Help converting Keithley Lab amplifier into mic pre

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ckeene
suffering 'studio suck'
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Help converting Keithley Lab amplifier into mic pre

Post by ckeene » Mon Feb 07, 2011 8:47 am

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
suffering 'studio suck'
Posts: 418
Joined: Mon May 12, 2003 2:15 pm
Location: Cambridge, MA
Contact:

Post by ckeene » Fri Feb 11, 2011 3:48 pm

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

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