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[AMPS] Alpha 78 Input matching unit

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Subject: [AMPS] Alpha 78 Input matching unit
From: (Rich Measures)
Date: Tue, 3 Mar 98 05:43:57 -0800
>I can remember I read in this reflector someone saying "I can´t achieve
>more than 840 Watts with very low grid current (something like 20 mA)".
>I experienced the same on 10 and 15 meters. From 20 to 80 I can run a
>little more than 1 KW.
>I use a TS850 to drive it.
>I think there is a problem with the input matching unit.
I agree, Arturo.  Grounded grid amplifiers need a tuned input network.  The 
Alpha 78 has none.  
>They use a resistor pack which total resistance is about 22 ohms and it
>is in paralell with the input.
>Maybe it was ok at the time the 78 was designed (high output electron
>tubes transceivers) 
Indeed, because electron-tube transceivers had a tuned circuit in the output.
>but now with solid state ones there must be some
>changes. I tried giving more output (140 W) to the 850 and I could
>achieve 1 Kw on 10 meters and 1.3 KW on 80 meters. That was only for
>test purposes.
>Is that the problem ?
sounds like it
>Does anyone know the reason of such input matching unit design ?.

The 78's designer ignored Eimac's recommendations to use pi-network tuned 
inputs, possibly to cut costs, possibly to make a more compact amplifier, 
possibly due to lack of understanding of how a grounded grid amplifier works.  
In any event, leaving out the tuned input is not good engineering practice.  
>Is there anything to improve ?
¿¿A tuner?? 
later, Arturo


R. L. Measures, 805-386-3734, AG6K   

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