At 07:16 AM 4/11/99 -0700, you wrote:
>
>In planning my 2 x 3-500z gg amp, I've been looking at schematics of similar
>units, and there are a couple of questions that have arisen:
>
>1) Some of them do not ground the grids directly, but do so thru a bypassed
>choke or resistor. Why?
>
>2) Some of them use the usual pi-network input circuit, but others have
just a
>parallel-tuned circuit (the Bill Orr handbook, for example). I know that the
>input impedance of the pair of tubes is quite close to 50 ohms, so the
impedance
>matching function of the pi is not needed, so I guess the circuit is used
just
>to provide a 'flywheel' and a return path for the RF plate current. But
wouldn't
>a parallel-tuned circuit have a relatively very high impedance compared to
the
>load provided by the tubes? How does this work?
>
Those old input parallel tuned circuits were "tank" circuits to add some
"Q, and did some impedance matching as well. Remember that those amp
projects/articles were written in the era of exciters with vacuum tube
finals that had their own adjustable output tank circuits. Therefore,
presenting a resistive 50 ohm match ot the exicter wasn't as critical as it
is today with the solid units. When solid state became prevelant, so did
pi input tuned networks.
If you want to use one of those old schematics, cross-out the parallel
tuned input circuit and replace it with your own pi network. Use the Pi
formulas in the ARRL handooks, 1995 and later editions preferably.
Phil
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