rating power MosFets?

[Mars Amp Repair]

Hello,
Has anyone found a method for 'rating' those IRFP0240 & P240 Mosfets that Crate & Ampeg use?

I have a bunch in stock that are not rated. From experience I know they don't play well in the sandbox together if they're not somewhat matched.

Fortunately, they are only about $3.50 ea from Loud.

glen

[rf7]

Or, if you're not a service center, they're around $6 each. And they take a week before they ship them out to you.

[fyl]

Has anyone found a method for 'rating' those IRFP0240 & P240 Mosfets that Crate & Ampeg use?


Try http://www.passdiy.com/howto/matching.htm

[MarkusBass]

Do you mean: IRFP9240 ?

Here: http://www.tme.pl/katalog/index.phtml?f_szukaj=IRFP9240&idp=1 you can buy them for less than $2.

Marek

[Mars Amp Repair]

Nice,
but are they rated? I've got a bunch that came with an aquisition, but none matched & I've stopped using them as they just create more time & trouble.

Loud puts an orange dot in the upper corner of their marked ones, but I recently worked on an Ampeg SvT6pro that also had the P-channel ones marked with a Blue dot.

I'm not certain if they need only to be matched for Crate/Ampeg, or if they also need to be within a certain ranking range to work well in those amps...g

[Steve Conner]

I'd suggest making a matching jig, with two power supplies (maybe 10V for the drain- doesn't matter much - and a variable one for the gate) and a source ballast resistor the same as what is used in the amp, so 0.47 ohms or whatever. The (-) of both power supplies return to the bottom end of this ballast resistor. You're basically making a replica of one section of the amp at idle.

You also need a big heatsink to keep the temperature constant, since the bias current varies a lot with temperature.

The idea is to put one of the known good FETs with the paint dot in there, and twiddle the gate bias till you get the idle current as stated in the service manual. Then take that FET out and try a bunch of your replacement ones, without changing anything, and pick the ones that run closest.

I hate the whole practice of using switching power MOSFETs in linear amps, but I guess it's here and we need to live with it... :(

[Mars Amp Repair]

Yeah, I meant IRFP...
Sounds like a lot of work for a $3.40 part, but I have enough of these that it might actually make sense to try it. Esp when I only need to replace one out of a bunch, it would be nice to just snag one from stock.

I've got a number of old sinks I can use. I also have one of those Tenma type triple current limit adjustable power supplies that would be perfect for this application.

Thanx for the suggestion. If I ever get to it, I'll share the results. thanx, glen

[MarkusBass]

I'm not sure whether the term "rating" is correct in this case. But if you search google for "MOSFET matching", you will find several sites that describe how to do it.
For example:
http://www.passdiy.com/howto/matching.htm
http://www.susan-parker.co.uk/zeus-mosfet-matching-1.htm
http://www.altavistaaudio.com/MosfetMatching.html

Marek