[Amps] high voltage fuses

[Fuqua, Bill L]

  Every one should read the actual specifications of the fuses they use. 
The rating does not mean the same on the non-slow blow fuses. It is not like
there is a definite threshold where the fuse conducts and when it blows. 
 And when exceeding the rated current it may many 10s of seconds to blow. 
73
Bill wa4lav

________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Carl [km1h at jeremy.mv.com]
Sent: Monday, October 07, 2013 12:23 PM
To: Jim Thomson; amps at contesting.com
Subject: Re: [Amps] high voltage fuses

Instantaneous peak current can be a lot more than 5-10% more than key down
and long term stressing of an underated fuse will eventually fail.

A tube arc/short instantaneous current will take out a 2-2.5X rated fuse as
fast as a smaller one.

All the commercial TX Ive ever seen have the fuse in the same area as the
tube and that includesmilitary ones Ive worked on.

Dentron used a pair of 1 Ohm 1/2W carbon composition resistors in parallel
at the base of the RFC as the fuse, seems to do the job for some failures
but not all.

I suppose the old style 220V cartridge fuse could be rebuilt for HV but I
would never suggest any bare wire with a simple covering at much over 2000V.
I suggest those who believe it is OK should see the results of a plasma arc,
nothing stops it until power is cut.

Carl



----- Original Message -----
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Monday, October 07, 2013 11:49 AM
Subject: [Amps] high voltage fuses


> Date: Sun, 6 Oct 2013 10:53:36 -0400
> From: "Carl" <km1h at jeremy.mv.com>
> To: "Pete Smith N4ZR" <n4zr at contesting.com>, <amps at contesting.com>
> Subject: Re: [Amps] high voltage fuses
>
>
> ###  say what ?   If you want any HV fuse to blow asap,  fuse it for no
> more than 5-10%
> higher than the amps steady state, key down current.  Just make sure to
> put the HV fuse BEFORE the
> glitch resistor.   The glitch resistor can go in the RF deck, or in the HV
> supply.  IMO, for max safety,
> put it in the HV supply, or as close as you can to the B+  of the filter
> cap.
>
> ##  A small ga wire, suspended between stand offs  works just fine for a
> HV fuse, with the provisio
> that you 1st slide some heat shrink over the wire, use anything as long as
> it barely slides over the
> fusing wire.  You don’t apply heat and shrink it to the wire either, just
> slide it over.  Or  use some
> 88/33 tape, etc,  and wrap the wire. Then when it blows open, you don’t
> get any vapor deposits.
> Those glass BUSS  HV fuses can be refurbished too,  Take the end off, and
> pour out the sand.
> Remove other end, clean it up and solder in a new fuse wire, and pour the
> sand back in.
> Re-assemble the mess.
>
> ##  The 5 kv, .75 – 1.0 amp  microwave oven  HV fuses work just fine.
> I also install  a 2nd HV fuse, between one leg of the plate xfmr sec and
> the
> FWD... or  FWB..   Either leg, only one fuse required.
>
> ##  Drake used a .82 ohm @ 1/2 watt CC  resistor for a HV fuse in the B+
> line.
> That works too, and blow cleanly in half.
>
> later... Jim   VE7RF
>
>
>
>
>
>
>
> Thats a bit too close for even a 1200W amp. Might be fine for something in
> the SB-200/AL-80 size amp.
>
> Size the fuse at 2 to 2.5 times the actual peak current, not steady state
> key down.
>
> Carl
> KM1H
>
>
>
> ----- Original Message -----
> From: "Pete Smith N4ZR" <n4zr at contesting.com>
> To: <amps at contesting.com>
> Sent: Sunday, October 06, 2013 10:30 AM
> Subject: [Amps] high voltage fuses
>
>
>> For what it's worth, I see that there are a lot of inexpensive 5KV
>> 0.75-1.0 amp glass fuses listed on ebay for microwave oven use.  I assume
>> these would also work in a legal ham amp?
>>
>> --
>> 73, Pete N4ZR
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