[Amps] 4CX350A Cathode Driven HF amp strange symptom

[Ken Harada]

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, sm0aom at telia.com wrote:
> The 4CX350A is susceptible to grid emission, which can give very similar symptoms.
> 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.
>
> 73/Karl-Arne
> SM0AOM
>
>   
>
>
> ----Ursprungligt meddelande----
> Från : jo1vrk at hayama.net
> Datum : 2023-05-24 - 14:20 ()
> Till : amps at contesting.com
> 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.
>
> Regards,
>
> Ken /JO1VRK
>
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