinear tapping), control grid and cathode. Before the noises began I saw that the screen grid and anode voltages were very close indeed, suggesting no current flowing in this valve's half of the output transformer primary. The cathode voltage stayed very close to zero. Since this is a cathode bias circuit (470 ohm cathode resistor) this was consistent with no current flow too, although I suppose the cathode might alternatively have been shorted to ground somehow. The control grid drifted somewhat negative and then returned to a potential very close to zero.

I'm speculating that when the valve is cold its cathode is electrically detached from the pin which carries the connection through the glass. As it heats up thermionic emission begins and it loses electrons to the grid, explaining the negative excursion there. The loss of electrons will drive the cathode positive. Eventually the action of the grid, as the cathode gets increasingly positive with respect to it, will be to shut off main current flow in the valve. After a surprisingly repeatable 55-65 seconds some sort of thermal deformation of the valve's metalwork then brings the cathode and pin into contact with one another. The transiently poor contact will create the noise. When the contact closes properly the voltages will snap to their correct values, with the anode roughly 2V negative of the screen grid and the cathode at 29V. This is what I measure.

Does that sound like a plausible explanation ? Fortunately (for me, but perhaps not for this discussion) the retailer has asked me to return the valve, which I've done, and promised a replacement if it proves to be faulty when they test it. So I'm afraid I can't carry out any more experiments. But it would be comforting to know if a single relatively simple issue could be the cause of the problem.

Cheers,

GJ
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Old 14th Sep 2021, 10:54 am   #2
Chris55000
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Default Re: What happens if a valve's cathode becomes electrically detached ?

Hi!

I'm totally in agreement with your explanation as to what's happening inside your suspect valve – the only other thing to add is that it's almost certainly caused by a defective spot–weld between the cathode tube itself and the fine connecting strip/wire leading off down the glass "pinch" to the base pins – only thing you can do with it is what you've already done, return it to your supplier for replacement!

There has been mentioned on here in the past, and sometimes in servicing books and P.T. magazine, "welding" tricks on internal duffy connections to valve and C.R.T. electrodes using the very high flyback pulse voltages from valve–operated line output stages on vintage TVs, but it's an extremely dangerous technique to attempt only as a very last resort when all else fails, as quite apart from the personal risks, you can destroy a perfectly good and irreplaceable line output transformer attempting it!

Chris Williams
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Old 14th Sep 2021, 11:35 am   #3
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Default Re: What happens if a valve's cathode becomes electrically detached ?

Quote:
Originally Posted by Chris55000 View Post
There has been mentioned on here in the past, and sometimes in servicing books and P.T. magazine, "welding" tricks on internal duffy connections to valve and C.R.T. electrodes using the very high flyback pulse voltages from valve–operated line output stages on vintage TVs, but it's an extremely dangerous technique to attempt only as a very last resort when all else fails, as quite apart from the personal risks, you can destroy a perfectly good and irreplaceable line output transformer attempting it!
Final anode and 'dag.

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Old 14th Sep 2021, 11:45 am   #4
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Default Re: What happens if a valve's cathode becomes electrically detached ?

I'd agree with the majority of this, except this bit:

Quote:
Originally Posted by GrimJosef View Post
As it heats up thermionic emission begins and it loses electrons to the grid, explaining the negative excursion there.
The anode and screen-grid being far more positive than the cathode, any electrons emitted will go through the control grid and end up on anode or screen. Otherwise, you'd see grid current in normal operation too!

Further, even if anode and screen were not highly positive, and the cathode did lose its electrons to the control-grid until the rise in cathode potential stopped further flow, the magnitude and duration of current flow would be so small you'd be looking at microamps and microseconds (10μA for 10μsec will elevate 10pF by 10V).

So, the negative grid for some seconds must be another mechanism.
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Old 14th Sep 2021, 11:59 am   #5
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Default Re: What happens if a valve's cathode becomes electrically detached ?

I had a similar problem in my STA15 many years ago. I had fitted a new quartet of Chinese EL34s and after a month or so started to get weird noises like you describe. I worked out which valve was responsible and a close inspection showed its cathode had 'slipped' down and was probably only making intermittent contact with its pin wiring. The supplier did send a replacement which worked fine for maybe 15 years before weird noises caused me to replace them with my last quartet of Mullards.
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Old 14th Sep 2021, 6:57 pm   #6
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Default Re: What happens if a valve's cathode becomes electrically detached ?

The negative excursion on the grid was relatively low-amplitude - less than a volt - and lasted only a couple of seconds or so. With 220k grid leaks it would only have needed a few microamps. I agree that a completely isolated cathode would have charged up very quickly indeed. But perhaps Chris's suspected defective spot-weld had some small residual conductivity ? Some tens of kohm at that point would have allowed the cathode voltage to rise high enough to switch the valve off with around a milliamp of current flowing down through the cathode resistor. I'd have struggled to see that on the 200V range of the DMM I was using to monitor the cathode voltage. It'll be interesting to hear what the retailer finds.

Cheers,

GJ
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Old 14th Sep 2021, 7:25 pm   #7
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Default Re: What happens if a valve's cathode becomes electrically detached ?

I would take a close look at the valve pins and reflow the solder before ditching the valve.
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Old 14th Sep 2021, 9:05 pm   #8
GrimJosef
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Default Re: What happens if a valve's cathode becomes electrically detached ?

I would too. Indeed I did take a close look. The EL34 has a glass button-base of course, so the wires are relatively short and dead straight. I could see the ends of all of them where they'd been snipped off as they left the solder blobs on the pin ends. The 'wetting' of the wire by the solder was good in each case.

Cheers,

GJ
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Old 14th Sep 2021, 9:52 pm   #9
Alistair D
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Default Re: What happens if a valve's cathode becomes electrically detached ?

No emission can also happen if the screen grid is not connected. you mentioned that you were getting some control grid current so I suspect that the screen grid is disconnected rather than the cathode. Hopefully the problem is due to bad soldering on the valve pins.

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Old 14th Sep 2021, 10:32 pm   #10
GrimJosef
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Default Re: What happens if a valve's cathode becomes electrically detached ?

I suppose that to begin with a disconnected screen grid would be raised to a somewhat positive potential by its capacitative coupling to the anode, but as soon as any electron flow from the cathode began there would be a tendency for the screen grid to intercept some of those electrons and, therefore, head downwards in potential, so throttling the electron flow.

As I say, the soldering on the pins looked fine under a magnifying glass, but since the valve is back with the retailer now I'm not in a position to re-check or to try doing anything about it.

The grid current did start and stop pretty quickly. In the light of kalee20's comments that could be explained by the HT, once it was established, sweeping the electrons through the control grid. The HT supply is solid-state rectified, but with a rather large smoothing capacitor, which doesn't charge instantaneously.

Cheers,

GJ
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Old 14th Sep 2021, 11:35 pm   #11
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Default Re: What happens if a valve's cathode becomes electrically detached ?

As it was part of a matched set, presumably it worked unremarkably before dispatch. So it must have been some vibration-induced defect that occurred during shipping.

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Old 15th Sep 2021, 8:06 am   #12
GrimJosef
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Default Re: What happens if a valve's cathode becomes electrically detached ?

My experience of valve matching is that I have to wait a little while for the valve to warm up properly before it's worth starting to take measurements. So perhaps the person doing the matching did what I do i.e. he powered the valve up and then went off to make a cuppa or whatever while it settled down. Since the fault seems to resolve itself perfectly after about a minute he may never have noticed it.

I ran the valve into a dummy load for more than an hour over a couple of days, taking audio measurements of the amp to confirm it was fixed. Everything behaved very well. It wasn't until I actually plugged it into a speaker and heard the brief squeal during the warm-up that I realised there was any problem.

Cheers,

GJ
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Old 15th Sep 2021, 3:50 pm   #13
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Default Re: What happens if a valve's cathode becomes electrically detached ?

O/C cathode connections in television CRTs was a problem in the 1960s, Mazda in particular. The usual cause of the fault was the connection between the cathode and the lead out wire. The tiny weld joining the cathode to the wire failed. If you tapped the neck the very temporary return of the picture proved the point. It could of course happen with valves. J.
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