Recently, I was presented with the opportunity to repair a "garden-variety"
8877 mono band amplifier; there was nothing remarkable about this amplifier, it
has 3200 volts on the plate, a 25 ohm @ 20 watt glitch resistor, a 1N3311B
Zener Diode (12 volts @ 50 watt) in the cathode return and, B- was above ground
by some amount of ohms. I am familiar with this particular amplifier as, I
repaired it last 3 years ago.
When it came in, the first sign of trouble was the vaporized/smoked resistor
(what ever value it was) between B- and chassis ground which, pointed to
excessive grid current in the 8877 tube circuit. I did a visual inspection and,
checked the bias diode which, showed good.
In an effort to get this thing under control, I installed fuse blocks in the
cathode return circuit and also in the grid return circuit so, I have a 1.0 amp
slo-blo fuse in the cathode return (shunted by 30 k resistor) and, a 250
millamp fuse between the 1 ohm resistor connecting B- to chassis ground.
With me so far? It's test time, I pull B+ off of the tube and hit the foot
switch and B+ comes up like normal (B+ is off until footswitch closes contactor
controlling primary winding) with, no fuses blowing.
Test #2, hook up the tube and with ammeter in line, key the amplifier to check
idle current. Grid fuse blows, Oooooh-Kay!
Test #3 with Grid circuit open, I check current in cathode circuit and, I am
running over 1 amp current and there is no fuse in the circuit between B- and
chassis ground.
So, I go back to your "garden variety mono band operator" and tells him that he
need a tube and, he give me one that he says was "weak" but "still good". Just
for "drill", I replaced the zener diode with a brand new one, just in case(?).
Sanity check time, I am getting the same results with a different tube so
either the second tube is also bad or, I am missing something.
Mel.
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