# Question about transistors and heat transfer



## ahardb0dy (Feb 19, 2012)

Hello, I have a question about how transistors mount inside an amplifier, old Art series PPI in particular.

The rear cover has those metal fingers that press against the transistors, is the purpose of the fingers to push the rear of the transistor (the metal part) up against the mica or Kapton tape so that the heat from the transistor "couples" to the metal heat sink of the amplifier?

Does any heat normally transfer from the plastic front side of the transistor through the metal fingers?

I'm modifying my PPI 5075DX to eliminate the metal fingers, so far I have done one side. I used 1/2" square tubing that bolts into the holes the rear cover used to bolt into, I drilled the holes on the top of the tube so the Allen head bolts would fit inside the tube, than I cut a length of 1/2" flat stock to fit over the metal tubing. I than drilled and tapped holes so the flat stock bolts down to the metal tubing and than drilled and tapped 3 holes in the flat stock so the rear cover can be bolted down to the flat stock, it sounds complicated but it isn't, and it works so far. Of course I had to get rid of the fingers on the rear cover to clear the metal tubing.

When all is bolted together from the outside you can't tell anything is different than stock and the way I am doing it, the rear cover can be removed at any time with out bothering the transistors.

How much pressure should be applied to the transistors? Should it be just enough so they are sitting flat against the Kapton tape and the inside of the amplifier case?


I was talking to someone who was staying with us this past week and he kept telling me he thinks the transistors use the metal fingers to transfer heat but I told him I think they just put pressure on the transistors so they transfer the heat to the heat sink..

I know either Mica insulators or the Kapton tape is an insulator, I assume so the transistor is insulated from the metal heat sink under them, but the heat still transfers using heat sink compound to the amp case/heat sink.

Is this correct?

Thanks


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## ahardb0dy (Feb 19, 2012)

Ok, so this is what I did so far, in pictures:

This is with the metal tube bolted down over the transistors, with the flat stock bolted down to the tube, the bolts sticking up are what attaches the rear cover to the flat stock:










This is with the flat stock un-bolted, laying on the edge of the amp, the tube is still bolted to the amp chassis :










This is the metal tube removed with the Allen head bolts sticking out the top side, flat stock in background:










Close up of the top of the tube showing hole the Allen bolt goes into, threaded hole the flat stock bolts down to next to large hole:










bottom view of the tube, had to make the holes slightly larger than the bolt threads as I had some alignment issues, larger hole allows some play:










Flat stock spacer I made because the Allen head bolts the hardware store had were to long, shortest bolts they had in 6MM:










the 3 parts and bolts laid out in the order they are when assembled:










Should I use anything between the metal and the transistors? I noticed on my Orion amp they use a similar method of holding the transistors down and it looks like they have something between the transistors and the metal. See below pic:


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## ATOMICTECH62 (Jan 24, 2009)

You did good.Yes some heat is transferred through the front of the transistor but its not much.Any way your clamps would work if it really did.
When I rebuild an amp with long runs of transistors like those I cut a long strip of cardboard to go between the transistors and the clamp.Like the Orion does.
You do realize there are two mosfets missing from the power supply.


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## chad (Jun 30, 2005)

ATOMICTECH62 said:


> You do realize there are two mosfets missing from the power supply.


LOL as are the traces for one of them, seek and destroy repair.

I may add that yeah, that chunk of metal will probably do BETTER but I'd add more screws and tapped holes to more evenly distribute the pressure. like the orion is done. The cardboard trick mentioned above will help a lot, they also have a closed cell foam designed for this application too.


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## ahardb0dy (Feb 19, 2012)

The 2 missing transistors are the 2 that got fried due to me taking the rear cover off to see what the tech did on the last repair, I guess the fingers weren't applying the proper pressure on those 2. 

I wasn't sure if I should use something between the metal and the transistors, that's why I asked and mentioned the Orion. I noticed the Orion has a lot more screws but it seems the transistors in the Orion are spaced further apart than on the PPI.


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## chad (Jun 30, 2005)

as log as it's even pressure, rock it. I can't remember which amps I worked on used the foam like I mentioned, I wanna say old QSC, but it escapes me.


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## TrickyRicky (Apr 5, 2009)

The transistors datasheet has the torque specs. I believe its 10lbf-in, but that's just my guess.

The heat of transistors is always dissipated from the metal part of transistor, the rear/drain.


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## ahardb0dy (Feb 19, 2012)

Today I cut a piece of fiber gasket material to sit between the transistors and metal tube.
I bolted the tube down and connected the amp to my power supply and an old Kicker 12" woofer and a small 4 " Boston Acoustic driver, both 4 ohms.

I ran the amp for over an hour, the heat sink got a little warm but the amp played fine, on the high channels anyway. The low side is still missing the 2 transistors.


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