[Amps] L-7 Failure, HV fuses

[Ian White, G3SEK]

R. Measures wrote:
>>Re HV fuses:
>>
>>I have seen in the literature descriptions of a HV fuse made by inserting
>>into the B+ line,   a 30 guage Cu wire approx 2" long.  suspended between
>>two ceramic standoffs in the amp.  Would that function as a near
>>instantaneous HV fuse. protecting the tube in the event of a heavy current
>>HV fault?
>
>**   2" of #30 Cu has about 0.017-ohms of R.   Since the ESR of 
>high-ripple I 450V electrolytics is c. 0.25-ohms each, in a typical 
>3200VDC anode supply, the peak discharge I would be 3200V / [(8 x 
>0.25-ohms) + 0.017-ohms] = c. 1600A-peak.  IMO, this does not provide 
>adequate protection.  As I see it, HV fusing is not needed if the 
>primary of the HV transformer is fused or circuit-breakered.  What is 
>needed is a device to limit peak discharge-I -- i.e., a sturdy-enough 
>glitch-R; typical values of which are 10 to 30 ohms.
>

Totally agreed - the difference between a resistor and a fuse is that 
the resistor provides instantaneous current limiting. A fuse does not.

That's why the glitch resistor is the key component. You need a fuse as 
well, but that can be a mains fuse in the transformer primary.

A "sturdy enough" has to be capable of dissipating tens of watts in 
normal operation, and also capable of withstanding the full HV from end 
to end if a major current surge occurs. In practice this means quite a 
large, long-bodied resistor.

Unfortunately a resistor of this size may be difficult to fit into an 
amp that doesn't already have one - but do it, any way you can.


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