[Amps] 4CX350A Cathode Driven HF amp strange symptom

[Victor Rosenthal 4X6GP]

Perhaps it's breaking into oscillation?

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/

On 24/05/2023 15:20, Ken Harada wrote:
> 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.
> 
> Regards,
> 
> Ken /JO1VRK
> 
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