weird tube mics

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Murdock

Well-known member
Joined
Jan 28, 2015
Messages
844
Location
Germany
Hey folks,

a few month ago I got two mics which looked quite nice from the outside. They were custom build by someone probably in the 50s or 60s. The capsule is also quite interesting as it is "spring tensioned". Never saw that before. The only manufacture engraving on the capsule was "Rojac".
From the inside they looked really bad. I mean the builder probably knew what he was doing but service friendly it was not... All parts were taped together. There was no internal structure. I had to cut the cables to even get all out of the shell.
I'll post some pics of the actual mics later.

Anyway, I redraw a schematic and I'm quite puzzled. Not that I know anything about tube mics, or electronics in general but I've never seen anything like that before. 



Sorry for the messy drawing and writing. I hope you can read everything.
The two questionmarks are probably "2,2nF" and "10nF" ceramic capacitors. Sorry for the wrong symbols.
I'm also not quite sure about the pinout.
The parts that seem odd to me are the grounded cathodes without any resistors in between and that of both, the primary and secondary of the transformer, one leg is grounded.
Can someone more knowledgeable tell me if that makes any sense? Would that mic even work like that?
I could not test it as the power supply is broken...
 
It is unlikely that there is a 2.2 M resistor between the capsule and the grid of the tube.
(This would mean a positive grid voltage!)
I suppose this is a capacitor.
The 2.5 M resistor between the voltage divider and the capsule seems too low for me, I would expect something over 50 M here.
 
Hey Ruud,

thanks for your thoughts!

Yes, as I mentioned, the 10nM and 2,2nM parts are probably capacitors.

I know now that the mic is "grid leak biased". I read a bit about it and it seems to be not so bad with low input circuits.

What do you think is the B+ in this circuit? Probably higher than the "normal" 120V, right?
 
Yes you are right, I immediately looked at the schematic...
I think 120 V would be a good starting point.
You should at least hear some sound.
A cheap Chinese power supply would be good enough, if you don't have the original power supply.
( https://www.thomann.de/nl/the_tbone_psu_sct2000.htm )
 

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