[Amps] Dual GS35b input impedance

[K7RDX]

I have built three GS-35b HF decks and tested at least twelve tubes and I
can verify with Jeff the fact you probably won`t find a pair of these tubes
that are real close to the same specs as far as grid bias,etc. I personally
would use seperate bias control for each tube if building a pair...There are
a lot better tubes out there for multi-tube decks.My opinion, of
course...Why not use a single YC-156?  Jim.
----- Original Message -----
From: "jeff millar" <wa1hco at adelphia.net>
To: "Tomm Aldridge" <KD7QAE at ARRL.NET>
Cc: <Amps at contesting.com>
Sent: Sunday, February 13, 2005 4:17 PM
Subject: Re: [Amps] Dual GS35b input impedance


> 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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