Hi Ian
>
> Tom, on the SB220, which direction is "open"?
Open is the same as "un-meshed". That normally is the direction of
full clockwise setting, if the knobs are installed correctly.
> > Tom Rauch wrote:
> > Next look at where you have the load control set. Try to open the
> > load control as far as possible with FULL drive applied, until power
> > just drops off. You might have to "walk" the controls up as you advance
> > power, always keeping the load OPEN just beyond the point of maximum
> > output.
Opening the load control a bit MORE than needed at full drive
power, and backing off the drive to rated or desired output:
1.) Increases negative feedback in the amplifier. Remember the
input and output are in series in g-g amplifier, and the feedback is
the ratio of the cathode and anode impedances. Lowering the
anode load impedance by opening the LOAD control up reduces
anode impedance which increases negative feedback that reduces
distortion generated in the PA. Adding more negative feedback is
also what makes grid current drop.
2.) Opening the load control up reduces peak voltages in the tank
and on the tube by loading the tank harder. This greatly reduces
the chance of arcing during exciter transients or antenna system
faults.
If you don't load the PA for full exciter power with modern exciters
and use an exciter that somewhat matches the PA's output for
drive requirements, you are asking for problems.
73, Tom W8JI
w8ji@contesting.com
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