Recently I fitted Harbach modifications to an SB221.
(I also fitted R Measures glitch resister).
With the Harbach kit came the following suggestion to fit a fuse to the HV
circuit.
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A HELPFUL HINT
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.
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.
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.
Instead of fitting an exposed wire, I tried an alternative of
a fuse contained inside a case: being 1.6 amp / 250 Volt 20mm long fast
blow.
I chose 1.6 amps because #32 to #36 wire has very little current capacity of
0.1 to 0.2 amps.
When power was turned on through the amp, without any aerial
or exciter power connected the 1.6 amp 20 mm HV fuse blew.
This has left me wondering what size this fuse should be?
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.
My questions are:
1. Could anyone please advise what current size the 250 volt fuse should be?
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.]
3. Where can suitable high voltage fuses be sourced from ?
Looking forward to your advice:
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.
> 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.
> A HV fuse is not needed in a SB-221 because the primary of the HV xfmr
> is protected with 10A circuit breakers.
> 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.
> While speaking into the microphone with an ahhhhhhhhhh, 330mA is plenty
> since the peak-I is about triple the indicated-I. .
> 250V-max fuses should not be used in applications with more V unless
> you are trying to start a fire.
> My advice is use no HV fuse.
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