I'm starting to test my 4CX1000A amplifier and I have noticed something that I don't
understand.
The amplifier has three operating states, which I call READY, ENABLED and
TRANSMIT.
In READY,
HV = 3400V
G1 = -89V
G2 = 0 (grounded)
Antenna relays disabled
In ENABLED,
HV = 3400V
G1 = -89V
G2 = 325V
Antenna relays enabled but de-energized
In TRANSMIT,
HV = 3400V
G1 = -60V
G2 = 325V
Antenna relays energized
The idea is that the QSK or VOX line switches between ENABLED and TRANSMIT. By switching
to READY during long periods of receiving, there is less chance of an 'event', or of a cat
stepping on the key and transmitting at full power, etc. Since bias switching is all
electronic, it is faster than screen switching, which uses a relay.
Now here is the mystery. In READY there is no plate or screen current. In ENABLED there is
a negative screen current of a few mA and also a plate current of a few mA.
Why is this current flowing? I understood that secondary emission is caused by electrons
hitting the plate and 'bouncing' or causing it to emit electrons itself. But the plate
current is very small in this condition.
I noticed the same phenomenon with much lower plate voltage, 2400V.
Could the tube be gassy? The tube is a pull, but I don't want to put my new one in there
until all initial testing (including with RF) is finished! I ran the heater all night to
getter it, if that matters.
--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|