[Amps] What current rating and type for a fuse in HV circuit of SB221?

[Jeff King]

Recently I fitted Harbach modifications to an SB221.

(I also fitted R Measures glitch resister).

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With the Harbach kit came the following suggestion to fit a fuse to the =
HV
circuit.

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A HELPFUL HINT

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To help protect your SB-2201221 from high voltage current transients, =
try
making and installing a simple HV fuse. It will protect against blown
resistors on the rectifier/metering board, destroyed plate current and =
grid
current meters and other BAD things.

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Take a small piece of circuit board material about 1.5" x W' and place a
solder pad on each end, leaving about a 1" gap between the pads. Find =
the
HEAVY BLUE HV wire coming from hole "J" of the left side of the
rectifier/metering circuit board that goes to the HV feed through =
insulator
"CK" to the left of the circuit board. Cut this wire in the middle and
solder the ends to the pads on your HV fuse. Take a piece of small wire,
about #32 to #36, and solder it between the pads on the fuse. If you do =
not
have the small gauge wire, you can get it from any 11OVAC extension cord =
of
the cheap variety - ONE strand is enough. Make sure the fuse is clear of =
any
other components or wires. This can also be adapted for the S13-2001201.

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NOTE: A good source of #36 wire is ONE strand from a discarded computer
ribbon cable.

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Additional protection from this fuse seemed a good idea.

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Instead of fitting an exposed wire, I tried an alternative of=20

a fuse contained inside a case: being 1.6 amp / 250 Volt 20mm long fast
blow.=20

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I chose 1.6 amps because #32 to #36 wire has very little current =
capacity of
0.1 to 0.2 amps.

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When power was turned on through the amp, without any aerial

or exciter power connected the 1.6 amp 20 mm HV fuse blew.

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This has left me wondering what size this fuse should be?=20

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The SB221 manual recommends a maximum SSB a max plate current of 0.33 =
amps.

This will be at say 2,800 volts: I.e. 924 watts.  That suggests the same
piece of fuse wire at 250 volts would carry at least 3.69 amps.

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My questions are:

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1. Could anyone please advise what current size the 250 volt fuse should =
be?

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2. Is there any problem using a 20 mm long fuse holder? [When the fuse =
has
blown there remains an internal gap between metal contacts of about 1/4 =
of
an inch.]=20

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3. Where can suitable high voltage fuses be sourced from ?

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Looking forward to your advice:

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Yours sincerely

Jeff King ZL4AI

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PS: I emailed Rich Measures who provided the following useful advice. =
Rich
also suggested the matter could be discussed on amps.=20

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> There is no such thing as a 20mm fuse that will open a 3000VDC circuit

> during a glitch.  32V fuses are typically about 20mm long.  3KV fuses

> are typically about 100mm long and they are filled with high-purity

> silica sand to quench the arc as they are opening.  --  note -- the

> metal vapor arc that  forms as the fusing metal melts has only 20 - 30

> V drop.

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> A HV fuse is not needed in a SB-221 because the primary of the HV xfmr

> is protected with 10A circuit breakers.

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>   In free air, #32 Cu will easily carry 2A.  However, in free air, no

> metal makes a satisfactory HV fuse.  The bottom-line is virtually no

> current limiting.  The advantage of a glitch-R is that it limits =
peak-I

> as the circuit breakers are in the process of opening.

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> While speaking into the microphone with an ahhhhhhhhhh, 330mA is =
plenty

> since the peak-I is about triple the indicated-I.  .

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> 250V-max fuses should not be used in applications with more V unless

> you are trying to start a fire.=20

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> My advice is use no HV fuse.

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