>At 04.44 1997-12-17 -0800, you wrote:
>>... ... ...
>>>I suspect that varying the ratio of screen volts to grid volts on the
>>>G2DAF may well get the 3/2 power curve due to each grid down to a square
>>>law for the overall transfer characteristic, although I admit I haven't
>>>tried doing a mathematical analysis yet.
>>
>>If the screen potential was being varied at an RF rate, maybe something
>>of the sort would be possible. However, in the G2DAF circuit, the screen
>>potential is varied at a semi-audio rate while the grid is varied at an
>>RF rate - so never the twain shall be in phase.
>>>
>>>The original idea of the G2DAF was to simplify matters by getting rid of
>>>the screen and bias supplies. However, better linearity might be
>>>available if some bias is available.
>
>* I doubt that, as G2DAF is designed for zero-bias operation. That means that
>* the amp will draw some amount of gridcurrent at all levels of drive.
True. With no grid bias supply, grid current is assured.
>* Taking this facts into account, I really don`t understand the "on-air-test"
>* made by Rich. How could you possible lower the drive until there were no
>* gridcurrent??
The two G2DAF-users temporarily lowered the drive in order to decrease
the grid current -- which decreased the splatter level. In a grid driven
SSB amplifier, more grid current means more distortion. However, Eimac
builds two tubes (4cx600JX & 4cx1500B) that can operate with some grid
current, yet remain fairly linear. The allowable grid current is
typically less than 500uA
>
>>Indeed, Peter, and a constant screen potential could well result in even
>>better linearity. . IMO, the original G2DAF proposal also included the
>>promise of Class-C-like efficiencies. .
>>- . When I was a teenager, I saw advertisements in Popular Mechanics
>>for plans for constructing more efficient carburetors for use in ordinary
>>automobile engines. Claims of over 100MPG were pretty common. There
>>were also advertisements for plans for constructing various and sundry
>>perpetual motion machines.
>>... ... ...
>>cheers
>>Rich...
>
>* As said before, I really do like your sense of humour Rich!
>* However, regarding efficiency, the resting Ip in a G2DAF amp is very low
>and * much lower than amps in AB1 or AB2 = Higher efficiency for G2DAF!!
>
The resting current in the Class AB1 amplifiers that I use is 0mA.
Within a fraction of a second of not speaking into the microphone, the
transceiver's VOX switches to Rx mode, and within 3mS thereafter the
amplifier switches to Rx mode. It's almost like talking on the
telephone.
- The efficiency of the AB1 amplifier is roughly 60%. If the efficiency
were an impossible 100%, the S-meter at the receiving end would increase
1/3 of one S-unit.
- There's an electronic-switched AB1 grid bias supply at:
http://www.vcnet.com/measures/D.a.04.GIF
cheers, Lars
Rich...
R. L. Measures, 805-386-3734, AG6K
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