Help converting Keithley Lab amplifier into mic pre

[ckeene]

Hello,

I just acquired 2 tube amplifiers. They're by the Keithley company, all tube with a 5 position multiplier switch. Model 102B. I am working on getting schematics for them and will post photos here when I get a chance.

The main issue with them is the front panel specs a very high input impedance: about 4 megs. There appears to be a large resistor right at the input. The previous owner said these were used to good effect as DIs, but I'm wondering if a can put a 150:50K transformer in front and make them mic pres.

I realize a schematic would be very useful right now, and I'm working on it! But in the meantime perhaps someone might have some experience with these units, or something similar.

Thanks,

Chris

[ckeene]

So I got a schematic today, which I'll scan a better image when I have access to a scanner,

The input is this:

http://img843.imageshack.us/i/keithley102b.jpg/

Basically, there's a 22meg resistor on the input which seems like it's very large for a mic pre. the manual says the actual input impedance is 400 megs. Any thoughts on how I can drop this?

I have a couple 150:50K transformers, which I assume I'll want to incorporate.

Thanks, and let me know if I should provide more info.

-ck