|[AMRadio] Re: Gates BC250L efficiency|
Brett.gazdzinski at mci.com
Wed Sep 29 18:30:45 EDT 2004
Well, if not going into a dummy load, the swr would go up.
Just because you run an antenna tuner does not mean
the amp is running clean and not wasting power in harmonics.
Measure the TR-7 by itself and its likely clean, and overdriving the amp
is likely the problem.
Modern solid state rigs likely wont allow much harmonics out unless
you tweak things inside for more power out than they are supposed to run.
Most will put out quite a bit of extra power if you tweak the alc circuit,
but it wont be clean.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Larry Taft
Sent: Wednesday, September 29, 2004 3:19 PM
To: Discussion of AM Radio
Subject: RE: [AMRadio] Re: Gates BC250L efficiency
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.
>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
> >From memory, a Q of 15 is 'normal'.
>A Q of 12 should be ok, who has harmonics these days?
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