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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 6th Jun 2008, 7:36 pm   #1
Tractorfan
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Default Cathode poisoning in valves.

Will the cathode be damaged if a valve is run with heaters but no other voltages? Some homebrew projects call for a triode on its ownsome and say to leave the other half of a double triode (for example) disconnected. With an ECC83 no problem as the centre tapped heater means that one half can be left cold but that's not always possible. In that case should the spare half be strapped to the "active" half or should it have an anode resistor to keep it emitting? If so, then what do you do with the grids? It's just that with valves slowly becoming rarer it seems a shame to let one be ruined for the want of a couple of resistors if my first question is answered in the affirmative.
Cheers de Pete
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Old 6th Jun 2008, 7:44 pm   #2
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Default Re: Another (possibly silly) valve question

I don't think any harm will be done. Many AM/FM radio designs disconnected the HT from the FM front end when the radio was switched to AM.

Paul
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Old 6th Jun 2008, 7:57 pm   #3
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Default Re: Another (possibly silly) valve question

If the HT is fully off, then there is no problem. It is used sometimes to keep valves 'warmed up' as Paul says.

Do make sure it's fully off though. HT in the wrong place can do damage. Think of a Pentode when there is nothing on the anode and full HT on the screen grid. The grid tries to be an anode, glows white hot and destroys the valve.

Cheers,

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Old 6th Jun 2008, 8:11 pm   #4
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Default Re: Another (possibly silly) valve question

Thanks for the prompt responses. It's just that I heard somewhere years ago that cathodes can be "poisoned" if they're hot but not emitting. Maybe I was thiking of something else.
Perhaps because the HT was disconnected from the FM front end when switched to AM that old radios are often poor on FM when we get them for restoration. I may be wrong though.
Cheers de Pete
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Last edited by Tractorfan; 6th Jun 2008 at 8:14 pm. Reason: Left a bit out.
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Old 6th Jun 2008, 8:26 pm   #5
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Default Re: Another (possibly silly) valve question

It's not the HT that makes the cathode emit electrons - just warm it up and off they go. The electrons emitted aren't pulled back to the cathode, they just wander around aimlessly. Perhaps they all pile up in the bottom of the valve and eventually short all the pins out........perhaps I'll think this through again.....I have heard of 'poisoned' cathodes but really don't know what they are, perhaps just lost their emissive coating.
Regards - Martin
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Old 6th Jun 2008, 8:31 pm   #6
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Default Re: Another (possibly silly) valve question

There's not much evidence that cathode poisoning ever happens to a significant extent in normal domestic valve circuits.

The subject seems to exercise the valve audio people a lot. I suppose if you've just paid 1,000 for a pair of PX4s you don't want to take any chances.

Paul
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Old 6th Jun 2008, 8:47 pm   #7
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Default Re: Another (possibly silly) valve question

Cathode poisoning does, apparently, exist...

There are various mechanisms that can cause it, including impurities in the electrodes themselves, but what is being queried here is poisoning caused by absence of cathode current.

Apparently, what happens is that the electrons that are emitted by thermionic action do not just wander off to nowhere in particular: they tend to form a charged cloud around the cathode ('space-charge' as some term it). This has an odd effect, namely it tends to repel the electrons emitted from the cathode, and they turn round and bombard it. Various studies have indicated that low-energy bombardment of cathode materials can lead to long-term damage.

I can only speak as I find... in high-power transmitters, due to cathodic poisoning it was considered a bad thing to leave the filaments of a TX switched-on long-term without any HT supplies. If you had a main/reserve type of TX installation, this was bad news, since in the event of failure of the main TX, you had to switch over to the reserve TX which had cold filaments. These were large valves, and could have warm-up times of several minutes - a long time for a broadcast service to be 'off'. So a wheeze was devised, known as 'black heat': here, the filaments of the reserve TX were heavily under-run, to the extent that they didn't emit visible light at all. But they were still jolly hot, for all that, and when needed, full filament volts would be whopped on, and they'd then be ready for service in about half a minute or less.

In a small domestic device you don't need to take such radical steps. All that is needed is to increase the device bias until it's just taking a sniff of cathode current - this strips away the space charge, and the effect doesn't occur.
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Old 6th Jun 2008, 9:03 pm   #8
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Default Re: Another (possibly silly) valve question

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Originally Posted by Ray Cooper View Post
In a small domestic device you don't need to take such radical steps. All that is needed is to increase the device bias until it's just taking a sniff of cathode current - this strips away the space charge, and the effect doesn't occur.
It certainly won't do any harm to do this if you are in any way concerned. I don't think you'd get any detectable deterioration in, say, an unused triode section of an ECF80 though, even after many hours of use. The valves used in big broadcast transmitters are a very different matter and I'll certainly defer to Ray's expertise there.

Paul
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Old 6th Jun 2008, 9:46 pm   #9
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Default Re: Another (possibly silly) valve question

I've experienced cathode poisoning with transmitters, in the 100W plus range - little used transmitters need new valves evry 4 months, busy ones, the PA valves last 12 to 18 months!

It can also cause problems in small signal amplifiers, Tektronix had a problem with Y amplifiers in some 500 series oscilloscopes - a different valve was developed to overcome the problem.

Jim.
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Old 6th Jun 2008, 10:05 pm   #10
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Arrow Re: Another (possibly silly) valve question

Cathode poisoning - see the attached link - scroll down to the last entry, "Failure Modes" . . . . .

http://en.wikipedia.org/wiki/Hot_cathode

Al / Skywave
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Old 6th Jun 2008, 11:07 pm   #11
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Default Re: Another (possibly silly) valve question

Good point Jim.

Which raises an important question for those of us involved in the transmitter at Ally Pally. Only on for a few hours a week...

Cheers,

Steve P.
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Old 7th Jun 2008, 9:15 am   #12
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Default Re: Another (possibly silly) valve question

Had a thought last night on this, and I'd like to know what others think. ECC85/UCC85 valves often need changing. Could this be why? On many sets, the heater stays up but the HT is switched via the wavechange switch.

Thoughts, anyone?

Cheers,

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Old 7th Jun 2008, 9:28 am   #13
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Default Re: Another (possibly silly) valve question

Quote:
Originally Posted by Steve_P View Post
Had a thought last night on this, and I'd like to know what others think. ECC85/UCC85 valves often need changing. Could this be why? On many sets, the heater stays up but the HT is switched via the wavechange switch.
If there was any evidence that cathode poisoning would be a problem, it would have been very easy and cheap for the makers to connect the anodes to HT via a large resistor when the radios were switched to AM. They could have referred to the improved valve life in their marketing. The fact that none of them bothered to do this speaks volumes.

Paul
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Old 7th Jun 2008, 9:47 am   #14
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Default Re: Another (possibly silly) valve question

I suppose it's still open to debate. Most AM/FM sets use something like an ECH81 as the AM mixer and most manufacturers disconnect the oscillator anode via the wavechange switch when using FM. However most sets work happily on AM even if they have been switched to FM for years.

It's interesting to note however that my Baird 301 has a high value resistor switched in to feed the anode of the oscillator section of the ECH81 when in FM mode. This indicates that some manufacturers where concerned about cathode poisoning.

When I first started learning about radio some 40 odd years ago, I was always told not to run a valve for very long periods without HT connected. Just how long 'very long' periods were, I don't know. I suppose now that valves are more difficult to obtain, it would be prudent to fit a high value resistor to the 'unused' anode so that at least it passes a small current.

Rich.
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Old 7th Jun 2008, 5:45 pm   #15
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Default Re: Another (possibly silly) valve question

I heard about cathode poisoning some time ago, this was in relation to CRTs being affected in this way with sets having an "instant-on" facility (leaving the heaters powered up).



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Old 7th Jun 2008, 6:03 pm   #16
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Default Re: Another (possibly silly) valve question

Bush also fitted a high value resistor in the HT feed to the two EF80s in the VHF41 - their first (and not very good) FM chassis. This disappeared on later sets such as the VHF61 which used an ECC85.

I think if cathode poisoning were a serious issue, many FM sets which had been operated for most of their lives on FM would show a poor E/UCH81 triode. This is not my experience, in fact the heptode is usually less good than the triode, possibly because it has spent most of its life biased nearly off by the AGC assuming most sets are tuned to a local station. This gives rise to all of the anode current being "sourced" from a small area of the cathode where the grid pitch (variable mu heptode) is at its coarsest.

I think the relative unreliability of the E/UCC85 is simply due to its running a bit hot for its envelope size. Fitting a blackened screening can makes a big difference and can reduce tuning drift as a result. It's a dead easy mod on Bush sets which use standard cans.

Leon.
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Old 7th Jun 2008, 10:40 pm   #17
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Default Re: Another (possibly silly) valve question

The last time this issue was raised it was pointed out that many valve comms receivers disconnected HT for stand by [eg Eddystone 358x] with no ill effects. Of course it may be a different story with high power TX situations! Maybe there has been confusion between the two modes in the past. Dave W
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Old 8th Jun 2008, 4:29 pm   #18
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Default Re: Another (possibly silly) valve question

Special quality valves were developed for computer use where they could be without anode voltage applied for long periods. e.g. E88CC. The Brimar book says it and others were developed to achieve
this; their other SQ valves were designed for high vibration tolerance.

In a comms set, switching to transmit mode is usually only for a few minutes at a time and then the valve is powered normally.

Pete.
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Old 8th Jun 2008, 7:09 pm   #19
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Default Re: Another (possibly silly) valve question

Coo, maybe it wasn't such a daft question after all . Makes a change as I usually manage to kill threads off instead of starting interesting discussion. Thanks to all, lots to think about there. It's just another of the random thoughts that pop into my mind for no particular reason from time to time.
BTW, My wife's just said to me: "While you're on your forum ask 'em how to pickle cherries!" But I suspect that might just be a bit off topic
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Old 8th Jun 2008, 7:45 pm   #20
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Arrow Re: Another (possibly silly) valve question

My research indicates that cathode poisoning is caused by the cathode coming into contact with traces of unwanted chemicals, e.g. oxygen, aluminium or various silicon compounds. Typically, these enter the valve envelope via leaks in the valve envelope, by over-running the valve (so that the various electrodes become stressed and emit impurities) or by deficiencies during valve manufacture. The result of any of these effects is decreased emmissivity from the cathode surface - known as 'cathode poisoning'.

Therefore, it seems that the question really boils down to this:
does powering valve filaments - without cathode current flowing - cause these impurites to become activated and thus attack the cathode?

For low-power valves, I suspect not - but I cannot find any authoritative statement anywhere to confirm or contadict this.

Al / Skywave.
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