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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