IMO The best way to do this is to use a passive grid driven mode for the
4CX250B's where you can adjust the noninductive grid resistor to get the
proper grid voltage swing based on the output of your driver and using a
toroidal transformer to maintain 50 ohm impedance to the driver (if needed).
This gives you a built in dummy load for tuning and no grief switching the
amp in and out. I have used this arrangement on every HF amplifier I have
ever built.
David C. Hallam
KC2JD
Message: 9
Date: Tue, 18 Jan 2005 10:33:30 -0500
From: "Will Matney" <craxd1@ezwv.com>
Subject: Re: [Amps] Small Amplifier
To: "Amps@contesting.com" <Amps@contesting.com>
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I have seen where some tried running the cathode driven. However they
weren't straight G-G as they used some screen voltage and a bias. They
don't run too hot this way in my opinion. But, they carry a lot of whollop
when running four as grid driven with very little drive. Most amateur amps
using this setup had an attenuator in the grid circuit to allow a
transceiver with standard output to drive them. The first one of these I'd
built, I can remember remarking, "Now that will walk the dogs and lead the
pups" :) If I were to build again, I would have to use a couple of
4CX800's though. Way less expensive and easier to fabricate.
Will
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