[AMPS] Peak reading vs True peak

[Tom Rauch]

> On a steady carrier, the HV sags more than on speech. The tube gain
> doesn't generally change appreciably, but the maximum output power sags
> because the available plate swing is less. 

Gain is reduced when HV sags.

I've seen as much as 10%
> difference in PEP between CW, 2 tone, and voice. You need an oscilloscope
> to see it. PSU regulation is the key. Consider - if you have a plate swing
> of 3kV - say 3500volts down to 500volts, and your PSU sags so that the
> swing becomes 3kV down to 500 volts, you now have only 2.5kVpk across the
> load resistance. The tube draws the same current, because the drive is the
> same. 

The tube also draws less current, because HV is less.

So 1 amp peak plate current giving 1500 watts at 3500volts will
> become 1250watts when the PSU sags. Actually, of course, you'd start to
> see clipping, but you get the principle.

Not so on the power. Nor would you necessarily see "clipping". The 
end result is gain reduction, not power limiting. Clipping normally 
comes from loss of match at the anode. In the case of a HV 
reduction with other parameters constant, the anode remains 
perfectly matched.  

Power output sags at the square of the voltage reduction. If you run 
70% voltage, power output is half (.7 squared) if everything else 
remains constant.

With a HV sag or reduction of 10% (to 90% of full HV), and other 
operating parameters fixed, power output sags 19% (to 90% 
squared of full HV rating).

That's why a plate modulator, applying voltage equal to the anode 
voltage, causes PEP power to reach FOUR times the unmodulated 
level. That's also why the SB-220 has a CW voltage about 70% of 
the SSB value, so the customer could legally tune up at 1000 
watts input (600 watts output) and then flip a switch and have 2000 
watts PEP input (1200 watts PEP output).



  
73, Tom W8JI
w8ji@contesting.com

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