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Old 6th Jun 2008, 9:47 pm   #7
Ray Cooper
Retired Dormant Member
 
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
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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