Hello Paul and Abbi, I built an 8877 amp and put in a high voltage fuse and a 50
ohm resistor on the B+ line. The fuse popped a couple of times when the tube was
first used but settled down after that. Rich will tell you its parasitics but
the
fact is that's just normal. The best protection you can put in the amp is a grid
trip circuit in case of bad tune conditions or you transmit on the wrong
antenna.
I set mine to trip at 100ma grid current. I also have a swr shut-down cct. in
line to be double sure. Be sure to load the amp heavy and it will play nice once
you get the bugs worked out.
gl es 73 Steve VE3TU
Paul and Abbi Elliott wrote:
> A friend and I are putting together an 8877 amplifier for 160 through 20 m.
> Power supply puts out 4200 volts no load. Plate transformer is very heavy
> (guessing about 100 pounds)--oil-filled filter capacitor is 25 mf. A 25
> ohm, 50 watt glitch resistor is installed in the amplifier.,
>
> We have been experimenting (on the bench, amplifier not connected) with a
> triggered spark gap to blow (and I do mean blow!) a fuse of #39 wire, about
> 3" long, when a fault is detected and thereby disconnect the power supply
> B+ from the amplifier. The voltage drop across a 2 ohm resistor in the
> negative power lead is used to control an SCR to fire, when the current
> drawn from the power supply rises to 1.5 amps, a regular automotive
> ignition coil connected to a small spark gap (gap #1). In very close
> proximity to gap #1 is gap #2. The fuse wire is connected in series in the
> B+ lead between the power supply and the amplifier One side of gap #2 is
> connected to the B+ lead on the amplifier side of the fuse wire. The other
> side of gap #2 is connected to ground, either directly or through a resistor.
>
> Under ordinary operation conditions gap #2 is an open circuit. When gap #1
> fires, the ionized air caused by gap #1 firing causes gap #2 to breakdown,
> shorting the B+ to ground through the fuse wire--fuse wire disappears, with
> a very loud bang, and high voltage is disconnected from the amplifier. Can
> detect no difference between the loudness of the bang with a 200 ohm
> resistor in the ground lead to gap #2 or with or just a ground lead with no
> resistance. Using a 350 ohm resistor in the ground lead of gap #2 caused
> the fuse wire to glow bright red (did not melt) before the 15 amp circuit
> breaker in the transformer primary lead (was using 115 volts for the
> testing) to open. We have fired this thing about a dozen times. Gap #2 is
> made of copper about 20 mils thick and shows no pitting or other damage.
>
> This device fires reliably--it does disconnect the B+ from the amplifier
> side of the line. But does it disconnect fast enough the protect the tube
> in case of a tube arc, seeing that, at this time, we are not attempting to
> shut down the power supply?
>
> Any help and educational comments and suggestions would be greatly
> appreciated. (Why are we doing it this way? Because my friend thought it
> might work and be cheaper than thyristors--but if it won't protect the
> 8877, then we are looking at a very false economy)
>
> 73
>
> Paul W5DM
>
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