VVR fun

[Milkmansound]

Installed a simple terminal strip VVR into an amp yesterday - as I brought up the Variac close to line voltage there was a big "bzzzzt" sound

of course the mosfet and diode were toast after that

turns out there was a hairline fracture in the mica insulator - enough for the B+ to arc through.

Fun!

[NickC]

Bummer!

[vibratoking]

Good job finding the fracture before you put in another FET. It could have gotten expensive.

[Milkmansound]

agreed - at $14 a pop its a lesson you don't want to learn more than once

as is mentioned in other threads, the back of the FET has the full B+ voltage on it. Its very important to isolate it!

I have since replaced everything that shorted and the amp has been running for hours with the VVR intact. Just that little piece of Mica

[mhartman]

agreed - at $14 a pop its a lesson you don't want to learn more than once

as is mentioned in other threads, the back of the FET has the full B+ voltage on it. Its very important to isolate it!

I have since replaced everything that shorted and the amp has been running for hours with the VVR intact. Just that little piece of Mica

I've used the ceramic insulators without problems. They seem to be more robust.

[johnnyreece]

I had one broken, as well. A little crack in the corner, and now there's a black spot on my chassis.

[Gaz]

Yeah, I use those big fat ones, like 3/32" thick. And Tim, that's way too much you're spending on those MOSFETs! Which one are you using? I use this one a lot, which is even a little overkill for smaller amps: http://www.mouser.com/search/ProductDet ... azGA%3D%3D

[ampmike]

Milkman,Love your site,I want one of those 5 watters for church,Perfect for that,It would also help my troubled back Nice work!!!!!!!!Mikey

[Milkmansound]

Yeah, I use those big fat ones, like 3/32" thick. And Tim, that's way too much you're spending on those MOSFETs! Which one are you using? I use this one a lot, which is even a little overkill for smaller amps: http://www.mouser.com/search/ProductDet ... azGA%3D%3D

Gaz thanks for the tip!

I am new to the VVR world and ordered 4 of the NTE 2973 mosfets that I think Dana uses. Now that I have my VVR legs under me, I will order some of those

@ampmike thanks! That little 5W is so small you can put it under a seat on an airplane

[xtian]

Yeah, I use those big fat ones, like 3/32" thick. And Tim, that's way too much you're spending on those MOSFETs! Which one are you using? I use this one a lot, which is even a little overkill for smaller amps: http://www.mouser.com/search/ProductDet ... azGA%3D%3D

Awesome, Gaz. Can you also point out the insulator you use?

[Milkmansound]

I bet this one would work:

http://www.mouser.com/ProductDetail/Aav ... eivwyZc%3d

almost as expensive as the FET

[Structo]

Yeah, those mica insulators are old school.
Use a good heat sink and a more modern insulator.

[Gaz]

Milkman, Those are exactly the insulators I use with an aluminum chassis. If it were steel I'd probably use a heatsink and some thermal coupling grease.

[vibratoking]

Yup, grease and heatsink with steel chassis or the knob quits working...the good thing is that the amp is running at full power when the knob quits...better than silence.

[Masco]

I use the product part number above from Mouser, but the product picture used above isn't the actual product. These are the thick white ceramic aluminium oxide type insulators. Make sure your mounting surfaces and drilled holes are smooth and clean, or these things will crack upon tightening.