[Boatanchors] AM Amplifier

[Gary Schafer]

Amp efficiency on AM when properly tuned.
The amp must be first tuned up at full PEP power level.

At 375 watts carrier out the PEP capability should be 1500 watts out.
Assuming the amp is 50% efficient at 1500 watts out then the efficiency at
375 watts carrier will be around 25%. 
(carrier efficiency is in the neighborhood of 1/2 that of full power
efficiency)

This will leave a plate INPUT POWER (with no modulation) of 1500 watts in
the tube(s). (375/.25 = 1500 watts)

Plate DISSIPATION in the tube(s) will be 1125 watts. (1500 input - 375
output = 1125)

So a pair of 3-500 are being pushed at this power level even if the tubes
are provide with proper full ventilation.

On voice peaks the efficiency of the amp will be at 50% and provide 1500
watts PEP output and will run a PEP input power of 750 watts. (1500 input -
750 output = 750 watts dissipation on peaks)

So as can be seen the efficiency improves as modulation is applied. Heavier,
processed audio will be easier on the tubes than light level audio. (still
maintaining below 100% modulation)

Average audio power with no processing will run around 20% to 30% which will
improve the tube efficiency to around 30 to 35% when modulating with a
fairly constant voice.
Note that this is highly variable depending on voice type and how much you
talk.

Reducing drive power to a lower level to try and reduce plate dissipation
(after amp has been tuned to 1500 watts) may not help much as efficiency
will also drop even more if you do this.

73
Gary  K4FMX