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Re: [Amps] step start

To: MU 4CX250B <>
Subject: Re: [Amps] step start
From: Gary Schafer <>
Date: Wed, 4 Oct 2023 12:45:23 -0500
List-post: <>
Jim w8zr,
I have been trying to get in touch with you regarding the stepper motor drive 
boards that you were building for amp tuning. Did you ever finish them?
Gary K4FMX

Gary Schafer

> On Oct 4, 2023, at 11:29 AM, MU 4CX250B <> wrote:
> Hi Paul,
> I agree generally with your take-always, though I still don’t see the
> merit of using so much filter capacitance in the HV supply.  I’m
> guessing you’ll be using a big bank of electrolytic capacitors, which
> is likely to be the most trouble-prone part of the entire amplifier.
> You’ll also be generating a lot of unnecessary heat in your bleeder
> resistors and, frankly, it would make me nervous having that much
> stored energy in a 2500V power supply: you could have a real death
> trap on your hands if the bleeder resistors ever opened up! Your HV
> supply will be more reliable if you reduce the filter capacitance to
> ten percent of your intended amount. (In this regard, it’s not a bad
> idea to follow the lead of commercial amplifiers, which use a much
> smaller capacitance than you’re planning.)
>      I’m not sure what screen voltage circuitry you have in mind, but
> if you want to use a “stiff” power supply which achieves its voltage
> regulation by being able to deliver much more current than is actually
> needed, then there are better choices. In my homebrew 160/80m amp, for
> example, which uses three parallel, grid-driven GU74B tetrodes, the
> maximum screen current of all three tubes is only 45 mA, and yours
> will probably be roughly similar. (Note that some amplifiers, the
> Collins 30S1, for example, use a circuit that grounds the screen grid
> and routes the full cathode current through the screen grid power
> supply. That is an unfortunate design, in my opinion, and should be
> avoided.)
> My homebrew amp uses a screen voltage regulator which supples 330V DC
> to the screen grids, with 1% regulation, and which costs about $10 to
> duplicate. It is important to protect the tube if there is flashover
> in the HV supply (or, worse, a screen-to-plate internal arc in the
> tube) If this, unfortunately common occurrence happens, the plate
> voltage of the tube will drop to zero and the screen grid will act as
> the plate, and bear the full cathode current. This is a situation that
> can destroy the tube almost instantly. A properly designed screen
> voltage regulator will protect the tube in the event of a flashover,
> and is a simple addition to a screen regulator circuit.
> To me, overbuilding high voltage power supplies (screen or plate) in
> order to gain a bit of voltage regulation isn’t generally advisable,
> since simple, inexpensive, and reliable alternatives are available.
> 73,
> Jim   W8ZR
> Sent from my iPad
>> On Oct 4, 2023, at 5:31 AM, Paul Decker <> wrote:
>> Hello group,
>> The take-aways I received so far are:
>> 1. Absolutely have a Plate soft start esp with 200 - 500 uF in the HV.
>> 2. Plate regulation is not as critical as screen regulation.
>> 3. Never apply screen voltage before plate voltage. Safe to apply at the 
>> same time. Best practice is to apply plate voltage before screen voltage.
>> That being said, I can design my soft start to start plate voltage. Once 
>> soft start completes and switches out the 25Ohm resistors from the plate 
>> primary lines, it can energize the screen primary lines to bring the screen 
>> voltage on.
>> Paul.
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