3n138 equivalent

3N128 is 20V and a wider Gm spread. 3N139 has slightly higher capacitances. 3SK33 is 25V and lower cap. 40467/40468 are 20V were considerably less rare a few years ago.. Pro-Electron subs would have different pin-out., but BFS28 at 20V would ~fit.

What are you trying to do? If this is for a repair of useful equipment, it's cost is probably less than the labour involved or (likely) the cost of replacing the equipment.

If it's for a new project or an app note, then perhaps another n-mos depletion-mode device or analog switch would work just as well, particularly at low frequency.

These parts are expensive probably because they are no longer mfred, and are static sensitive - diifficult to store (for 20 years), ship or install. If it arrives in a simple plastic or paper container, send it back.

RL

A dual gate mosfet? Or is it so old it only has one gate? I can't remember.

To a large extent, it seems like small-signal RF mosfets have faded out. They were big at one time, but then people learned more about transistors and realized some of the problem with bipolars were they weren't being used properly. Once they got over the mentality that the key to transistors were low power, it seems like bipolars got better at handling strong signals, and the need for mosfets (that were once touted as being so great for strong signal handling) faded.

So basically you are looking for an RF dual gate mosfet, and one that won't cost a lot. It's like small signal germanium diodes at this point, the key being germanium rather than part number.

One place that has a small selection of mosfets, and is set up for dealing with hobbyists, is "Dan's Small Parts", and you might look over his webpage at

at the very least, it might give you an idea of what might be available to search for those numbers at some other parts place.

Michael

It is for a repair, and you are correct, the labor and replacement cost far outweigh the cost of the FET. However, I don't know if it's bad, I don't have any experience checking these types. Pinout isn't important, the board has room for bent pins.

Thanks, Steve

This is for an audio oscillator, single gate. I'll give the site a shot.

Thanks, Steve

You can turn a dual-gate mosfet into a single-gate mosfet by connecting the two gates together. Of course all of the other parameters may not be the same. If you post a schematic of your application circuit then people may be able to suggest a substitute that will work there.

Chris

Actually, I did some testing this afternoon, and the original fet is fine. The problem lies in one of the op amps, as I posted in the CIC replacement parts post. It would be nice to have a backup, but since it's still working, I think I'll just keep using it.

I appreciate the help though,

Thanks, Steve