> 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
>
>
To: <amps@contesting.com>
>Date: Fri, 16 Feb 2001 12:17:03 -0500
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>
>
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>From: W0YR@aol.com
To: <amps@contesting.com>
>Date: Fri, 16 Feb 2001 08:14:27 EST
>Subject: Creepy Amplifier
>
>
>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.
>
AØ is not legal on HF.
cheers
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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