[Boatanchors] AM Amplifier

[WA5VGO]

Gary,

Very eloquently explained.

I haven't been trying to say that it won't work or it's a bad idea. I 
guess there are three points I was trying to make.
1) The efficiency is poor.
2) Unless you have an amplifier with excellent cooling and a heavy duty 
power supply, you'd better take it easy.
3) You've got to over couple the amplifier so that you have a minimum of 
6dB of head room.

Darrell

On 7/8/2018 2:06 PM, Gary Schafer wrote:
> 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
>
>
>
>