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Re: [Amps] step start

Subject: Re: [Amps] step start
From: Victor Rosenthal 4X6GP <>
Date: Sun, 1 Oct 2023 23:15:52 +0300
List-post: <>
I mentioned the need for a 3 minute delay before turning on plate and screen voltages, but Jim is right -- you CAN turn on plate voltage immediately. Just keep the bias on so the tube is cut off for at least 3 minutes.

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

On 01/10/2023 22:43, MU 4CX250B wrote:
Nice project, Paul. I built my first tetrode amp a couple of years
ago, and I was dubious I could carry it off, but it worked well and
was fun. I have website at with photos and schematics.

  Re your questions, a 300V drop in plate voltage won’t be a problem.
That’s a pretty typical  voltage drop and isn’t anything to worry
about. Re your soft start question:  for a tetrode amp, you should
always have the plate voltage turned on BEFORE  turning on the screen
voltage. In other words the screen voltage should never be on by
itself. Doing so will quickly destroy your 4cx1500b. To be on the safe
side, I would turn on the plate voltage and the cut-off bias voltage
when the amplifier is powered up and just leave them on. Once the
plate voltage is on, you can then safely turn on the screen voltage.
On my homebrew tetrode amp, I made a little interlock circuit that
monitored the plate voltage and shut off the screen voltage if the
plate voltage dropped out for any reason.
Jim w8zr
Sent from my iPad

On Oct 1, 2023, at 12:19 PM, 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 

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.


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