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Scott Dorsey
 
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randomexpress wrote:
I've seen some tube mics(Korby?) with 5703 tube- a small triode.
Some even suggested that this tube could replace AC 701 in KM 54,56 other
neumanns.


It's a nice tube, the 5703. It was in production in the Raytheon plant
in Massachusetts until the late 1980s, and there are a lot of them out
there because they were used in a lot of military equipmnent.

Is this possible?
If so--how is it done?


Well, the plate voltage is in the right ballpark. I think that the filament
voltage may be different so you may need to replace the zener in the filament
supply. You will almost certainly need to change the cathode resistor so that
the tube is biased properly (and I don't recall what grid-cathode voltage it
wants but the Raytheon Tube Handbook will tell you).

It will sound different than the AC701. I think this is a good thing
personally.

The 5899 pentode is another possibility. Even strapped into triode mode
it will have much better top end than the AC701k. However, it is very
microphonic because of the tight element spacing required to fit a pentode
into that size package.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."