[Amps] 4CX350A Cathode Driven HF amp strange symptom

[Ken Harada]

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