You may need higher voltage AC to supply the -140 VDC grid bias
suggested. Can you replace this transformer or increase it's output AC
voltage?
73
Jim W7RY
On 10/29/2013 12:02 PM, Vic K2VCO wrote:
The voltage can be varied by changing a resistor, up to at least -120v
if I wanted to. There should be at least 8 ms between the signal to go
into TRANSMIT and when the RF drive appears, and the input relay
(which switched the bias) operates in less than 3 ms. So there
shouldn't be a problem.
On 10/29/2013 10:47 AM, Paul Christensen wrote:
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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