W0YR@aol.com wrote:
>
> 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
How much higher are we taling about - 1%, 10% 100% or whatever ?
One possibility (I admit remote) is that the current is not actually
increasing, but instead the meters shunts have got hot and therefore
drift. I would not have thought there was any reason for an amp builder
to run the shunts at anywhere near the point at which this would happen,
but it is possible.
Perhaps thermal movement in the tube causes one electrode to get nearer
or further away from another and the resulting field changes causes
this. Possible I'm sure, but that's all I could guess at.
--
Dr. David Kirkby Ph.D,
email: drkirkby@ntlworld.com
former email address: davek@medphys.ucl.ac.uk
web page: http://www.david-kirkby.co.uk
Amateur radio callsign: G8WRB
--
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