Vic, what frequency? Does the plate current drift?
Screen current is extremely sensitive to the degree of loading adjustments.
Could it be that there is a capacitor on the output side of the plate
tuning circuit that is heating due to rf and changing value?
Does the amplifier act the way you described on all bands?
73,
Gerald K5GW
In a message dated 11/7/2013 5:16:12 P.M. Central Standard Time,
k2vco.vic@gmail.com writes:
I've finally applied full drive to my 4CX1000A amplifier. Here is what I'm
seeing:
I have tried two tubes, both pulls. In one case I can run about 1kW out,
and
with the other, maybe 1200. What happens when I try to push past this point
is this: when I hold the key down for 10 seconds or so, the screen current
starts to drop, then goes negative, and when it hits -35 mA the screen trip
circuit kicks in. I haven't seen even a flicker of grid current.
The screen circuit is well-regulated at 325v, so this isn't case of
runaway
caused by increasing voltage. What it seems like to me is that the screen
is
heating up and the secondary emission is rapidly increasing. Screen current
before this happens is reasonable, 20-25 mA, so I am not exceeding its
rating.
I have a couple more tubes to try, but I have a bad feeling that I am going
to end up buying a new tube if I want 1500 watts from this.
Do the experts think my diagnosis is correct?
--
Vic, K2VCO
Fresno CA
[1]http://www.qsl.net/k2vco/
References
1. http://www.qsl.net/k2vco/
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