>> From: "Ian White, G3SEK" <G3SEK@ifwtech.demon.co.uk>
>> Subject: Re: [AMPS] Screen Supplys
>
> Hi Ian,
>
>> If you look at the datasheets, you will see that a string of high-
>> voltage zeners cannot have a output impedance less than a few hundred
>> ohms - a load pull will confirm this. A few hundred ohms is big enough
>> to have a very noticeable effect on the IMD.
>
>I certainly agree with you Ian.
>
I recommend using a string of10v to 30v 5w zeners. They have minimal
dynamic R. When a sufficient capacitor is connected in parallel they
seem to work okay.
>Zeners make poor regulators for screen supplies unless you use some
>type of gain loop to reduce the effective ESR of the supply, and
>hold the current in the zeners constant.
In my (2) 7580W amplifier, the screen current moves from positive 5mA to
negative 3mA during modulation, so there is little change in current
through the zener string.
Rich---
R. L. Measures, 805-386-3734, AG6K
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