Re: [Amps] Dual GS35b input impedance

[Tony King - W4ZT <amps@w4zt.com>]

I discussed this problem with a friend of mine who suggested a very
simple way to pick your pair of tubes for similar emission.  He
suggested using my filament jig but this could be done in any amp
chassis.  With proper filament voltage on one tube, apply only a few
hundred volts to the anode of the tube. Note the anode current. Doing
the same thing with the other tube(s) will give you an idea of how well
their static balance is.  Of course this wont tell you anything about
how they will respond to drive but adjusting static bias probably wont
either.

73, Tony W4ZT

K7RDX wrote:
> 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@adelphia.net>
> To: "Tomm Aldridge" <KD7QAE@ARRL.NET>
> Cc: <Amps@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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