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Re: [Amps] HV Fuse

To: "'amps'" <>
Subject: Re: [Amps] HV Fuse
From: <>
Date: Mon, 23 Sep 2019 09:43:14 +0200
List-post: <>
HV fuses are still produced:


-----Original Message-----
From: Amps [] On Behalf Of Carl
Sent: Freitag, 20. September 2019 23:50
To: Paul Christensen; 'amps'; Mike McCarthy, W1NR
Subject: Re: [Amps] HV Fuse

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.
> W1NR
> 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
>> 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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