[Test-Equipment] tube input impedance

[J. Forster]

That's a standard differential amp. You MIGHT be able to swap FETs for the
tubes, BTW.
-John

Randy Zelick wrote:

> The input tubes are 9-pin miniature triodes. The signal (+ or -) is
> applied to the respective grid through a 100 ohm resistor. The junction of
> the grid and the 100 ohm has a 10 megohm to ground. The cathodes of the
> two input tubes are tied together with a 100 ohm pot (each cathode to one
> leg). The wiper of this pot goes to a transistor circuit but I think it is
> just a source of 1.3 V. The pot is "INT BAL", which I believe is internal
> balance. Looks to me like it sets the gain of the two tubes to be equal.
> The plates go off to 5 K plate load resistors (decoupled from the 300 V B+
> bus) and also to the base of a 2nd stage amplifier transistor. There is
> also 15 pF to ground from the plates.
>
> I'll let the list know how the tests progress!
>
> Cheers,
>
> =Randy=
>