Tayda J201 and 2n5457

[RobA]

Thanks Rob.

Another question. How do you know when one is good? Is it if the hFe is in range?

Sorry, I missed this question earlier. I just build one of the JFET matching circuits on breadboard, usually the one from ROG because it's the easiest to knock together and it works well enough, and measure both the Idss and the cutoff voltage.

Some weird things can happen here though. Like the set of 2n5457's I had that were way out of spec but sounded really good in one of the ROG Fender circuits. The gain from the JFET was pathetic, but this somehow played into the circuit well and actually sounded really good overdrive and distortion wise. I could have designed a nice amp sim around them, but I've only got like 10 or 20 of them and it would have been pointless. I'd really like to know what the JFET's really are, but I can't find anything that actually matches them spec wise. My guess is that they were a set that measured out-of-spec and where scrapped and supposed to be trashed but were lifted from the trash and remarked and sold. They could be anything though. I've even seen reports of BJT's being remarked as JFET's and being sold. That doesn't seem to be the case with these though.

[billstein]

Thanks Rob.

Another question. How do you know when one is good? Is it if the hFe is in range?

Sorry, I missed this question earlier. I just build one of the JFET matching circuits on breadboard, usually the one from ROG because it's the easiest to knock together and it works well enough, and measure both the Idss and the cutoff voltage.

Some weird things can happen here though. Like the set of 2n5457's I had that were way out of spec but sounded really good in one of the ROG Fender circuits. The gain from the JFET was pathetic, but this somehow played into the circuit well and actually sounded really good overdrive and distortion wise. I could have designed a nice amp sim around them, but I've only got like 10 or 20 of them and it would have been pointless. I'd really like to know what the JFET's really are, but I can't find anything that actually matches them spec wise. My guess is that they were a set that measured out-of-spec and where scrapped and supposed to be trashed but were lifted from the trash and remarked and sold. They could be anything though. I've even seen reports of BJT's being remarked as JFET's and being sold. That doesn't seem to be the case with these though.

Thanks. Of course as usual this leads to other questions.

I just put together Jacob's Jfet tester
http://www.madbeanpedals.com/forum/index.php?topic=3098.15

You were talking about two measurements, it looks like this gives one. Would that be the cutoff voltage? Looking at a J201 datasheet it looks like a good component would fall in-between  -0.3 and -1.5. Is all this correct.

Thanks everybody for furthering my education in all this.

[RobA]

Yeah, it looks like that tester is measuring the Vgs(off). This works out fine from everything I've seen and measured for JFET matching because the Idss and the Vgs(off) move together. So, if two JFET's match in one parameter, I've always had them match in the other. For the Fetzer valve calcs though, you use both Idss and Vgs(off). For me, I was looking to see if they matched the specified values as well as building some sets to play with in phase shifter circuits.

Those values you have are the same as I've got in the spec sheet for the J201.

[billstein]

Yeah, it looks like that tester is measuring the Vgs(off). This works out fine from everything I've seen and measured for JFET matching because the Idss and the Vgs(off) move together. So, if two JFET's match in one parameter, I've always had them match in the other. For the Fetzer valve calcs though, you use both Idss and Vgs(off). For me, I was looking to see if they matched the specified values as well as building some sets to play with in phase shifter circuits.

Those values you have are the same as I've got in the spec sheet for the J201.

Thank you Rob for being so helpful. 

[RobA]

You're welcome and I hope you can find some good J201's (for not too much cash). It's a pain that they are getting to be so hard to find now.