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Re: [Amps] step start

To: MU 4CX250B <>
Subject: Re: [Amps] step start
From: Paul Decker <>
Date: Mon, 2 Oct 2023 08:14:05 -0400 (EDT)
List-post: <>
I was thinking the step start could be put onto both the screen and the plate 
voltage transformer. Since the screen dumps into a 1 Henry, it does take some 
time for that to charge so they both should ramp up.  

It's easy enough to have the soft start only on plate and when the soft start 
kicks out, turn on the screen voltage.


> On 10/01/2023 3:43 PM EDT 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.
> 73,
> Jim w8zr
> Sent from my iPad

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