Amp on Fire!

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I'm trying to figure out what I did wrong the FIRST time I built my Leach amp...

I have bare aluminum heatsinks for the TO-3 transistors. Right now I am using rubber insulators and aluminum screws to fasten the transistors to the heatsink. I know this is wrong because I am shorting the +Vcc and -Vcc to the grounded heatsink.

I know this is a newbie question, but how do I electrically and physically connect the transistor socket (on other side of heatsink) to the collector of the transistor without touching the heatsink?


Assuming that I'm visualizing what you're describing correctly, the solution is easy and cheap. They make little plastic cylinders that slide over the base and emitter pins of the transistor and keep them from contacting the heasink. The transistor itself sits on a mica insulator that takes care of the collector. There are newer insulators that some folks here like. Go find one of the threads on them. Berylium oxide, I think, and silicone, and probably something new by tomorrow.

If you tighten the screws too much on silicon rubber
insulators, it's very possible that the sharp corners of the holes in the heatsink will bite through the insulators and short the supply rails to ground.

Mica insulators almost can't be damaged by this method; at any
rate, put on new insulators and test the supply rails with an
ohmmeter before you procede further.

Also, the sockets themselves should have shoulders that align
with the holes in the heatsink and precisely space the mounting
screws so they won't touch the heatsink.

Been there before, believe me.
Transistor insulators

For the base and emitter pins of the transistors, I prefer heat shrinking tube. For the collector, I use a soldering terminal tag (that washer with a solderable extension...). To make everything safe, you can even use plastic screws !!!

Good luck!
For me mounting TO3 transistors has been simple. A parts dealer here sells kind of "TO3 kits" which contain two screws, two spring washers, two nuts, one washer with solderable extension, a mica insulator and two plastic washers.

Now, it's mounted like this. The diamond-shaped mica insulator has a 4mm hole in each end, and two 3mm holes for emitter and base pins. These are marked onto the heatsink, and drilled. I just use a 4mm bit for all 4 holes. And as Prof. Leach recommends, I spin a bigger drill bit by hand in every hole to remove any burrs and sharp edges.

I coat the transistor's "bottom" with silicone grease, put mica insulator in place and then spread some silicone grease to the insulator's yet uncoated side.

Then the transistor is put in place. The plastic washers go into the two holes where the screws will go (that is, on the opposite side of the HS from transistor's viewpoint). Before that, the washer-with-solderable-extension goes next to one screw's butt _on_ the plastic washer which 1. aligns the 3mm screw in the 4mm hole so that it, carrying collector current, will not touch the grounded heatsink and 2. prevents the extended washer from touching the heatsink, being between them.

Then the spring washers and nuts are put on the transistor's "top" side and fastened. Just for additional security, I put short, about 1/3" pieces of insulator from a wire to the emitter and base legs, so that they will not touch the heatsink 1. if the transistor happens to move/misalign itself before being fastened or 2. they get bent accidentally.

This will also gain some forgiveness if the holes in the heatsink were misplaced by just one mm in the first place.

I've started buying the mounting parts separatedly now, because that way it's much cheaper.

-Kimmo S.
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