On Thu, 5 Jun 1997 13:24:16 -0400 (EDT) W6TG@aol.com writes:
>In a message dated 97-06-05 10:19:50 EDT from W8JI, you write:
>
><< In the practical world, ALC is actually MORE necessary in a class
>AB1
> PA. The slightest grid current will send the PA into distortion
> quickly with most AB1 grid driven designs.
>
> The correct way to install ALC in an AB1 PA is to sample PEAK RF grid
>
> voltage, and activate the ALC as peak positive voltage approaches the
>
> quantity of negative bias voltage.
>
> In other words, if the bias voltage is -50v, ALC should be activated
> whenever positive RF voltage peaks approach or reach 50 volts. Any
> ALC sampling grid current will be "too late" to do any good, so the
> system has to be a peak voltage detector.
> >>
>
>I would use the power output control that is on most all the
>transceivers
>these days to not drive the AB1 amp into grid current.
>Terry - W6TG
Not very easy to accomplish with some of the newer rigs.
Of particular concern is the prevalence of 4CX250 type tubes on the
VHF/UHF bands; they are a boon and a bane at the same time.
The lack of ALC circuitry in almost every published article for these
tubes creates a problem for many VHF/UHF ops who have to operate on the
band along with overdriven amps.
I am not even sure if the typical VHF multimode xcvr has any provision
for ALC input; perhaps Ian would have that info.
The inclusion of ALC circuits in various VHF oriented publications would
be a start in the right direction since AB1 tetrodes are more common at
those frequencies.
73....Carl KM1H
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