Here's a user report on Chinese-made HV fuses, widely available on eBay
(search on "900 mA HV fuses" ) These fuses, intended for microwave ovens,
come in several current ratings (900 mA is the largest), and are 6.5mm
diameter, by 40mm long.. They are rated for 5000V. Physically, the fuses
resemble an ordinary 3AG fuse, but are about 50% longer. They fit fine in
PCB mounted 3AG fuse clips, provided the clip spacing is lengthened. Like a
3AG fuse, hey have metal caps on each end of a clear glass tube. An internal
spring keeps tension on the fusable link. The fuses are very cheap, costing
less than a dollar each.
I used one of the 900 mA fuses while checking out a homebrew amp running
three GU74Bs. Although the NOS tubes had been well conditioned (by running
them with filaments for 50 hours) one of the tubes evidently arced
internally, blowing the fuse and tripping the power supply magnetic circuit
breaker. with a boom. When this happened, the plate current was about 1.5 A,
roughly 65% over the fuse rating. I believe the fuse opened because of a
flashover in the tube and not because its rating had been exceeed. If it
had opened because of high steady-state current, there's no reason the
circuit breaker would have also tripped.
The boom came from the fuse, which exploded its glass tube into tiny
pieces.when it blew. (The sound was not quite as loud as the sound made by
the 0.82 ohm fuse resistor in the Drake L4B power supply) More importantly,
the fuse did satisfactorily open the 2900V B+ line to the GU74Bs, which was
my major concern.. I plan on some running some timing tests on these fuses
with a bench low voltage supply to get a feel for their operational current
ratings. There is no data supplied with the fuses, but it appears their
rating is the maximum current the fuse will sustain indefinitely without
opening. Based on my limited experience, I'd think a single 900 mA fuse
would be about right for an 8877 amplifier (1 Amp max cathode current), or a
pair of 3CX800s (1.2Amp max). Smaller rated fuses would be best for, e.g.,
a pair of 3-500Zs. I'll probably use two of the 900 mA fuses for my three
GU74Bs. For that amp, the plate current trip point is set at 2.1A
One caveat, regarding tetrode amplifiers. If the HV line is fused, then
it's essential that the amp circuitry kill the screen voltage as soon as the
fuse blows. In my homebrew amp, I insert the fuses immediately before the
plate choke. A voltage divider samples the HV on the downhill side of the
fuse, and if the fuse opens, the regulator immediately shuts down the screen
voltage. And finally, be sure and wear safety glasses if you are working on
an RF deck and the fuse is visable and uncovered. If it blows, tiny pieces
of glass fly everywhere. It's probably a good idea to slip a loose-fitting
piece of fiberglass spaghetti over the fuse to catch the pieces.
73,
Jim W8ZR
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