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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