It’s rare that a fixit job makes me feel like a rockstar, but this one made me feel invincible, for about an hour or so. A client brought in a pair of beautiful Sony C-57 tube mics for rehabilitation. As far as I can glean, they use some of the same components as their highly regarded C-37A, but in a slim sculpted housing reminiscent of the Shure 300 series of the mid-50s.
Because of the constricted environment, Sony used a miniature triode wired directly to the circuit board. This tube, the 6D-H3, is apparently impossible to find, so when one dies, the mic is essentially useless. This is exactly what had happened to one of the mics (the other was usable but noisy, which apparently was par for the course with this model). I despaired of finding a replacement tube, but I was heartened when I saw a messageboard post by David Royer (of Royer Labs fame) offering to try and fix a broken C-57 and see if he could retrofit a substitute tube. I dropped him a line and got to work on the power supplies and cables.
I kind of wish I had taken a snapshot of the mic cables; they literally crumbled in my hands, and the thought of +/- 265 volts passing through them was more than a little frightening. I replaced them with new Mogami, and rebuilt / renewed the old rubber strain reliefs as needed. The power supplies were beautiful to behold both inside and out, but removing the ancient (and very sturdy) Nichicon can capacitors took a loooong time and quite a bit of patience. After adding proper 3-prong grounding and bypassing the noisy low-cut circuitry (does anyone ever really use it?), I was anxious to get the actual mics up and running.
Eventually it became apparent that, generous offers aside, Royer Labs had better things to do than repair mics they had never made in the first place. At the same time, a dim Christmas-size bulb of insight flickered to life in my head: If the mic used many of the same components as the C-37A, why not try to build the same circuit inside the C-57? One advantage of this is that, unlike the C-57, the C-37s tube (6AU6 strapped for triode) is readily available; a matched pair of N.O.S. RCAs set me back $10.
The main challenge was going to be real estate: The inside of a C-57 is an extremely constricted environment, and excess heat could damage the sensitive capsule (I learned that a later Sony mic, the C-800, used a passive Peltier cooling system for this very reason. However, with the circuit board removed, there was just enough room to squeeze in a 6AU6, positioned upside-down so as to allow the shortest possible cable run from capsule to grid.
I constructed a tiny wiring harness out of surplus tube socket pins, shrink-tubed the hell out of the connections so as to avoid a short-circuit in the cramped environment, crossed my fingers and slowly brought the power up.
Wha la! After a false start or two optimizing the grounding arrangement, the new circuit worked perfectly, and was noticeably quieter than the original. I was still concerned with heat dissipation, but after many tests ranging from a few minutes to many hours, the exterior of the mic housing was slightly warm to the touch, about what I would expect from a vintage tube mic.
In retrospect, what I did with these mics was NOT rocket science; working cleanly and carefully, I mated a proven circuit with a compatible and well-designed power supply. But as anyone who knows me knows, I can’t pass up saving old junk, especially of the audio variety (come hang out in my basement some time and you’ll see what I mean). But in turning these beautiful (and beautiful sounding) antique mics into something usable (and safe), I really felt like I had knocked one out of the park.