[Amps] HV Fuse

[Paul Christensen]

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