[AMPS] New Triode Board from G3SEK

[Ian White, G3SEK]

measures wrote:
>> Inconjunction with a HV fuse, this provides a high degree of protection.
>>A thyristor crowbar circuit will provide even more protection if you
>>want it.
>
>A thyristor with a withstanding potential of 4kV?

No, a stack of 1000-1200V devices, triggered by opto-couplers. There's a
description by OE5JFL in http://www.qsl.net/ok1cdj/dubus/9002-7.pdf

Excess anode current switches off the mains by a solid-state relay, and
also fires the thyristor stack to short the capacitor. The stored energy
in the capacitor is dumped into a resistor, plus the resistance of a
snubbing inductor. 

I haven't built this myself, but have watched OE5JFL crowbar the 3kV
supply with a screwdriver. There's just a little spark at contact, and
the supply quietly shuts down. No fuss, no bangs, no fuses.
Most important, it passes the test recommended by Siemens for use with
their big PA tubes: you can short the 3kV supply with 2-3in of 3A fuse
wire, and the wire will NOT blow. Now *that* is protection!

Anything else is a compromise, but the glitch resistor is actually a
pretty good one for amateur-size tubes.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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