general question about JFET's and pedals-

[plexi12000]

i was looking at some vero layouts and such....one overdrive that uses jfet's, stated to adjust so and so resisitors, so drain voltages are around 4.5 volts.

other jfet powered pedals made no mention of drain volatge.   I was wondering if "4.5v" is generally what you should shoot for? or does this vary depending on the circuit?

Thanks!

[hymenoptera]

Can you specify exactly which two circuits, for comparison? I'm assuming both are similarly configured input buffers? Or not??

Two schematics to compare would help. There's a lot of things you can do with a transistor.

[slacker]

Generally if the FETs are configured something like the Fetzer Valve http://www.runoffgroove.com/fetzervalve.html then they'll need tweaking to bias them properly and unless the design says otherwise 4.5 Volts on the drain is a good starting point, the circuit should at least work. It always worth experimenting though, too high or low a voltage and the circuit won't work but there's a range of Voltages in between where it will work but might sound different or distort differently that you might prefer.
There's plenty of circuits with FETs that don't require any adjustment, they'll just work so like hymenoptera said there's no definite answer without knowing which circuits you're talking about.

[Gus]

Look for the Vishay app notes on JFETS.  IIRC 102 is the one about biasing
Search here and the web for with words like "JFET design"

It depends on what you are using the jfet for.   When I use JFETs in microphones I select them from a bag of 1000 I bought.  Different measurements get different resistor values etc with some circuits.
Oktava 219 microphones appear to have two numbers written on the transformer for two resistors that values change with the JFET used.
Older Neumann U87s have selected source resistor values for the JFET used

For guitar effects using 9VDC you have a the 9VDC limit you need to work with

Changing the drain resistor can change the output resistance of the JFET stage might matter might not matter.

I don't like using JFETs in 9VDC guitar effects unless I need to.

A link to an older thread.   I did not design the circuit I saw something like it in a Fender Harvard preamp section part of a schematic many years ago
In another thread I believe another person built a version with +18VDC -9VDC I have not found the thread yet.
http://www.diystompboxes.com/smfforum/index.php?topic=93817.0

[tubegeek]

Look for the Vishay app notes on JFETS.

Thank you Gus!

[plexi12000]

well..i would love to build one of Joe Davisson's overdrives.  the sounds clips i've heard are fabulous.  just what i'm looking for.

his Vulcan (jfet version) is recommended for 4.5 volts with J201's.

MAN....i wish someone made pcb's for these!  hahaha-- i'm beginner level, it would be a huge help. 

[Kipper4]

The jfet Vulcan was about the only one that got away when I had a jelly bean overdrive frenzy.
If you decide to build the Vulcan you may be wise to use 100k trimmers from +V then you can bias the fets better. If you like it ,replace the trimmers with a fixed value later.
It might be a bit noisy on a breadboard.
Have fun my friend.

[plexi12000]

hey, that's an outstanding idea, Kipper4!  thank you!! 

[PRR]

> I was wondering if "4.5v" is generally what you should shoot for? or does this vary depending on the circuit?

You come home on your motorcycle, late at night, and want to put it in the garage.

Say the garage is 9 foot wide.

Leaving the big light on attracts bugs, so you put a small light to "aim at" when you pull in.

Where (left-right) do you put the light?

If you put it 1 foot from the left wall, your left side will get banged-up. Same if you put it close to the right wall. Considering you may come home tired or stoned, you want a LOT of clearance for minimum damage.

The "optimum" place within a 9 foot width is 4.5 feet from either side. However 4 foot one side and 5 foot the other side is close-enough for all but the worst nights.

That's true when garage and bike are symmetrical. I forgot to say you have a side-car, sticks out a few feet to the side of where the driver's eyes are. After a few near-misses and hard-knocks, you might discover that 3 feet one way and 6 feet the other way is the best place for an eye-guide light for *this* rig.

"Usually", when you first put-together a JFET amp stage, it comes out like the "1 foot from wall" case. You can't hardly get a signal through without hitting. Your *first* step (in most 9V situations) is to trim it closer to 4.5V (4.0V to 5.0V much the same).

Source resistor and external loading are "side-cars" on the idealized stage. For maximum clearance in a specific design you might aim a little different. In most cases you can start from mid-point, play LOUD signals, and trim for least signal damage.

That's for "clean". In a Pedal Forum, sometimes we want dirty. Setting the drain "off center" produces a different distortion sound which may be musically interesting.

[chi_boy]

well..i would love to build one of Joe Davisson's overdrives.  the sounds clips i've heard are fabulous.  just what i'm looking for.

his Vulcan (jfet version) is recommended for 4.5 volts with J201's.

MAN....i wish someone made pcb's for these!  hahaha-- i'm beginner level, it would be a huge help.

Umm, might be able to help with that.....

I made a small run for myself.  I have a few extra boards left.

[BubbaFet]

> I was wondering if "4.5v" is generally what you should shoot for? or does this vary depending on the circuit?

You come home on your motorcycle, late at night, and want to put it in the garage.

Say the garage is 9 foot wide.

Leaving the big light on attracts bugs, so you put a small light to "aim at" when you pull in.

Where (left-right) do you put the light?

If you put it 1 foot from the left wall, your left side will get banged-up. Same if you put it close to the right wall. Considering you may come home tired or stoned, you want a LOT of clearance for minimum damage.

The "optimum" place within a 9 foot width is 4.5 feet from either side. However 4 foot one side and 5 foot the other side is close-enough for all but the worst nights.

That's true when garage and bike are symmetrical. I forgot to say you have a side-car, sticks out a few feet to the side of where the driver's eyes are. After a few near-misses and hard-knocks, you might discover that 3 feet one way and 6 feet the other way is the best place for an eye-guide light for *this* rig.

"Usually", when you first put-together a JFET amp stage, it comes out like the "1 foot from wall" case. You can't hardly get a signal through without hitting. Your *first* step (in most 9V situations) is to trim it closer to 4.5V (4.0V to 5.0V much the same).

Source resistor and external loading are "side-cars" on the idealized stage. For maximum clearance in a specific design you might aim a little different. In most cases you can start from mid-point, play LOUD signals, and trim for least signal damage.

That's for "clean". In a Pedal Forum, sometimes we want dirty. Setting the drain "off center" produces a different distortion sound which may be musically interesting.

Nice analogy, PRR!

Zen and the Art of JFET Biasing. err... Motorcycle Maintenance.

[hymenoptera]

You come home on your motorcycle, late at night, and want to put it in the garage.

Say the garage is 9 foot wide.

...

Analogies like these are golden, and should be required reading for all beginners.