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[AMPS] Tuned input

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Subject: [AMPS] Tuned input
From: (Paul Hewitt)
Date: Mon, 9 Jul 2001 21:06:57 -0700
Doesn't the Alpha 89 use a remotely switched input
network with 25 ohm connecting coax?

Paul Hewitt

> In a new design, the pi-net tuned input
> can be right next to the cathode
> circuit.  But, when working with
> amplifiers that were built years ago, one
> must live with the existing limitations.
> In the Swan Mark II, it was easy to add
> a new wafer and mount the relays and
> pi-network under deck - right next to
> the cathode where it should be.
> But, when I modified my Alpha 76 -
> which has no tuned input to use 3CX800A7
> tubes, I studied the mechanical layout
> for hours trying to include a tuned
> input switched by the existing band
> switch.   I could not figure out how to
> do it, since it is very tight inside
> and the bandswitch uses a different
> degree of rotation between contacts
> than any others I had.  The Alpha 76
> normally uses a toroid input to match
> inpedances, but that would not have
> any of the benefits of a tuned input.
> After much internal conflict, I
> finally decided to abandon the tuned
> input on the 76.  It works fine and is
> clean enough.
> Using a remotely switch pi-network at
> the rear input connection of the
> amplifier is not ideal but it is much
> better than none at all.  In fact, the
> overall length of the coaxial line
> between the tuned circuit and the cathode
> may not be much different than the
> length of some commercial amps.  As I
> recall, my old HT-41 has just a few
> inches between the input connector and
> the cathode, so a remotely switch
> pi-network at the input connector would
> add significant improvement and would
> be the only reasonable approach to try
> to improve the old battle-axe.
> 73,  Colin  K7FM
> --
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