[Amps] step start

[MU 4CX250B]

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 <kg7hf at comcast.net> 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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