Date: Mon, 15 Mar 2010 10:58:27 +1300
From: ZL2AAA <zl2aaa@paradise.net.nz>
Subject: Re: [Amps] GI7B Question - Input Matching
I think you will find it's nearer 35ohms.Morrie
>
> I have been reading about the GI-7B Russian made tube. It seems that the
> input impedance of two tubes is around 100 ohms. Could you not just use a
> 2:1 transformer to make a match from 50 ohms to 100 ohms? What am I
> missing here.
>
> Lee - K0WA
## it won't work anyway. The purpose of the PI tuned input is to provide
flywheel action.. as well as Z matching. The tube only conducts for aprx 210
deg,
NOT 360 deg. When u drive the cathode POS, you are driving the grid NEG,
and the tube doesn't conduct at all ! The tube can't source anything, it can
only
sink current.
## Put a scope on the cathode, and you will clearly see the non linear
distortion
[no tuned input, or a 2:1 xfmr] IMD goes to hell. Plus it's harder to drive
the amp.
## The PI net, tuned input.. [the C2 cap.. closest to cathode], will look
like a
LOW Z to harmonics. [then the tank eff goes way up] . Ideally, you want the
C2
cap as close as you can practically get to the cathode. IF the cathode Z is
very close
to 50 ohms, you can easily get away with 50 ohm coax between C2 cap... and
the
cathode. You can also parallel 2 x 70 ohm cables to make 35 ohm coax, or 2 x
75
ohm cables to make 37 ohm coax..or 2 x 50 ohm cables to make 25 ohm coax.
## Henry radio, et all, tried the xfmr approach, with disastrous results.
Jim VE7RF
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