Thanks to everyone for their help. The consensus was that the cutoff bias needs to be
higher, and this is indeed the case. -115V completely cuts off the tube.
On 10/29/2013 10:24 AM, Steven Katz wrote:
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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--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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