[Amps] Small mystery

[peter chadwick]

Vic,

The negative screen current is normal in a 4CX1000. See the Eimac data sheet.

In 'ENABLED', you need more negative bias to cut off the tube, and it's a good idea to get into cut off (but only just) to minimise wideband noise generation. The data sheet says typically -115 volts for cut off.

vy 73


Peter G3RZP








========================================
 Message Received: Oct 29 2013, 05:22 PM
 From: "Vic K2VCO" <k2vco.vic at gmail.com>
 To: "Amps reflector" <amps at contesting.com>
 Cc: 
 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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