[AMRadio] AM Usage with Linear AMPS

Patrick Jankowiak recycler at swbell.net
Tue Nov 23 00:33:43 EST 2004

The plate efficiency at full output under practical conditionsis 
usually of the order of 50 to 65 per cent. With less than the 
full output, the efficiency is proportional to the driving 
voltage. When the signal to be amplified is a carrier wave 
modulated 100 per cent, the carrier amplitude is half the peak 
amplitude to be handled. The efficiency for the unmodulated wave 
is then half the maximum efficiency, or 25 to abour 32 per cent 
under ordinary conditions. The average efficiency of a linear 
amplifier used with amplitude-modulated waves is hence relatively 
low, because ordinarily the wave is fully modulated only a small 
part of the time; even when fully modulated, it is at or near the 
crest value for only a small fraction of the modulation cycle. 
The peak output power that can be developed by a tube operating 
as a linear amplifier is approximately the same as the power 
developed by the same tube in Class-C amplifier operation, or it 
may be slightly greater since the linear amplifier does not 
operate at peak level continuously. Since the peak power of a 
fully modulated wave is four times the carrier power, a tube used 
as a linear amplifier is capable of developing something between 
one-half and one-fourth as much carrier power as the output 
attainable from the same tube operated as a Class-C amplifier. 

A pair of 6146's operating at 600VDC put out 100 watts single 
tone. This is the maximum that amplifier would make. The current 
was about 260mA.

peak/single tone:
600VDC * 260mA = 156 watts input.
100 out, 56 dissipation.
64% efficiency.

Carrier conditions:
600VDC * 135mA = 81 watts input.
25 out, 56 dissipation.
31% efficiency.

I realize this is pushing 6146's, better would have been 80 watts 
PEP and 20W carrier.
The point is the efficiency as taken by meter readings. These 
values can probably be validated on a host of existing 2x 
6146-equippped gear.

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