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[Amps] 4CX350A Cathode Driven HF amp strange symptom

Subject: [Amps] 4CX350A Cathode Driven HF amp strange symptom
From: Ken Harada <>
Date: Wed, 24 May 2023 21:20:11 +0900
List-post: <>
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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