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FET substitute?

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Mark Black
Posted on Wednesday, January 26, 2005 - 11:43 pm:   

Anyone happen to know a cross-reference for a FET marked as follows:

SF
51176
619

It's a TO-92 case, used in the tremolo circuit of a 2475-130 amp.

Thanks!
Mark Black
Posted on Thursday, January 27, 2005 - 10:54 am:   

Oops - looks like I found the answer elsewhere in this forum.

If I read correctly the FET that should have been there is a 2N4091 - don't know what that other one is or why it's in there...

Thanks again -
plugger

I also found a TO-92 FET

I also found a TO-92 FET marked SF 51176 in the tremolo circuit of my 2275-130 (210 HD-130) amp while I was troubleshooting a noisy tremolo (would introduce distortion as the intensity dialled past ~4).

I was surprised, as I had expected to find a 2n4091 there as indicated in the schematic.

Two strange things: Firstly, The SF 51176 is very hard to find any info about. It's marked with a National Semiconductor logo, but I can find much else about it anywhere, can't find it mentioned in cross-references or anything like that. Searching online, it's like it never existed!

Here's the second strange thing: I tried replacing it with a TO-18 2n4091 I ordered as a replacement part in case it was the FET causing the distortion problem, but I couldn't get the 2n4091 to work in that position! Tremolo wouldn't function at all (as if the FET was not gating). Biasing issue? I tried the tremolo trimmer in all positions, but no go.

I'm wondering whether there is a change in value of one of the other components that works with the "mystery" FET, but doesn't work with the 2n4091. I didn't see anything obvious, but I haven't yet checked exhaustively by tracing through the circuit board and comparing to the schematic.

The other possibility is that I got a bad 2n4091, of course, but limited checking with a multimeter doesn't indicate an obvious problem (the gate is acting as a diode to both source and drain.)

Anyone else come across this mystery part? Are there any other FETs besides the 2n4091 (which are now obsolete and hard to source) that are known to work in this position? (I have some MPF102 on order for another project so I will try one of these when I get them, even though it doesn't look particularly close to the 2n4091 spec).

BTW, the problem turned out the be cap C25, which I replaced with a 0.1uF 63V greencap, and so the distortion issue is solved. But I'd still like to know what the deal is with the FET... I'd like to have some confidence that I can find a suitable replacement if it ever does decide to pack it in!

BTW, thanks to Steve and everyone for this forum (and the old archived one)! Truly a goldmine of info for MM amp owners...

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