[VHFcontesting] Why the Old Fashined Screen Supplies "Just Don't CutIt"

[Tom Carney]

The screen supply for a linear amplifier has to be stable (IE regulated) to
minimize inter modulation distortion.  In an old class C amplifier, the
screen supply is usually just a resistor in series with a power supply.
Since the sreeen current varies during the power cycle, you get a voltage
that varies during the cycle.  This will cause a significant increase in
IMD.  Thus the need for a regulated screen supply.

As for the second question, I would guess the mid 60ies for HF, later for
VHF.  My mid sixties handbooks show amplifier designs that could be biased
for either linear of class C service in the HF section while the VHF
amplifiers are often class C designs.  No hard and fast rule.

73,  Tom  K6EU


 ----- Original Message -----
From: "DUGAS, JASON A. (JSC-EP) (NASA)" <jason.a.dugas at nasa.gov>
To: <vhfcontesting at contesting.com>
Sent: Sunday, March 06, 2005 6:23 PM
Subject: [VHFcontesting] Why the Old Fashined Screen Supplies "Just Don't
CutIt"


> Hello all,
>
> I've been doing some reading in the VHF/UHF DX Book.  I have ahead of me,
a
> homebrew amplifier and a power supply to build.  In reading some of the
> various power supply details in the book, they mention that the old screen
> supplies used for Class C (modes AM, FM, and CW) won't work for Class-AB
> service in SSB.
>
> I'm still learning more about tubes and amplifiers here.  I would like to
> hear some discussion on why, when building a amplifier screen supply, that
I
> can't use the old screen supplies?  I assume regulation is at the heart of
> this statement.
>
> Also, when I am reading some of the older literature, at what year in the
> anthologies, should I assume that an amplifer was NOT designed for SSB
> operation?  All I know is that SSB was around when I was born.  Anything
> before that is beyond my recollection!
>
> Thanks,
>
> Jason
> KB5URQ