Vic,
The negative screen current is normal in a 4CX1000. See the Eimac data sheet.
In 'ENABLED', you need more negative bias to cut off the tube, and it's a good
idea to get into cut off (but only just) to minimise wideband noise generation.
The data sheet says typically -115 volts for cut off.
vy 73
Peter G3RZP
========================================
Message Received: Oct 29 2013, 05:22 PM
From: "Vic K2VCO" <k2vco.vic@gmail.com>
To: "Amps reflector" <amps@contesting.com>
Cc:
Subject: [Amps] Small mystery
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/
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