Heatsinking TO-3's for small power amp

Ive got a small 15WPC amplifier that is ready to put in an enclosure. The output transistors are in TO-3's and I'm looking for some method to heatsink them in a 1U rack unit. The idle current is not to high but when run full open for a while they o/p trans get hot. When in an enclosure I know it will be a bigger problem

It seems I have seen heatsinks that are about 1U high (1.75") and mount to the backside of the rack stretching halfway or so across the width, leaving enough room for i/o connectors. Does anyone know of something like this with TO-3 mounts (4) on it.

How about any other ideas? The TO-3's are not directly mounted on the circuit board, so I do not have that restriction.

Maybe a quiet fan? or an air circulation system? Obviously it doesn't have to be to extreme with this power level.

Thanks all,
Brian

Hi Brian,

It may be a bit tricky to find a heatsink which is only 1U high pre-drilled with 4x TO-3 cutouts. If you don't fancy drilling it out yourself (use a mica heatsink washer and a sharpie as a drilling guide) it may be easier to mount the devices horizontally in the case and cut vent holes in the front, side and rear panels. I use a chassis punch (Greenlee etc) to punch a 25mm or so hole, and then cover the hole with metal gauze held in with pop-rivets and washers for a neat finish.

A 15W per channel amp shouldn't run that hot, and if the internal heatsinks are adequate, natural convection thru the chassis and vents should provide enough heat sinking.

If the internal heatsinks are thermally coupled to the mass of the chassis too that will help. Don't forget insulating washers for the power transistors if you bolt heatsinks to the chassis.

One thing to remember with "passive" heatsinks, is that their efficiency depends on the external temperature of the equipment. IOW, if the equipment is sitting in a rack between two bits of valve gear, its' external ambient temp will be much higher than if it was sitting in a rack with 1U of air above and below it.

Probably the best thing to do is build the amps, run them out of the case with heatsinks at the sort of output you'll be running them at, and see what happens over a period of time.

I'm not keen on fans in audio gear, due to audible and electrical noise. But clever layout, heatsinking and vents can avoid them.

:thumb:

Mark