Help biasing jfets

[MaheshSK]

   http://guitar-fx-layouts.42897.x6.nabble.com/Need-layout-td17687.html#a17751
Powered it up yesterday.
 Only getting a loud hum and crackles... Biased it approximately according
to the notes.. Any help guys? Will post detailed
voltage readings by tomorrow if i get time... And i
used BF245C from Fairchild. Any help guys?? I have some 2SK30's too..

[induction]

Did you reverse the BF245? The pinout is the opposite of 2N5457.

[Frank_NH]

Yes, check your pinouts since the BF245 is the reverse of the 2n5457.

From the data sheet, it appears that the BF245C have a relatively large Idss (12 - 25 mA), but if you can get it to bias, then go for it.  I would spring for some 2N5457s or J202s.

Anyone going to experiment with a tone stack for this effect?  I've actually been looking at active Baxandall and James tone stacks after looking into the reasons for the EQD Talon's lack of treble (the Talons uses an altered version of an active Baxandall).  Frankly, I think one of the various versions of the BMP tone stack with a recovery stage and mids switch is a better option.

[MaheshSK]

Thanks induction. I mounted the fets correctly.. This is how i bias fets i put the positive wire on pin 1 of the trimmer and negative wire to the drain pin and adjust the trimmer till i get 4.5-5 volt reading. Is this the right way Iam new to all this biasing thing.. Any way of using 2SK30's ??
Same here Frank . Tone stacks in the contribution section looks promising....

[Frank_NH]

Here's how I bias.  With your DMM, put the + probe on the drain and connect the - probe to ground (use an alligator clip).  Adjust the trimpot for 4.5 - 5 V.  If you can't get that, then you'll need to substitute  different jfet.

The 2SK30 is a MOSFET and likely won't work properly in this circuit.

[MaheshSK]

Thanks Frank. Will try it out tommorow .I think 2SK30's are jfets from Toshiba save it as a sub for jfets

[rocket88]

I've been looking at the tonestacks myself for some ideas and projects I've got. Here's what I've been thinking. Any effect that has a tonestack already and has a recovery stage of some sort after it, you should be able to remove the one in the effect and place one of these minus the LPB boost that comes after it. IE: take the talons, remove the tonestack and put in the marshall one in it's place. Theoretically it will work. You just have to make sure the impedance is correct, 38k, before the tonestack and you should be good to go.

[induction]

MaheshSK wrote

This is how i bias fets i put the positive wire on pin 1 of the trimmer and negative wire to the drain pin and adjust the trimmer till i get 4.5-5 volt reading. Is this the right way Iam new to all this biasing thing..

I suspect a typo here. Pin 1 of the trimmer and the drain of the jfet are connected by a jumper, so you should not be able to get a significant voltage difference between them. They should have the same voltage unless there is a problem with the build.

When we bias jfets, we are trying to set the voltage of the drain as compared to ground. Follow Frank's biasing recipe and let us know what you find.

[MaheshSK]

I meant the top most pin.
        1
2        
        3
will try n get back cheers

[Travis]

To read the voltage properly you need to put the red probe on the drain and the black probe to ground. The voltage will change as you adjust the trimmer

[MaheshSK]

Hey guys. Did the biasing as you all told.. the loud hum is gone now but still no sound . How about 12 or 18volts ??

[Travis]

Sounds like a misplaced component, bad cut, solder bridge, or wiring issue

[induction]

Can you post some photos of your build? Hi-def closeups preferably.

[MaheshSK]

Ok guys will try to post pics... Mean while
http://guitar-fx-layouts.42897.x6.nabble.com/3Times-champ-build-td17998.html
Could you guys adjust with this ?? Sadly i dont have a good camera..

[MaheshSK]

Hope this helps...

[MaheshSK]

[MaheshSK]

[motterpaul]

In my experience (IME) it is pretty hard to substitute FETs and get them to respond the same as what the original specs call for.  If it says 2n5457 (IME) that is usually what you need to use. Unlike many other transistors, Jfets just seem to have much less tolerance and more specific requirements.

I have been down the same road where I have tried subbing different fets, and they just don't work in some cases. Sometimes you can sub one in a circuit, but when it is all three it gets much harder.

If you can't get it to the usable range with the trimpot, you can try a bigger trimpot. But (IME) it still won't have the results you are hoping for, even if you get them into the right bias voltage zone.

Your build looks good to me as far as I can tell (I can't really see everything). One reason it helps to drill out the holes you plan to cut is so you can see them when you turn the vero board over to the component side. 1 wire and 1 cut are different from the original design, but not in a way that should matter.

My guess is that if you got 2n5457s in there it would work just fine. Once again - just based on my experience.

I would try reflowing some of the solder, and clean off the extra solder (solder wick is good for that). Try pulling on all leads of components (lightly) to see if they are tight, and also test row to row and between cuts with a continuity tester to make sure there are no unwanted solder bridges.

Next I would try an audio probe to see how far it gets.

[MaheshSK]

The problem is I cant find any other jfets here... I think i can find some Toshiba jfets what do you guys think ??

[MaheshSK]

What would be the biasing for
12 or 18volts ?? Help me guyz..