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Re: [Amps] step start

To: Paul Decker <>
Subject: Re: [Amps] step start
From: MU 4CX250B <>
Date: Sun, 1 Oct 2023 12:43:13 -0700
List-post: <>
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 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
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