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Re: [Amps] 4CX350A Cathode Driven HF amp strange symptom

To: amps@contesting.com
Subject: Re: [Amps] 4CX350A Cathode Driven HF amp strange symptom
From: Ken Harada <jo1vrk@hayama.net>
Date: Tue, 10 Oct 2023 18:06:18 +0900
List-post: <mailto:amps@contesting.com>
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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