To: <amps@contesting.com>
> Date: Sat, 26 Jul 97 04:32:54 +0000
> Ciao to all.
> I read on the Ameritrons' AL80B technical info that on this Amp is
> installed a "electronic bias" that is a known circuit and a " Dynamic ALC"
> that allows "high level, low distortion RF processing", doubling the
> average SSB power.
Hi Vince,
That's advertising fluff. Because the ARRL Handbook says
RF compression causes up to 3 dB improvement in "talk power", that
got put in the ad.
How much improvement it really makes depends on the ALC response time
of your rig, and how you run the rig.
> I'm interested to know more about this solution, in order to install it on
> my SB-220.
Here is what the circuit does. Instead of sampling RF voltage at the
tube cathode, it samples grid current. Grid current is referenced
against an unregulated sample of rectified and filtered power
transformer secondary voltage.
One advantage of this system is if the transformer or power line
sags, the peak grid current is reduced.
The other advantage is if you set the ALC control to allow (for
example) 150 mA of grid current, you can never exceed 150 mA of grid
current under any tuning condition. That reduces your worry or
concern about arcing the tank circuit, overdissipating the grid, or
overdriving the PA from nearly any cause.
Any time the grid current starts to approach 150 mA (the circuit has
built-in hysteresis), negative ALC voltage appears.
> I believe that could be some sort of amplified grid current feedback to
> transceiver, instead of the standard capacitor partition of input signal.
That is correct. A more advanced board used in commercial PA's
samples anode current, grid current, peak RF negative anode voltage,
and reflected power for ALC and warnings. It has a latching
fault circuit with a three strike timers. (Within one minute. One
fault it resets with a quick LED warning, two faults it warns longer
and still resets, three it shuts the PA off.)
You can e-mail me for further questions if you like.
73, Tom W8JI
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