[Amps] Small mystery

[Paul Christensen]

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 at jmr.com>
To: "Vic K2VCO" <k2vco.vic at gmail.com>; "Amps reflector" 
<amps at 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 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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