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Re: [Amps] 4CX350A Cathode Driven HF amp strange symptom

Subject: Re: [Amps] 4CX350A Cathode Driven HF amp strange symptom
From: Ken Harada <>
Date: Thu, 25 May 2023 19:51:21 +0900
List-post: <>
I have placed a bleeder resistor to screen grid circuit.  Should I do the same for control grid as well?

Ken / JO1VRK

On 2023/05/25 2:59, wrote:
The 4CX350A is susceptible to grid emission, which can give very similar 
This is due to the very small spacing between control grid and cathode,

It the tubes are used, this becomes quite common.
One possible remedy is to use low-impedance  supplies for both screen and 
control grids.


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Datum : 2023-05-24 - 14:20 ()
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Ämne : [Amps] 4CX350A Cathode Driven HF amp strange symptom

Hi Amp enthusiasts,

I am building a HF linear amp with 2 x 4CX350A with cathode driven
circuit,  but getting a strange symptom.  Ep=2000V, Esg=300V (regulated
with Zener and FET), Ecg=-50V to -20V (by LM337SP).  Before applying any
RF, I try to adjust no signal anode current by varying the Ecg.  Going
up from -40V, it gradually increases from 0 to approx 50mA, and in a
sudden it jumps up to 300mA (around at -30V).  Moving back to deeper
Ecg, it remains 300mA and suddenly drop to 0 - showing hysteresis.

I wonder what is happening to the tubes/circuit and where I should
check.  Any suggestion would be greatly appreciated.



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