[AMPS] Peak reading vs True peak

[Tom Rauch]

> Tom says:
> 
>  >You are viewing short term fractional cycle behavior and trying to 
>  >use it to explain long term averaged current and power.
> 
> Yes. Just like Chaffee analysis does.
> 
> But the original point was that the gain goes down. My argument is that
> the power output goes down, but the 'gain' is marginally affected. So what
> we see is gain compression; a linear amplifier has constant gain over the
> full input power envelope. If gain is affected, then running a kW
> amplifier at 10 watts out would see just as much percentage drop in output
> when the  plate volts is reduced as at a kW. In other words, that would
> say that gm is highly dependent on plate volts. But look at curves for a
> tetrode, and it ain't so. Even with a triode, the mu doesn't change that
> much for the plate voltage changes we are talking about.
 
As I said, there are limits. You can reduce HV so far that a tube 
becomes non-linear in its response to a HV change.

But if the HV changes and the tube is not moved into a non-linear 
portion of the curve, gain and power output is reduced by the 
square of the HV change. The SB-220 changes 3 dB in gain, and in 
power output, for a reduction of anode voltage to 70%.

The error from ideal in the SB-220 Rich measures is caused by 
ESR in the power mains and in the transformer. The HV sags a 
little less than calculated by turns ratio and by UNLOADED voltage 
change because voltage drop is more in the SSB position than in 
the CW position. That's because the 220 draws more power from 
the power line in the HV position, and so the available voltage is 
less.

Rich is correct. Gain is reduced when HV is reduced.

Within the limits of the metering, if Rich takes the time to 
backtrack what he measures as a change, you'll find the SB-220
follows the rules of any other PA. Power output and GAIN changes 
by the square of the anode voltage change.

You just have to be careful that the meters are  steadily loaded so 
meter response isn't an issue.




 

   
73, Tom W8JI
w8ji@contesting.com

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