[Radio Wrangler]

Low thermal resistance heatsinks for TO-5 devices just didn't work. Most of them gripped the transistor around the cylindrical part of the cap which is not well-connected thermally to the die. A few gripped them around the flange where the cap is cold-welded to the header, which is a little better.

One oscilloscope family used TO-5 packages for the Y-amplifier output stage. The transistors ran appreciable power and needed heatsinking as well as possible. They got mounted on Beryllium Oxide washers shaped like those plastic PCB mounting pads, sitting on top of an aluminium heatsink with three holes for the three wires. But BeO is best avoided.

If good thermal contact is needed for a TO-5, the place to heatsink is on the underside, right where a 4th lead would have been, because the die is right above there.

But for a bias-control sense device, there doesn't have to be much power dissipated, so the aim is to get the transistor at the same temperature as the heatsink and protect it from ambient.

I've just been looking in the attic because I remembered having some stud-mount TO-5 sinks which mount the transistor upside down with its legs sticking upwards through a hole in a cap-nut. They relied on anodised insulation. I've not found them, and I don't remember there being as many as four. I did find a new CV450, so I'll take that along to Norman's doo for the TV people.

For temperature sensing, I'd go with a tapped hole, a screw, a washer, a top-hat bush, a silicone rubber insulating washer, and I'd pick a common TO-220 transistor or diode.

David