[Amps] TL922 efficiency

[Manfred Mornhinweg]

Hi Vic,

there are several things that might explain what you see, and very 
likely it's a combination of several.

 > On 40 meters, I am measuring the following into a dummy load:

The dummy load might not be exactly 50 ohm with zero reactance. Most are 
pretty good, but do measure it.

 > Output: 1200 watts (several reliable wattmeters agree).

I have noticed many wattmeters showing significantly higher power than 
real. The manufacturers seem to do this to please the users/customers... 
Try to measure the power output with an oscilloscope, if you have that 
possibility.

I assume you are measuring with a clean carrier? Otherwise your power 
meter might be showing PEP, while your DC meter shows average current!

 > Drive: 65 watts.
 > Plate current: 580 ma.
 > Plate voltage 2600 V under load (3 kV no load).

You calculation is fine, but built-in meters are often inaccurate, 
specially after some years.

All these factors might be adding up. But it's also possible that all is 
fine and accurate, and that you are simply driving the amp into slight 
saturation. The maximum theoretical efficiency of a class B amp is 
78.5%, without saturating it. Practical imperfections limit any real amp 
to a lower efficiency, class AB costs some efficiency too, but as soon 
as you drive it into saturation, the efficiency goes up again. So, 76% 
isn't impossible at all.

Manfred

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