[Test-Equipment] tube input impedance

[Randy Zelick]

Hey thanks all for the good number of helpful suggestions. I've learned a 
bunch of tube theory.

Here are my next steps...

I will experiment with the 26A1 to see if I can increase the Zin somewhat.  
This will take a couple of weeks, though, because I have to get some
ancillary stuff going to do a proper test, and I want to get the
physiology prep ready to see how much loading it can tolerate.

Meanwhile, I will look into building a matched-pair FET probe as some have 
suggested. If anyone has a ready-to-go design, do let me know. The 
requirements would be 2 Hz - 10 kHz bandwidth, unity gain, a gigohm (or 
more) input impedance. I would try to put it in a small Pomona box and run 
it off batteries. A shielded twisted pair cable would take the outputs of 
each FET back to the Tek amp, which would provide differential 
amplification and filtering. 

Also, for a few folks who were interested in more details of the 26A1 
input, it goes like this:

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=

-- 
R. Zelick				email: zelickr@pdx.edu
Department of Biology			voice: 503-725-3086
Portland State University		fax:   503-725-3888

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