> >Can anyone help me with copies of the following articles. I'm having
> >trouble finding them locally.
> >
> >cheers, Paul
> This is an article I missed, Paul.
> However, the article should be interesting because the grid's DC bias
> voltage in a Class AB1 amplifier is normally adjusted so that, with
> full-throttle grid drive, the grid draws ZERO grid current. This means
> that the exciter can not overdrive the Class AB1 tetrode (or pentode)
> into the distortion-producing grid current region Thus, ALC is not only
> not necessary, similar results could be achieved by turning down the
> microphone gain AFTER the amplifier is tuned up under maximum signal
> conditions.
That's not necessarily correct data. Exciter's power capabilities as
well as the PA's grid circuit efficiency varies from band to
band, plus someone is likely to eventually connect a different radio
to the PA.
While Rich's statement is correct IF the PA is designed and adjusted
to NEVER draw grid current, that's an almost impossible situation to
find in the real world.
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.
ALC is VERY important in an AB1 PA Paul, more so than in class AB2
PA's. The PA needs ALC unless the exciter has exactly the
same output from band to band day after day, the PA has the same
input system loss from band to band, and everything is adjusted
perfectly all the time.
73, Tom W8JI
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|