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
It could also be a damaged screen from overcurrent in the past.
Both are rather common tetrode problems where screen current isnt constantly
monitored as a seemingly simple QSY can drastically affect it.
Also running the screen voltage at or close to maximum rated is not a good
idea and I would also give up the idea of 1500W from one
tube...reliably...even Alpha couldnt do it (-;
Carl
KM1H
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