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Re: [Amps] step start

Subject: Re: [Amps] step start
From: Victor Rosenthal 4X6GP <>
Date: Sun, 1 Oct 2023 23:06:19 +0300
List-post: <>
Plate voltage regulation on a tetrode is not very important, because the screen voltage has a much greater influence on the plate current. 93 uf is quite a lot for the plate supply, imho more than you need.The screen regulation is critical with this tube, because the screen current has to be kept within narrow limits to protect the screen (it will go negative easily). It is also important for reasonable IMD characteristics. You should consider using some kind of active regulator circuit in the screen circuit. Soft start is useful to protect diode and relays in the power supply, or tube filaments or heaters. It is not relevant to plate or screen voltage. One important point: NEVER turn on screen voltage before plate voltage! Make sure this can't happen by accident. The screen will go up like a fireworks display if it does. Either turn them on together, or turn on the plate voltage first, followed by the screen. And make sure there is a 3-minute delay from when the heater is powered on until plate and screen voltages are applied.

Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5

On 01/10/2023 21:19, Paul Decker wrote:
Hello all.

I'm building a 4cx1500b amplifier. This is the first tetrode high
power amplifier I've tried to do.  I'm using about 300V on the screen
and 2500 on the plate.

For my power supply I'm using a 1 Henry choke input on the screen
supply dumping into 200uF This seems pretty stiff.  For the plate
supply it's a voltage doubler outputting 2700 V dumping into 93uF.
Not quite as stiff, about 300V drop under full load. I have plenty of
capacitors, so I can add more capacitance if needed.

Two questions, 1. is the plate voltage regulated enough or do I need
more plate regulation and 2. for soft start,  Should the soft start
bring both the screen voltage and plate voltage up or should I let
the screen voltage be and just soft start the plate voltage.

Thanks for any insights you all might have.

Paul KG7HF

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