Just wondering if there was an opinion about mounting TO-3 package output transistors. Is it better to use thermal compound / paste or pre-cut thermal insulator pads? Or do both perform about the same?
Hi,
Paste properly done is always better. * Pads are quicker and more consistent, but worse.
rgds, sreten.
* Mica insulators may or may not be involved. If not its even better.
Paste properly done is always better. * Pads are quicker and more consistent, but worse.
rgds, sreten.
* Mica insulators may or may not be involved. If not its even better.
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Thanks. I ordered a set of 4 TO-3 transistors to replace a mis-matched set on a Dokorder 800X receiver. Great sounding receiver, but having 2 different pairs means that one channel is louder than the other, in this case the right channel. Not sure why it was done that way, but I suspect that the owner or repairman just wanted a quick fix and the particular transistors were available.
Hi,
Mismatched output transistors in nearly all amplifiers will
have no effect on the output volume. Sorry to say but
it is unlikely you are fixing the problem.
rgds, sreten.
Mismatched output transistors in nearly all amplifiers will
have no effect on the output volume. Sorry to say but
it is unlikely you are fixing the problem.
rgds, sreten.
Pads are the only option in your case, since it sounds like you're planning on using only thermal compound. If you do this there will be fire. You must use mica sheets too!
I recommend the pads, they are a much better solution. Mica is uneven in quality and thermal compound is always applied wrong, even by experts. Pads can't go wrong, as long as your heat sink is burr free.
I recommend the pads, they are a much better solution. Mica is uneven in quality and thermal compound is always applied wrong, even by experts. Pads can't go wrong, as long as your heat sink is burr free.
You're likely right. But there's also a big difference in heat between the 2 sets of transistors, so I plan to change them anyway.
Thermal compound is there to exclude air from the interface.
The heatsink to To3 interface is metal to metal. It is a shortcircuit to electrical current.
If your To3 needs to be isolated electrically you must use an electrical insulating film. That could be a Thermal pad, or a mica sheet, or an Alox pad.
Any of the "hard" films/pads must use a very thin layer of Thermal compound to exclude air from both interfaces, i.e heatsink to mica and mica to To3.
A soft Thermal pad does not need Thermal compound. The soft material squeezes into the tiny surface imperfections to exclude the air from the interface.
The heatsink to To3 interface is metal to metal. It is a shortcircuit to electrical current.
If your To3 needs to be isolated electrically you must use an electrical insulating film. That could be a Thermal pad, or a mica sheet, or an Alox pad.
Any of the "hard" films/pads must use a very thin layer of Thermal compound to exclude air from both interfaces, i.e heatsink to mica and mica to To3.
A soft Thermal pad does not need Thermal compound. The soft material squeezes into the tiny surface imperfections to exclude the air from the interface.
the difference in volume could be caused by a variety of reasons, some of them could be,
1. volume control pot (if dual gang) could be a source if gangs are not matched especially at listening levels.
2. amplifier gain setting resistors could be mismatched in both amps.
3. speaker could also be a source if the sensitivity of drivers is different on both channels. This can be easily confirmed/eliminated as a source of variation by changing left speaker to right channel and vice versa.
edit: just read your recent post, so you could eliminate reason 3.
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Thanks everyone. I just might mount the 4 output transistors onto a pair of extruded aluminum heatsinks as the Dokorder has them mounted directly onto the back panel with a shield over them. I have a Sansui 3000A that does the same thing, and it conducts heat everywhere on the back panel. Not a good design but typical of the 60s.
Here's the power amp part of the schematic. All 4 output transformers are NPN. And when's the last time you saw a mono speaker out? I could run it to an EV corner speaker if I had one.
The best solution is no mica washers and put the output transistors on two heat sinks to separate collector voltages. But use thermal grease to fill in gaps. I tend to use wooden chassis for my amps so can get away with two isolated heat sinks.
I did this on old Maplin 225WRMS amp and it barely ever got warm even when thrashed.
I did this on old Maplin 225WRMS amp and it barely ever got warm even when thrashed.
Nigel, thanks. I would have never thought about isolating the collector voltages. It makes sense to me. I found a little raised TO3 heatsink on eBay that should do nicely:
SS415 to 3 Holes x1 Aluminum Black Heatsink Heat Sink Audio Amplifier | eBay
SS415 to 3 Holes x1 Aluminum Black Heatsink Heat Sink Audio Amplifier | eBay
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