Steve Thompson wrote:
>>
>>Years ago in one of the British ham magazines there was a circuit where you
>>discharged the reservoire cap in the hv with a thyristor that was in
>>paralell. When there was flash-over in the pa tube ( valve hi!) you got a
>>pulse that fired the thyristor.
>
>In the version I remember, the thyristor was triggered by the anode current
>spike, but it grounded the screen grids (so as to protect the socket
>capacitors), not the HV supply.
>
Steve's memory is correct, but there is also a circuit for a HV crowbar.
It uses a string of thyristors to discharge the reservoir cap, and a
solid-state relay to disconnect the mains from the transformer.
I've seen it working, and numbers of Europeans have copied it in both
single-phase and 3-phase.
The checkout test is to short the HV through a few inches of 3A fuse
wire - which does NOT blow! The old RCA test was to short the HV on to
aluminum cigarette foil without blowing a hole through it. All you see
is a small spark, and the supply quietly shuts down.
I don't have a direct URL, but go to http://www.dubus.org, then to the
"archives" page, and click to download article 9002-7.pdf
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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