David wrote:
I hadn't thought about the uA739 for a while. It was one of the
original low-noise amps - we used to use it for phono preamps.
If you could live with the anemic output drive, they were great
opamps for the time. I preferred the 749, which had an
open-collector output (no 5k internal pull-down resistor). I often
used external current source pulldown, bias network, Schottky Baker
clamps, and Class AB complementary emitter-follower outputs. Once
when 749s were unavailable but 739s were plentiful, a Fairchild
engineer taught us how to open-circuit the 5k resistor cleanly to
turn 739s into 749s. The plastic DIPs were prone to
sealing/passivation problems, so we always bought the ceramic parts
(I pretty much always customized the compensation, so we couldn't use
the 8-pin hermetic parts).
Re: glass piston caps, I have been able to sometimes get what I need
from Surplus Sales of Nebraska
You can get ceramic piston trimmers from the usual primary
sources. For example:
http://www.spraguegoodman.com/pdfs/660.pdf
http://www.eyou.com.au/product/johanson-5202-rf-air-trimmer-capacitor-08pf-10pf-/
http://www.johansonmfg.com/pdf/Air-Capacitor.pdf
Eastern European clones show up regularly on ebay at much lower
prices. The ones I've bought have been made to high standards and work well.
I think glass piston trimmers are still available from primary
sources, too, but I don't have a link handy.
Best regards,
Charles
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