[Test-Equipment] tube input impedance

[Randy Zelick]

Hello there,

I am contemplating modifying a Tek 2A61 high-gain differential pre-amp for
a physiology experiment.

This antique plug-in (for 560-series scopes) has an input impedance of 10
Megohm, and a maximum sensitivity of 10 microvolts per division.

The input circuit is a pair of 5842 triodes (one for + and one for - input
lines). Each has a 10 Megohm resistor from the input grid to ground which
sets the overall input impedance.

What I want to do is increase the input impedance to the maximum I can get 
away with. A gigaohm is not too much, if this can be realized.

So....

Does anyone have ideas about this? What is the Zin for such a tube? Can I 
get away with no grid-to-ground resistor or will the grid float positive?


Thanks,

=Randy=

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

mailing:
P.O. Box 751
Portland, OR 97207

shipping:
1719 SW 10th Ave, Room 246
Portland, OR 97201