Advice on VVR install

[dude]

Two inch square (old processor PC) heatsink and fan, install in a steel 5E3 chassis below, which way would be best for air flow?
Fan will be install on top of heat sink, which way, blowing air down on heatsink or pulling air out?
Chassis install with tubes down as shown.


al

[PRR]

Which way was it on the PC? Those guys usually know their heat.

[dude]

Good question, old PC, don't know. But there's lint or dust on the top of the fins of the heatsink, perhaps blowing down toward the heatsink. I've blown two mosfets in this amp so I'm trying this old PC processor fan.


I think most fans in PC's blow onto the element to keep it cool but I don't know, so I asked.

[92Volts]

Not a big difference. I've worked on PCs and the main reason to worry is if multiple fans serve one component or airflow channel-- you want them moving air in one direction, not fighting each other.

Blowing down (at the heatsink) should keep heat out of the fan itself, though it won't matter much.

Blowing up (away from the heatsink) should keep heat off of the chassis below the heatsink, though it won't matter much.

[dude]

Finished the VVR, works fine but after a few minutes a loud hum developed and stayed. I've had this problems before and posted here years ago a fix. For some reason if you tighten the small screw holding the mosfet to chassis tight, the hum is the results. I remembered this problem and loosened the screw a bit, hum gone. I think I mentioned this to Tubenit years ago and it worked for him too.


So, just a reminder if you get a hum, loosen the mosfet bolt a bit ,doesn't have to be tight. I assume being too tight may cause a slight short...?  I did use a plastic washer on the small screw, still the hum if too tight.


al

[92Volts]

There have been whole papers written about how tight MOSFET bolts should be! https://www.fairchildsemi.com/application-notes/AN/AN-4166.pdf

They just talk about thermal performance, but I'm sure you could eventually break it. I don't have a torque wrench that measures that low, I just aim for "medium" hand tightness (with a screwdriver, not wrench).

I like TO-220F packages which don't need insulators on the bolt or heatsink. Apparently the thermal performance is worse than regular MOSFETs with properly installed insulators, but it gives fewer opportunities to mess something up 

[PRR]

The power MOSFET must be tight to its heatsink.

If that causes hum, you have some other PROBLEM. It shouldn't do that. You might be right your insulator is poor. But note that HIGH voltages are applied on these insulators in many TVs and PC power supplies, without odd problems.

[dude]

I hear ya, I get a slight buzz at times, comes and goes. Then I loosen the Mosfet a hair more (still pretty snug) the hum leaves....? Other have had this problem on another forum years ago when they first came out for voltage reducing. Install in a 5E3, with a small P/C heatsink and fan as in above picture, Mosfet snug but not overly tight, used plastic washer on top of small screw, no grease. Amp played well, VVR worked well then I started to hear static, pulled guitar cord, still there. Figured tubes as it played well for twenty minutes before the slight crackle or intermittent noise came, similar to a bad tube. Replace one tube at a time, not the tubes..? Opened amp and loosen mosfet then retighten, not too tight as I would have but a quarter turn looser. No noise...? I used the clear plastic pad below, maybe I should try the gray rubber like pad...?  Seems if I loosen it anymore it would be too loose!


Maybe your right, another problem. The only thing I can think of is the insulator. The NTE-2973 I used came with a clear plastic insulator, I have other insulators that look and feel like a rubber substance, maybe I should use one of those...?   Should I use insulating grease and where?


Sincerely, al