[Amps] Small mystery

[Steven Katz]

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 at 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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