carvin r600 mosfets

[Mars Amp Repair]

Hi folks,
Am working on a Carvin R600. All the lit on the web (all 5 different revisons) indicates the power xistors are bipolar, but these are mosfet.

According to Carvin, they no longer stock these parts but gave me some substitute information...unfortunately, I can't find them either.

The power mosfets are MTW33N20 subbed by MTW20N20. The P channels are MTH20P20 subbed by MTH14P20. I don't find any of these numbers from any suppliers.

Has anyone had any success with any other subs for these guys? thanx, glen

[Enzo]

Go to Mouser and enter just the 33N20 in the search. WOuld any of those work for you?

Woould an IRFP250 work? I am in the process of putting a bunch of them in a Carvin F1200 in place of STW33N20. Don't know yet how well it will work. These are in TO-247, are yours?

I have a Behringer amp here that uses a single IRFP250 (TO-247) in the negative side and a pair of IRFP9640 (TO-220) in the positive side. One wonders at such a lopsided thing, I'ver never seen the like. I guess they don't make an equivalent part to the 250 of opposite polarity.

[Steve Conner]

Yes they do, it's the IRFP9250. Behringer probably used those IRFP9640 because they were cheaper, or because they stopped making 9250s.

MOSFETs have a standard numbering scheme. Something called xxx33N20 is a N-channel MOSFET rated for 33 amps and 200V in switched mode service. It's more or less interchangeable with any other MOSFET of similar voltage and current ratings, so I concur with Enzo, the IRFP250 and IRFP9250 probably would work, if you can even get the IRFP9250. P-channel MOSFETs are a dying breed these days.

However, all of these MOSFETs are designed for switching power supplies, and using them in linear amps is always going to be a lottery. The amp maker probably does a lot of tests to characterize them for linear use himself. Ecler used to test every one that came in the door and grade them into matched sets. Everyone else seems to just use large ballast resistors and hope the things will hold up.

The real problem is that the amp maker doesn't know when the MOSFET maker will make some little change to his process, that means the parts fail in linear use while still meeting the switched-mode spec. Carvin probably went through a bunch of substitutes, discovered none of them could take the heat, and redesigned the product with bipolars...

[Mars Amp Repair]

HMMM,
Yeah, I have some of the IFRP's, too. I haven't much experience with the MOSfets...most I see are in Ampegs & they seem indestructible...usually other issues in them.
I had to replace just one in a Crate, but those really need to be closely matched as the one I put in hogged all the current. I ended up ordering them from Crate & was fortunate that they matched the originals & I only had to replace the one.

I also noticed in this Carvin that on the sink side of all these Mosfets was a ranking number. I guess this is pretty common with the M-fets.

Great info on the mosfets Conner...that helps immensely.

I have to think my MTW's are the same as your STW's...Let us know how the IFRP's work in your Carvin...thanx All...glen