[Amps] HV Fuse

[MU 4CX250B]

I don’t have a problem with HV fuses in parallel, Paul. I’ve used two
and sometimes three of the 900 MA microwave oven fuses in parallel. If
there is a circuit overload, all three of the fuses will blow. The one
that blows first will then divert current to the other two which will
quickly follow.  I have not measured the time delay, but it’s probably
milliseconds.
73,
Jim w8zr

Sent from my iPhone

> On Sep 20, 2019, at 1:38 PM, Paul Christensen <w9ac at arrl.net> wrote:
>
> I don’t think fuses in parallel are a good idea :-). But probably you weren’t serious
>
> Vic, I am serious!  Actually, I would use them in parallel to determine the overcurrent point then try and fine a single fuse of adequate size.  That said, I have seen as many as three used in parallel in another amp design.
>
> Paul, W9AC
>
>
> . 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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