[Amps] HV Fuse

[Vic Rosenthal]

Hi Paul,

I don’t think fuses in parallel are a good idea :-). But probably you weren’t serious. I don’t know what the present rectifier modules are,  but rather than trying to protect them with a fuse, I would just make up a few strings of 6A10s, which are basically indestructible. Then you size the glitch resistor so it won’t blow up before the primary fuse blows in the event of a flashover. A nice 50 watt vitreous enameled one of 30-50 ohms should work.
By the way, I don’t know what the tubes are in that amp, but I presume one or more triodes. A fuse in the plate circuit of a tetrode will cause the screen to ignite like someone hit it with the heat ray in War of the Worlds, unless there is a screen trip circuit.

Victor 4X6GP 

> On 20 Sep 2019, at 22:04, Paul Christensen <w9ac at arrl.net> wrote:
> 
> Yesterday, I lost all four HV bridge rectifier modules in my Alpha PA-70V
> vapor-cooled amp.   After replacement, HV returned but whatever caused the
> fault event also took out a 2N3053 switching transistor that controls a 8.2V
> Zener bias diode from the EBS circuit.  I used the opportunity to change it
> to a more robust transistor in a TO-220 package that effectively doubles Ic
> and Vceo.  The transistor is already configured as a "discrete Darlington"
> with a 2N4401 ahead of it so the shock didn't get very far on the EBS PC
> board.    
> 
> Going back through the list archives, I see mention of using a HV fuse just
> ahead of an amp's glitch resistor.  VE7RF recommended a second HV fuse
> placed between one leg of the HV transformer secondary at the C-input filter
> cap.  Sounds like a nice way to potentially save the HV bridge diodes.  
> 
> I have sized the glitch fuse adequately and will use a HV microwave oven
> type as discussed in the archives.  However, because of some unknown
> variables, it's a bit tricky to calculate the HV transformer's secondary
> fuse.  I can use PSUD II software but primary and secondary HV transformer
> ESR are unknown variables.  
> 
> I'm also a bit skeptical that a fuse on the HV secondary will work as
> intended due to the extreme initial C charging current (35 uF/5KV filter
> cap), even though step-start will limit the initial inrush and that too
> factors into the equation.  I could just keep stacking 900 mA HV fuses in
> parallel until they don't blow but that isn't an intelligent approach to a
> solution.  Any guidance here?
> 
> I don't see sand-filled ceramic HV fuses in this category and I dread the
> thought of cleaning up glass shrapnel in an amplifier.  I'm thinking that
> Teflon heat-shrink tubing placed only over the glass portion should keep the
> shattering damage under control.
> 
> Paul, W9AC 
> 
> 
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