Vic,
Within some limits, can you vary Eg1? I think you are using Ian's board
set, but I don't know to what degree Eg1 and Eg2 are adjustable.
My first thought is that you've hit a near ideal target on Eg1 since there's
only a small amount of Ip. I may not want complete cut-off (or below) bias
in ENABLE. The more negative Eg1 in ENABLE, the more non-linear artifacts
you may notice in fast VOX (or leading edge CW waveform distortion in QSK)
on the initial RF envelope when finally going from the ENABLE to TRANSMIT
bias stages.
Paul, W9AC
----- Original Message -----
From: "Steven Katz" <stevek@jmr.com>
To: "Vic K2VCO" <k2vco.vic@gmail.com>; "Amps reflector"
<amps@contesting.com>
Sent: Tuesday, October 29, 2013 1:24 PM
Subject: Re: [Amps] Small mystery
I think you need more negative bias (cutoff) on G1 to pull it down into
complete non-conduction. I always used about -140Vdc.
WB2WIK
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Vic K2VCO
Sent: Tuesday, October 29, 2013 10:22 AM
To: Amps reflector
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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