maybe they're teflon and yes dwb I too think it's seeing bias voltage whenever the power is attached.

I used the plastic screws, because the holes are to small for standard insulators. I need to install a relay/switch so that yes the bias will be turned off when in standby. First mosfet amp I ever built, so yeah I have a few items to address yet.

The #4-40 screw insulators is what i was referring to. They are too big for hole in Mosfet. As these need to be electrically isolated from heatsink.

Last night, I installed a relay so that when keying circuit is activated, it turns on relay and supplies voltage to biasing circuit. That way there is only voltage applied when circuit is transmitting. Anymore ideas or suggestions are welcome.

Crusher

Thanks 43, never really thought of installing it, did not seem like a problem at time but as i said, it was something i overlooked. That amp likes about a 2 watt DK and works nicely. Put about 12 hours on it now and seems to work fine. But I know what GRTS meant about nylon screws, as they heat up, plastic tends to expand and eventually pressure is taken off of transistor and can't dissipate heat nearly as well and poof. Magic smoke. LOL

Someone had told me at one point that IRF550A mosfets would make for a nice small amp. Boy were they wrong as so was I for not looking at datasheet. Just in case anyone else was thinking about trying something slightly bigger in the same family so to speak.

The issue with the screws can be solved by drilling the holes of the transistor tabs lager and using the teflon shoulder washers or by placing a flat metal strap across the transistors with a single screw through it to the heat sink.

Why use MosFets. When you could put in 4x2879's and get quite a few more watts out of it.





Owner of the World biggest Galaxy radio.
4X2879's 550-600watts. PEP.

LOL, if I want a big box I will build a big box for myself. But I have more fun just building and experimenting with the stuff. Trying new stuff out.