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@telus.net>
To: <amps@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@jeremy.mv.com>
To: "Pete Smith N4ZR" <n4zr@contesting.com>, <amps@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@contesting.com>
To: <amps@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1432 / Virus Database: 3222/6229 - Release Date: 10/07/13
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|