[AMPS] Ameritron AL80B: Dynamic ALC?

Sat, 26 Jul 1997 21:55:43 +0000

> 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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