[Amps] Small mystery

[Vic K2VCO]

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 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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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/