Cinema Engineering Co 50K variable attenuators

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lohi

Member
Joined
Mar 10, 2006
Messages
15
Location
Winters, CA
I have a few Cinema Engineering Co 50K variable attenuators. I'm wondering if I can use one as an attenuator in a Pultec MB-1 preamp, between the input transformer (150-15Kohm) and the grid of the tube. Would the impedence be about right? I'd probably have a switch to remove it from the circuit altogether. Does anyone have experience with these old, old attentuators? I think they were a part of a passive mixer.
Keith
 
50K is too low, technically speaking, to go between a 15K transformer secondary and a tube grid. Any impedance at the grid less than 10x the secondary impedance of the input transformer will cause loading-down of the mic. But try it, see if you like it--and if you don't, no harm done. And yes, do put in a bypass switch, since you don't want that load shunting the grid when it's not needed.

Putting a pot right after the input transformer in a mic preamp is not good practice, technically, but some people have done it and liked the results and I can't be bothered to argue with them anymore :wink:

Those old Cinema stepped pots are good. Even if you don't use them for this project, they'll surely come in handy sooner or later. I doubt they came from a passive mixer; they were probably used as an interstage gain control in a program amp.
 
Thanks Dave, that's sort of what I figured intuitively. Really, I think I wanted to use them because they look so good. This preamp is going to have variable gain, and be used mostly with mics anyway. I'm probably just gilding the lilly.----Well, how would 50K be as an input for an unbalanced direct in for guitar or bass? Still kinda low? Anyway, thanks for the reply. I'll keep them around. Keith
 

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