AMP: Harris RF-110A (Pair of 4CX-1500Bs)
PROBLEM: Plate current and power output creep up after about 15 seconds under
"keydown" condition.
The amplifier will "run away" with itself until the overloads kick in.
Scenario: Amp is excited to about a kw output with carrier. Key down for 30
seconds. Power slowly increases to 1400 watts and would keep on increasing
until the overloads kick it off. When I let up on the key, the idling
current is high and then it slowly returns to its normal state.
Have measured the grid bias, no change under "heat up conditions." Same for
the zener regulated screen voltage supply. (I had thought the zeners were
heating up and causing the screen voltage to rise, thus causing the increase
in amplification. Apparently not so, no measurable increase in screen
voltage.)
I do not have extensive experience with ceramic tubes. Is this runaway
output because of "soft" tubes? They REALLY put out. Or maybe it is thermal
runaway, though the powerful air system on this military amp is working ok
(and certainly makes enough noise).
Under SSB conditions everything appears normal -- obviously due to the
markedly lower duty cycle.
Anybody got any ideas?
Mike
W0YR
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