[Amps] 4CX350A Cathode Driven HF amp strange symptom

[Ken Harada]

Hi,

It was indeed an oscillation.  I have then given up Ground Grid / 
Cathode driven circuit, and modified it to GK/Grid Driven with input 
load resistor.  It works perfectly fine & stable, with driving power as 
low as 5W.  FL2100Z chassis and transformer provides good platform for 
4CX350A.


Regards,

Ken Harada - JO1VRK

On 2023/05/24 21:51, Victor Rosenthal 4X6GP wrote:
> Yes, it could be at VHF or even LF.
>
> 73,
> Victor, 4X6GP
> Rehovot, Israel
> Formerly K2VCO
> CWops no. 5
> http://www.qsl.net/k2vco/
>
> On 24/05/2023 15:42, Ken Harada wrote:
>> Then my wiring is not good?  But I don't hear noise on a receiver 
>> beside the amp - though I should watch different frequency to detect...
>>
>>
>> On 2023/05/24 21:39, Victor Rosenthal 4X6GP wrote:
>>> 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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