[R-390] OT: Pass Transistor Question

[Barry]

I'm sorry to keep asking OT questions and if someone could suggest a better forum for this (and if Don would rather me take this 
elsewhere), then please do so; however, I think this one should be an easy answer.

I want to mount the TO-3 pass transistor(s) and TO-3 regulator on heatsinks that are open to the backside of the cabinet.  The problem 
is that the cases of the various devices are not common and they are at something other than ground potential so I'd like to insulate 
them from the back panel.

I think the traditional way to do this is to use a mica insulator under the device (between the case and the heatsink) with insulating 
sleeves around the screws and possibly mica washers on the other surface (to insulate the mounting screw from the back plate in my 
case).

The thing I'm not sure of is whether mica is thermally conductive.  I assume it is since this seems to be the way this is done but I 
haven't built like this before so I wanted to ask.

The heatsink "kits" I've seen have the mica to go under the TO-3, insulating sleeves for the screws, but I don't see what insulates 
the screw from whatever it is mounted to.  I assume another mica washer would work but they don't seen to include them.  Was this the 
way it's done?

I got the stud-mounted rectifiers and they have two mica washers along with an insulating sleeve for the stud so mounting them so that 
they are electrically floating is no problem.  I'm just having trouble figuring out how to do the same for the TO-3 devices.

I bought some nice threaded ceramic standoffs that I can use to mount small aluminum plates which would work, but it seems cumbersome 
and I think there are better methods I could use.  Looking at the back of my Astron RS-35M, it appears the transistors are mounted 
this way, but without digging in to it, I can't see exactly how they're mounted.

Thanks and, again, if this needs to be taken off-list, please just let me know.

Barry - N4BUQ