|
General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
|
Thread Tools |
12th Jan 2021, 10:01 pm | #1 |
Octode
Join Date: May 2003
Location: Belper Derbyshire
Posts: 1,909
|
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
__________________
Interests in the collection and restoration of Tefifon players and 405 line television |
12th Jan 2021, 10:09 pm | #2 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
|
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.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
12th Jan 2021, 10:16 pm | #3 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
|
Re: Serious valve failure !!
or plugged into the wrong hole!
__________________
Kevin |
12th Jan 2021, 10:41 pm | #4 |
Moderator
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
|
Re: Serious valve failure !!
A PC92 is a 7 pin base, all the other valves are likely to be 9 pin.
|
12th Jan 2021, 10:43 pm | #5 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
|
Re: Serious valve failure !!
depends where it was plugged really
__________________
Kevin |
12th Jan 2021, 11:00 pm | #6 |
Heptode
Join Date: Apr 2008
Location: Eastbourne, East Sussex, UK.
Posts: 671
|
Re: Serious valve failure !!
I'd agree this sounds like lightning damage!
|
12th Jan 2021, 11:30 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
|
Re: Serious valve failure !!
It wouldn't need to have been a direct strike, either. Nearby would do.
__________________
....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
13th Jan 2021, 12:56 am | #8 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
|
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?
|
13th Jan 2021, 6:33 pm | #9 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
|
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. |