[AMRadio] Re: Gates BC250L efficiency

Larry Taft ltaft at charter.net
Wed Sep 29 15:18:57 EDT 2004

I made a set of interesting measurements with my spectrum analyzer on a 
Drake TR-7 driving a L-4 amp.  Set the output at 1kw through a W-4 
wattmeter into a dummy load.  Harmonics looked good, down quite a 
ways.  Crank the TR-7 up a bit to the L-4 and the power went up to 1100 
watts.  The fundamental stayed the same, it was the harmonics that went 
up!  The power meter didn't care what frequency the energy was.  Now I 
always run a MN2700 pi network antenna tuner with any of the Drakes.

Larry  K2LT

>Efficiency is often a function of the tank circuit Q.
>Lower Q often gives more efficiency.
>It can be quite dramatic if you are far off.
>Grid bias needs to be way beyond cutoff, and with a
>lower Q, efficiency should go up.
>I did some experiments on the push pull 812 rig when it was done.
>I was getting 200 watts out of it, until I shorted 2 or 3 turns
>on the final coils.
>Power went up 50 watts!
>I removed the turns to prevent weird stuff going on, but still
>only got the 250 watts out, so the shorted turns were not
>causing problems in my application.
>Maybe try shorting some tank coil turns as a test and see what happens.
>If you left a lot of turns in the coil, your Q could be above 20 or more.
> >From memory, a Q of 15 is 'normal'.
>A Q of 12 should be ok, who has harmonics these days?
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