Thanks Mike. I already talked to John about this one. It's not the 990C version that works at multiple voltages, it's the earlier 990 version (no "C"), that only works at the specified voltage. This one happens to be a 24V version.
To quote John:
"The clear 990 is not a "C" version. If it was a "C" version, it would say "990C" (or, "990C+" for the latest variation -- see the 990 data package for details). The clear 990 is the "original" version, so it is designed to work at specific power supply voltages. On the bottom of the clear 990 it will show the power supply voltages it is intended to work at, either "12V", or "15V", or "18V", or "24V", meaning +/-12VDC, +/-15VDC, etc. The "C" version has additional circuitry so that it can operate over a wide range of power supply voltages, anywhere from +/-12VDC to +/-24VDC.
The clear 990 and the 990C should work identically, assuming you run the clear 990 at the proper power supply voltages.
They were manufactured from 1979 until about 1988 when I decided to use an aluminum potting shell for each 990. The two large output transistors of the clear 990 are plainly visible. For the "C" version those two transistors were moved to the edge of the circuit board so they could be in direct contact with the wall of the potting shell for maximum heat-sinking. It is not as fun to look at as the clear 990s, but it is a better package thermally."