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Re: [Amps] Dual GS35b input impedance

To: Tomm Aldridge <KD7QAE@ARRL.NET>
Subject: Re: [Amps] Dual GS35b input impedance
From: jeff millar <wa1hco@adelphia.net>
Date: Sun, 13 Feb 2005 19:17:54 -0500
List-post: <mailto:amps@contesting.com>
I've seen a variety of values for GS-35 input impedance over the range 25 to 100 Ohms. It varies between tubes because the grid bias varies with these russian tubes. I tend to believe the input impedance runs towards the low end of that range. It runs lower than 8877 and the like due to the high grid current. Cathode drive impedance is proportional to Eg / (Ip + Ig).

Definitely halve the impedance for two tubes in parallel. It might well end up 12-25 Ohms. Running high plate voltage will increase bias and increase input impedance.

jeff, wa1hco

Tomm Aldridge wrote:

I am now at the stage of designing my tuned input for the dual GS35b amp that is in process here. There seems to be a general concensus that the tube presents a 50 Ohm impedance when cathode driven in GG configuration. Can I assume a 25 Ohm input impedance as a good starting point for my input filter design?

Regards

KD7QAE
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