[AMPS] Tetrode Amp articles needed

[Ian White, G3SEK]

W8JI wrote:
>In the practical world, ALC is actually MORE necessary in a class AB1 
>PA. The slightest grid current will send the PA into distortion 
>quickly with most AB1 grid driven designs.
>
Very true - but how much of this is due to the tube characteristics, and
how much to the power supply? Many power supplies designed for AB1 have
very high output impedances (several K or more), so the flow of grid
current shifts the operating point dramatically. In such cases you can
expect distortion.

Ironically, much of this additional impedance comes from the circuits
intended to generate enough ALC voltage when grid current begins to
flow. This turns the need for ALC into a self-fulfilling prophesy.


>The correct way to install ALC in an AB1 PA is to sample PEAK RF grid 
>voltage, and activate the ALC as peak positive voltage approaches the 
>quantity of negative bias voltage. 

Yes, but even then it's a mistake to rely too heavily on ALC, because
there will always be overshoots.

Like Rich, I tend to run my own tetrode PAs open-loop, with no overall
ALC feedback, and instead use speech clipping and careful setting-up of
the whole system to control the drive level. On the other hand I also
sympathize with Tom as a designer of amplifiers that could be used by
any operator with any driver, and therefore has to provide ALC.

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

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