Re: [Amps] L-7 Failure, HV fuses

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

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
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