On Tue, 2006-10-10 at 19:45 -1000, Ken Brown wrote:
> Wow! I've misunderstood this for years. I thought the the ALC voltage
> was produced by the amplifier, and fed to the exciter to limit drive. I
> never knew that the exciter generated an ALC voltage to change the
> amplifier bias.
>
> DE N6KB
> > The ALC biases the RF to avoid clipping. In an amplifier, the exciter ALC
> > is applied as a bias to avoid the same thing. The Argo
> > does not provide ALC to an amp. If the amp can create an internal
> > ALC by biasing, that should work.
> > Most amps I know of use outside (EXCITER) ALC to do this.
> >
> >
>
News to me. Maybe Breune, Pappenfous and Shoenike got it all wrong in
their tome on SSB 45 years ago....
Generally one does never change the bias in the PA (though we did
modulate the grid bias in the Collins 821A-1 PA along with the cathode,
screen, and plate, something about slowing the electrons at modulation
and RF peaks so they didn't dent the anode so much, sure made for
interesting PA parasitic oscillations at 8 Hz) because its set and
regulated for best linearity. Changing it to change the tube gain only
works when the tube has a grid designed to be a gain control tube. Some
like 6BA6 but 6AH6 didn't.
Truth is, ALC is generated in the exciter, but near the output to
control the drive to the internal PA to keep it from clipping, and then
ALC can be generated in the PA to control the gain of the exciter to
keep it from clipping.
It is conceivable to put a voltage controlled attenuator at the input of
a PA that is controlled by ALC voltage generated in the PA to keep the
exciter from overdriving. I've seen that in circuits for transverters
published in G3SEK's VHF/UHF DX Book.
Oh yeah on that Collins PA, at modulation and RF peaks the electron
accelerating voltage was about 60 KV, and the electron velocity about
0.1 C.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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