How do you mount your transistors ?

What type of cases? In my A75 amps which run now for almost 7 years without failure I used silicon rubber insulators (no grease). For my upcoming Aleph project I will use beryllium oxide insulators and I suppose I have to use grase on them.
I tap the holes in the heatsinks and use screws to hold the devices. I think Nelson Pass mentioned somwhere on the forum the torque value. Then I p2p wire the output stage.
 

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Yes, I also would like to know what is the proper way of mounting plastic transistors (.... yes... for Aleph 2). Some of the questions are:

1. Insulator type. Always recommend vendors and give part numbers if possible...... this way, I can look it up on the web site ... and in the future .... I'll know where to go to look.
2. Grease type. Do you use just a little or a lot per transistor? Do you apply grease on both side of the insulator? Vendor and part number too .... please.
3. How do you actually mount it on the heatsink? Do you have to use machine screw and nut or just machine screw only. Doesn't the screw come loose after a while (if the nut were not used) because of the heat expansion when it's on and cooling contraction when it's off. I've asked this question before somewhere else .... I would like to hear more opinions. Is there any particular type of screw that is preferred?
4. Do you mount the transistors on the middle of the heatsink as opposed to one side of the heatsink? I think I know the answer but does it really make a difference since eventually the heat will be spread out to the whole heatsink.?
5. Is the hole on the plastic transistor pretty standard? What is a typical diameter for the hole?
 
Harry Haller posted a pic of the internals of a power amp (Musical Fidelity?). The power transistors were TO-220 or T0-247 directly soldered to the PCB and were not screwed individually to the sink but clamped to the sink by aluminium bars that screw into the sink. I like this idea as you don't need the PCB and sink mounting holes to line up exactly. There's room for misalignment - the transistors automatically fit the PCB and will be clamped as long as they fit somewhere between the clamping bar and the sink.

I don't remember the thread or I'd direct you to the picture. Maybe Harry will remember ... I'm sure he's lurking out there somewhere.

Regards
13th Duke of Wymbourne
 
OK here we go... i usually use mica and a TINY bit of thermal grease unless the sink is internal and can be left "floating" or the package is completely plastic. Common steel screws do the fixing, using these small plastic separators to keep the package from connecting to the heatsink through the screw... thats pretty much it.
 
I am more interested actually in the screws people use. Opening machine 3M screw holes is not always easy. I was wondering if I can use instead steel srews that are use in the steel metal cases. I think this would work if I open a little smaller holes before and then the screws open their way.
 
Peter,

"5 minutes epoxy" ..... you mean glue it and not be removable (unless you use a crawbar)? Is epoxy a good conductor of transfering heat? Won't the heat cause the glue to melt (esp. for Aleph 2 type of amp )? If that's what I think you meant, it should be real easy to install like you said!

Hey, I'm curious, does anybody installed it this way?
 
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promitheus said:
I am more interested actually in the screws people use. Opening machine 3M screw holes is not always easy. I was wondering if I can use instead steel srews that are use in the steel metal cases. I think this would work if I open a little smaller holes before and then the screws open their way.

I use a slightly longer screw, nut and a washer. Just drill a hole slighly larger than the screw, deburr carefully and you're set.
 
Lisandro_P,

You use a nut because of .... ? I was thinking of screws coming loose if nuts are not used. Come to think of it, if one drills a hole through the heatsink, won't it spoil the appearance of the amp - esp. if you mount the heatsink outside of the case. If this is the case, what would you do?
 
fcel said:
You use a nut because of .... ? I was thinking of screws coming loose if nuts are not used. Come to think of it, if one drills a hole through the heatsink, won't it spoil the appearance of the amp - esp. if you mount the heatsink outside of the case. If this is the case, what would you do?

I think he meant screwing directly to the case (you drill the hole sligthly less wider than the screw's diameter then force it in); i've seen this done quite a few times and done it myself too, but it's hard, needs good screws and i just feel safer with a nut there.

I don't understand what u meant with "drilling holes through the heatsink"; i mean, if you want to mount devices reilably you'll HAVE to use a screw or some sort of clip-on method... :confused:
For the record, my amp uses TO-3 cased transistors, so the case (and their screws) are always visible on external sinks; i just live with that. It doesn't look that bad either :) It depends i guess, but usually, a screw shown over a heatsink is not that bad (for me atleast). You could paint the screw head to match the heatsink, f.ex.
 
I am planing in making an Aleph with external heatsinks. I am not so sure what to do with mounting the transistors. I haven´t tried making holes for M3 screws which is the best but hard to do. For this amp I dont want to drill holes through them and you also can´t do that because the heatsinks dont have any flat surfaces on the outside in the first place. One good idea I was thinking of doing is to use alu pieces and press the transistors down on the heatsink. In this way you can use M4 or M5 screws which are easier to use.
 
Hi Promitheus,

when you use external heatsinks for an Aleph they normally are at least 8-10mm thickn. It´s quite easy to drill a 2.5mm hole about 7mm deep an tap some M3 tread in it. This way you won´t see anything on the outside and you don´t have to fumble with M3 nuts. The other option with a thick aluminium bar is also possible although you should use a screw between every fet otherwise it´s not garanteed that all are pressed equally to the heat sink.

I use kapton isolators for the TO-247 and TO-3 cases cause they have a very low thermal resistance (0,07K/W) and you don´t need to use grease.

william
 
I've yet to build my own amp (I promise to start soon ...), but there's one thing about mounting transistors that still has me confused. You insulate the transistors with grease, a washer, whatever, but then you screw them down. A screw is made of metal, and it makes contact with the transistor AND the heatsink. Wouldn't that defeat the purpose of insulating in the first place?

Have I overlooked something?
 
Screw & Nuts

HPotter,

Where do you get your hardwares normally? I have so much frustration trying to get some screws and nuts in Toronto. I tried home depot, canadian tire, sayal, active surplus etc.. either they have the screw but out of stock with the nuts or washers. Or they do not have the right length. To make the amp look really ugly I managed to use all mix and incorrect subsituted mix bag of screws and nuts, some are pan head some are flat head, some are brass, square, cross, even wood screw had to do too sometimes. Not happy at all.

Chris
 
lordvader said:
I've yet to build my own amp (I promise to start soon ...), but there's one thing about mounting transistors that still has me confused. You insulate the transistors with grease, a washer, whatever, but then you screw them down. A screw is made of metal, and it makes contact with the transistor AND the heatsink. Wouldn't that defeat the purpose of insulating in the first place?

Have I overlooked something?

the plastic transistors we generally use in output devices are in TO-3P / TO-247 package i cant quite remember off the top of my head but i believe that the TO-247 has an insulated mounting hole and the TO-3P has a live mounting hole thus requiring a sleave for insulation. Then again, it may be the other way around.
 
You don't have to insulate screw in TO-247 because the metal doesn't touch the hole. For TO-3 you need plastic washers with sleeves to isolate the screw.
I don't recommend using bar to hold the transistors. It's just extra work and material because the amount of holes is almost the same. The alternative to using M3 screw (which for that purpose really sucks because of fine thread) is using SAE screws which I believe are available in Europe as well. The choice here would be #6-32 which is very easy to tap. The hole size is 7/64".
I get my screws from aircraft industry but they are easily available at Home Depot or any other building supplies depot. You can even get stainless steel which might be preferred for non magnetic character.
As for using epoxy;) I only used it on small transistors which were not a part of the output stage.