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[AMPS] PA Recommendation Wanted

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Subject: [AMPS] PA Recommendation Wanted
From: (Ian White, G3SEK)
Date: Sat, 7 Jun 1997 20:11:01 +0100
Rich Measures wrote:
>>A peak detector with a hold circuit (perhaps driving an LED) would be 
>>a good idea. 
>Why not adjust the grid bias V so that the grid can not draw current with 
>max. drive applied? 
Not all tetrodes require grid bias voltages that conveniently match up
to the available drive power and the grid swamping resistance. Adjusting
the grid bias may work for Rich's amplifiers because they use big
tetrodes that require more than 100V of grid bias. Since the peak RF
voltage from the nominal 100W exciter is already pretty close to the
manufacturer's recommended bias voltage, a relatively small adjustement
to the bias voltage may make sense. But that strategy obviously can't
work for tetrodes that only need 35-50V of bias.

>>I also like Ian's suggestion of building the grid supply 
>>like the PA is for AB2 (regulated with a stiff shunt regulator), even 
>>though you plan on running it on AB1. 
>Eimac says you don't need a regulated bias supply..  

Rich, please can you quote exactly what Eimac says on this topic, and

Similarly, please can you quote what and where Eimac says that the
correct method to configure a class AB1 amplifier is to adjust the grid
bias for zero grid current, and the adjust the screen current for the
recommended ZSAC.

>From my experiences, 
>a 10k to 20k ohm output-R bias supply works just fine for tetrodes with 
>handles.   A 'stiff shunt regulator' supplying a zero current circuit 
>makes zero sense to me.

All this depends on the promise/assumption/belief that grid current
NEVER, NEVER flows. If grid current ever does flow, a high supply
impedance will cause severe distortion. 

That's a very high-risk strategy. Even if the exciter only has rare
power overshoots, they could cause severe splatter (high-order, wide-
band, transient IMD).

AFAIK there are no specific licence regulations in Europe about in-band
IMD, but we do have fierce rules against poor-quality signals in VHF

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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