NTE 457/J201 question

[jayp5150]

Okay, 1st of all, I have checked the FAQ, and have googled, but I need to make sure I understand this correctly.

The Gfs (or Yfs) of a JFET is basically a gain figure, correct?  Orman's info on the FAQ said that 1000 would be low, 3000 would be moderate, and 12000 would be smokin'.  

So, I have built the Big Daddy, and had to use an NTE457, which I know is not a direct replacement, but found out that it would work in previous posts.  My build does not seem to have the same grind as the sound clip, so I was trying to hunt down a j201 (or mpf102) locally, but I check the datasheet, and the gfs of the j201 is only 500--the NTE457 is 3000!  Is this correct, or am I missing something?

Basically, I was assuming a higher gain fet would give the extra "push" I was hoping for.

The only other mod to the pedal was a .01uf input cap.

Thanks in advance.

[WGTP]

I have used the NTE's in a Mu Amp and they definetly had more gain.  However, I don't know that it transfers to more "grind".  Sometimes with Jfets it seems too much gain just makes them smooth out and compress, IMHO.  Make sure the bias is correct with the different Jfets.  I was messing with a Jfet Vulcan stage and couldn't get any treble out of it and found that the bias had dropped to 3v.  Re biasing fixed it.  (a new battery probably would have too)  

[RLBJR65]

My local RadioShack carries the MPF102's they are alot more expensive than elseware but if your in a pinch might try there.

[jayp5150]

I guess voltages would have helped :oops:

Okay, the socket reads 9.16 at the drain, but when I plug the jfet in, it drops to 1.58!  s=1.11

Using the ROG layout, what affects that besides the 15k resistor from the 9v?  Does the 15x to ground and the 2.2uf electro have anything to do with that?

Any reads on biasing (should've been the 1st question, I guess).

Sorry, I'm still getting the technical stuff down.

Thanks.

[KORGULL]

jayp5150 wrote:

Any reads on biasing (should've been the 1st question, I guess).

A couple weeks ago somebody mentioned that there was a good article on biasing at R.G.'s site. I spent a couple hours there but did not find it. Anyone know where it is? Got a link?

[jayp5150]

Right after I posted that I found something there.  Do a search for biasing fets, and in that list should be "fooling with fets."  

I didn't get a chance to really look it over yet, but I think some of it will take me a minute to "get."

Has anyone built this circuit and know what the voltages are supposed to be?

Thanks.

[WGTP]

If you look at some of the other ROG stuff, they use a 50K trim pot (at the drain) to bias the Jfets in the different stages.  In most instances, you want 4.5v going into the cap to the next stage (easy way to find it).  The by-pass cap doesn't seem to effect the bias much.

There are several articles around about subing MPF102's for J201's.

Good Luck  

[jayp5150]

Okay, I finally found something.  It was in a previous post, but for anyone that missed it, here you go:

http://www.vishay.com/docs/70595/70595.pdf

Thanks, guys.  I'll let you know how it turns out.

[jayp5150]

hmmm, guess that didn't work right,  you'll have to copy & paste it.

Sorry.

[jayp5150]

I guess I'm considering my Big Daddy finished (for now)

I tried to bias the fet a little better (still using nte457), but if I lowered the drain R at all, I got a squishy, gated noise with the gain at 10.  According to WGTP, my voltages were low (source was 1.2, drain was 1.6), but when I got my drain V up around 4 or so, it sounded horrible.

The pedal sounds great as is, but I guess I was expecting a bit more gain (the sound clips were "hairier/grainier,moregrinding", et al than my finished product).  It's has a sweet classic rock sound, and in front of my 5150's crunch, with the BD's gain at 12o'clock, it was a great lead boost--very thick and searing then.  The volume of this thing is outrageous.  It hits unity at around 10o'clock.

I'm going to have to build me one now.  This one's a gift for my cousin.  Put it in a Fossil watch tin.  I'll try to get pictures soon.

Thanks to everyone here that helped, and has helped me learn this stuff.

I'm still open to suggestions on the fet type and biasing (I can get mpf102's locally, and I can order nte458's from Parts Express, which is right down the road from me--any experience with either of those in this circuit?)

Thank again.

[WGTP]

Hmmm.  Are you using the Jfet as a buffer or a gain stage?  I really think the gain stage should be between 4 and 5v.  Is it the NTE458 or 457?  Sounds like somehting isn't right.

[jayp5150]

In the Big Daddy, it's a gain stage.  the schem is at ROG.  It is a NTE457, I can get the 458, but I have to order it.  I can also readily get a MPF102 (which has less gain thatn the 457).

I did not change the source R.  It is 2.2k w/ a 2.2uf bypass cap, the gate R is 4.7M, and the drain is 15k, per the schem.  All I did was try to lower the drain R, and anything below the 15k did the gate-y thing.

I was going to pick up some MPF102's today and just plug'n'test tonight.

Let me know if all this sounds out-of-whack.

Thanks.

[WGTP]

Hmmm.  Some of the Jfet's have different pin outs and the J201 is different than the NTE458.  Other than that, I'm at a loss.   :?

[jayp5150]

That's cool.  Looks like I'm just at the "plug & play" stage.  I'll get the final report/pics when I'm done.

[jayp5150]

Okay, I finally went and picked up a MPF102, and some trim pots.  I put a trm on the drain, plugged in the MPF102, and set it for 4.5 volts (well, 4.44, close as I could get).  This had the source at about 1.87, and it sounded great.

I'm not sure if I had a out-of-whack NTE457, or if they just don't work well with this build, but no complaints now.  The Big Daddy is very nice!

Tinight I'll be able to test it on my 5150 (it's at the church--I need a practice amp, the bass amp doesn't cut it :cry: ).

Thanks for everyone who helped me out with that.  I know it was small, but I'm still getting this down, so it's very appreciated.

I'll try to have pics soon.

[B Tremblay]

I'm not sure if I had a out-of-whack NTE457, or if they just don't work well with this build, but no complaints now.  The Big Daddy is very nice!

Great!  I'm happy to hear that it worked out well for you.  Those "equivalents" can sometimes be way off from what you'd want.  If you used a socket, get some J201 and try one out.  You'll need to re-bias, but you may find the sound more pleasing than the MPF102.

[jayp5150]

I will try that, thanks.  this one's a gift, so I'll get some j201's for mine, when I build it  :twisted:

Thanks again.