> 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.
Hi Mike,
Look at the individual plate currents....is each rising the same?
Look at the RF at the input to the amp....is it steady all the time?
Will look in the manual today and see what I can figure out.
(((73)))
Phil, K5PC
> 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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