Hi Arturo & all..
You're right - at the time the '78 was designed and for most of its
production life most transceivers had tuned-output, vacuum tube amps so the
"flywheel" of a tuned-input PA wasn't necessary.
I think you'll find that the length of drive cable between transceiver and
amp input can make a big difference. Frankly I can't remember the optimum
length, but if yours isn't somewhere close to 5-6 feet you might try that
length of RG-58/U & see what happens. Maybe someone else on the reflector
knows the optimum length for sure?
73, Dick W0ID
-----Original Message-----
From: Arturo Gargarella [SMTP:AGargarella@sipetrol.com.ar]
Sent: Monday, March 02, 1998 9:28 PM
To: amps@contesting.com
Subject: [AMPS] Alpha 78 Input matching unit
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.
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) 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 ?
Does anyone know the reason of such input matching unit design ?.
Is there anything to improve ?
Thanks in advance for the help
73 de Arturo, LU6ETB
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