[Amps] HV fuse

[Ian White, G3SEK]

Bill Fuqua wrote:
>I have thought about fabricating a device to protect tubes. Eimac 
>claims that if your crow bar circuit will protect a piece of  #30 
>copper wire under short conditions it will protect the tube.

RCA's equivalent was "Must not burn a hole in the foil from a cigarette 
packet."

>   What if you stretched a piece of #30 bare wire thru the center of a 
>piece of well grounded copper tubing. If your tube should flash over 
>the #30 or maybe #32 wire would vaporize and the vapor would 
>immediately conduct the current to the surrounding tubing rather than 
>to the tube. or put your sacrificial current limiting resistor in the 
>center of the tube so it would not only limit the peak current but also 
>produce a plasma conduction path to ground.

I don't think that would work... but it's a big step towards inventing 
the thyratron crowbar  :-)

What *does* work, if you want to go that far, is OE5JFL's SCR crowbar:
http://www.qsl.net/oe5jfl/flashover.htm

On the other hand, for tubes up to the 1500W class, Eimac's and Rich's 
recommendation is a good one:
>> limit I-peak with a suitable HV glitch R and use circuit breakers, or 
>>250v fuses, in the primaries of the PS transformers.

That is essentially the advice of Eimac Bulletin 17 on 'Fault 
Protection':
http://www.cpii.com/eimac/eiapps.htm

According to Bulletin 17, 1500W tubes are at the crossover between the 
QRP region where glitch resistors are good enough, and the QRO region 
where you should begin to consider a crowbar. However, practical 
experience proves that glitch resistors can do  good job at the 1500W 
level and beyond.

It all depends how far you want to go, how much energy you have stored 
in the filter caps, and how much investment in tubes you're trying to 
protect.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek