[AMRadio] Re: Looking for Linear Amp

[Jim Bromley]

Paul Sokoloff, WA3GFZ, wrote:

>  I am looking for an Ameritron AL-811 or 
> any good amplifier of medium power (200-300 
> watts AM) that would also do 160 meters.  
> Any opinions on the Ameritron.  I am not 
> familiar with the unit, but it looks like 
> it will fit the bill

No, it won't, but through no fault of the 
amplifier.  You need a *lot* more plate
dissipation to run 200 - 300 watts out.

Here's my thinking:  First, carrier-level
efficiency for a linear amplifier running
AM is 33%, best case.  This means that at
250 watts out, you will be burning 500 watts
off the plates of the tubes.  Think 3-500Z.
But you really shouldn't run a '500 at max
plate dissipation full-time.  Think two 
3-500Z's.

Now for the power supply.  It has to crank
out 750 watts average power, with peaks of
about 1500 watts - 3 kV @ 500 mA.  Quite 
doable with just about anything out there
and no real problem.

The Ameritron amplifier you should be looking
at is the AL-82.  Or anything that runs a pair
of 3-500's.  I run about the power you are 
looking for with my old Heath SB-220.  After
a typical old-buzzard AM transmission, the
500's have quite a bit of color on the plates
and the shack temperature has gone up 10 degrees.

Bottom line:  You need a lot of plate dissipation
to run low-level-modulation AM.

Jim, K7JEB
Glendale, AZ