Jfets discussion

[full shred]

Hi to all.

Here is a good topic to discuss about the quality of jfets.
The most commonly used in pedals is j201. BUT i red in this forum http://www.ssguitar.com/index.php?topic=464.0
that j201 is not the best one that should be used. They say that 201's are just easy found and used just because they are the standard.

so what other substitutes are there that can be used?
1. What fets are more naturally sounding?
2. What fets have more gain?

any thoughts or opinions?

[atreidesheir]

Be aware RG is directly writing about using j201 as a buffer and listing good subs. He is a very good source of info about most things we discuss around these parts.
The j201 is getting rather expensive due to being out of production, but so are the other jfets.
The j201 has unusual biasing.

RG said in that thread:
"The parameters of JFETs vary about 5:1 to 10:1, so they are quite difficult to get exact biasing points on and exact gains. This is one reason they never got the wide application of bipolar transistors. In a source follower, though, the actual JFET is not as critical. You want a Vgsoff that's a significant fraction of the 9V battery, but not most of it or bigger than 9V. One around 4.5V is good for a 9V source follower."

OK. The wonky biasing is why you see variable resistors (trim pots) in the runoffgroove and several other jfet preamp emulators. The biasing of the j201 can be all over the place, so the variable resistor will get you to 4.5v point or very close where you get the best sound most of the time.

Some of our great friends here at FSB hate the guessing game of biasing 201's. There are some persons whom I respect that love other jfets in their circuits. Bajaman loves the 2sk117 for lots of uses. They are great, but much higher gain that j201, so you have to tweak the circuits when you replace the 201. But if higher gain appeals to you for some reason, there you go. But be aware higher gain transistors can lead to a muddy mess is you are not careful.
here is the 2sk117 datasheet. [url]http://www.google.com/url?sa=t&rct ... 5608,d.aWc[/url]
The 2sk170 is the low noise version of the the 2sk117, but I have never had a problem using 2sk117. There is plenty of references to these transistors using the FSB search function, or use google to search the forum. I cannot recommend others because I do not use them much.
So a great argument in favor of the j201 is the history and the depth of collective knowledge about its performance. I use them. Spend $0.10 for the trimpot and go with the easy build.

[full shred]

Thanks atreidesheir!!! It was a good reply and got me to understand a few things!

[MoonWatcher]

Some of our great friends here at FSB hate the guessing game of biasing 201's.

Bajaman loves the 2sk117 for lots of uses.

The 2sk170 is the low noise version of the the 2sk117, but I have never had a problem using 2sk117.

Not sure I'm included in the "great friends" category, but also don't enjoy j201 biasing.

I also get p/o'ed a bit to hear about people sorting those things like germanium transistors, so they can get them to drop in w/o a trimmer.

The hoard and sort thing just drives the prices up. At least that would be my guess.

I have a modest pile of 2n5457 types, and maybe some -58's and -59's.

But always have been interested in other types as again I've heard things from builders who don't hold the j201 in high regard for other reasons as well.

The 2sk117/170 has been obsoleted as well. Not available in the big box warehouses any longer. Where are people getting them from? I don't mean to sound ignorant. Just never got to checking for secondary sources.

I'm starting to see more and more things that combine bipolars and jfet, so it would be fun to play with them.

What's left that is still available? If the gain is too low, can't we come up with variations on the mu amp thing? Bootstrap them? Add a p channel and make an inverter or something?

I guess it's clear that I've really tried to stay out of the jfet thing when possible. Don't always want to do that, though. Been awhile since I built something with fets.

[aquataur]

They all seem still available in good quality from the original manufacturers - except in surface mount cases.

I guess whoever is mass-producing electronics these days is using an automatic component placement procedure, where through-wire components are just a hindrance. They are just catering for that market. Moreover, simulation is becoming better and better so less and less circuits are actually being breadboarded. This is a sad side effect and the death spell for the plated through component. Again a buck-driven decision and, as always, perhaps a bit short-sighted on their side.

Even for IC´s this becomes increasingly true.

So yes you get your MMBFJ201 galore... if you can find a retailer that sells you less than a 5000 pcs roll
I have seen a couple of retailers that sell the MMBFJ201 (SMD), as many pieces as you like, except all of them abroad where shipping becomes exorbitant.

-helmut

[ppluis0]

Hi folks,

What about the MPF102 ?

Anybody can tell me what can be expected if use this jfet to replace the aforementioned ones ?

Cheers,
Jose