[AMPS] gain and peak meters

[Tom Rauch]

Hi Peter and all,

I had a chance to review some circuits this weekend. 
I want to correct an error I made. 

When I said gain was proportional to the square of HV change in a 
grounded grid amp, that was incorrect except for the case of a tube 
with zero driving impedance or if you consider drive power that 
actually makes it to the grid remains constant. 

What really happens is this, as anode current increases negative 
feedback increases. That reduces the drive applied to the grid even 
if exciter power remains constant.

That's because the GG amp has current feedback. That feedback 
level is set by the ratio of anode to cathode impedance.

So the lower the cathode impedance compared to anode 
impedance, the closer the amplifier becomes to following the 
square of voltage change for gain change. A 3CX3000F1 has much 
closer to a 1:1 change than a 3CPX5000, which almost follows the 
square of anode voltage.

On the second issue of reading peaks, a scope is totally 
unnecessary. Rise time of the envelope is limited by transmitter 
bandwidth. With a 3 kHz bandwidth, it takes about .000166 
seconds to reach the peak of the RF envelope, and that time again 
for that peak to decay back to zero.

Almost any op-amp or even a simple transistor amplifier used with 
a directional coupler could follow that peak and store it. It's 160 
microseconds, and the storage cap only has to follow that up in a 
sine curve of that length or LONGER time period. If we select a 
charging time constant several times faster than 160 milliseconds 
with a long discharge time, we have the problem totally whipped no 
matter what the waveform.

I think you made a mountain out of a mole hill on that one, by not 
considering the transmitter bandwidth restrictions and how they 
affect the envelope.

I read medical equipment that uses a low microsecond duration 
square wave envelope (that has over 200 kHz of bandwidth) on a 
good peak storage meter, and it comes out exactly the same as a 
scope. If a 200 kHz wide signal can be read accurately, so can a 
narrower signal.

    

  
73, Tom W8JI
w8ji@contesting.com

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