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Re: [Amps] HV Fuse

To: "Paul Christensen" <>, "'amps'" <>, "Mike McCarthy, W1NR" <>
Subject: Re: [Amps] HV Fuse
From: "Carl" <>
Date: Fri, 20 Sep 2019 17:49:53 -0400
List-post: <>
Time to make your own from an open one or start from scratch..


----- Original Message ----- From: "Mike McCarthy, W1NR" <>
To: "Paul Christensen" <>; "'amps'" <>
Sent: Friday, September 20, 2019 3:38 PM
Subject: Re: [Amps] HV Fuse

I have an old Henry that has a Bussman HVB fuse. The larger HVJ is rated
for 5KV. They are sand filled cardboard, not ceramic. Available from
Mouser but are quite expensive. I'd look around for better pricing. I've
never seen "small" high voltage fuses that were not cardboard.


On 9/20/2019 3:04 PM, Paul Christensen 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

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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