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Old 12th Jan 2021, 9:01 pm   #1
high_vacuum_house
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Smile Serious valve failure !!

Good evening,
On going through a box of secondhand telly valves I found a Mullard PC92 valve with balls of metal rolling around inside. Looking through the glass between the lower mica and the base it can be seen that a major flashover had taken place inside. One leg of the heater where it enters the cathode has been vaporised completely and its connection to the base has melted back nearly to the glass. The cathode strip to the base connection has almost disappeared and blowing a small hole in the bottom of the cathode tube where it had been welded on.

I Bet this valve lit up rather well during its demise. Surprisingly the vacuum is still there! I wonder if it went heater to cathode short with no fuse protection.

Has anyone seen any valves that have this much punishment in their collection?

Christopher Capener
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Old 12th Jan 2021, 9:09 pm   #2
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Default Re: Serious valve failure !!

The PC92 is an RF amplifier of very low power. Either a very serious fault developed with the heater chain shoving mains through it or perhaps the aerial was struck by lightning. I can't think of any normal situation where a low power RF amp would be destroyed in such a way. I'm thinking that it was probably in grounded grid mode so the aerial input would have been to the cathode.
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Old 12th Jan 2021, 9:16 pm   #3
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Default Re: Serious valve failure !!

or plugged into the wrong hole!
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Old 12th Jan 2021, 9:41 pm   #4
Michael Maurice
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Default Re: Serious valve failure !!

A PC92 is a 7 pin base, all the other valves are likely to be 9 pin.
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Old 12th Jan 2021, 9:43 pm   #5
McMurdo
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Default Re: Serious valve failure !!

depends where it was plugged really
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Old 12th Jan 2021, 10:00 pm   #6
Jonster
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Default Re: Serious valve failure !!

I'd agree this sounds like lightning damage!
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Old 12th Jan 2021, 10:30 pm   #7
Herald1360
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Default Re: Serious valve failure !!

It wouldn't need to have been a direct strike, either. Nearby would do.
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Old 12th Jan 2021, 11:56 pm   #8
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Default Re: Serious valve failure !!

Possibly at or near the chassis end of the heater chain, a tempting but fusible link en route to the neutral-earth strap at the consumer unit from the lightning's point of view?
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Old 13th Jan 2021, 5:33 pm   #9
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Default Re: Serious valve failure !!

I have a vague recollection of a PC 92 being used in the EHT section of a very early Korting CTV, the one with both EHT and LOP transformers and two PL509 bottle. That was 40 + years ago, so I may be wrong.
nast high voltage things could happen there.
Les.
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