Biasing Help Please

[OSeanEC]

I am trying to bias two 2n5457's.I have a simple radio shack multimeter with settings for DCA,ACV,DCV,OHM and what looks to be a diode setting.Bias on the 2n5457's is supposed to be 4.5v.
My questions are;

What setting do I put my multimeter on?
Where do I put the negative side of the multimeter?
Where do I put the positive side?

Thanks in advance.I've searched since last night but I need very simple instructions and whatever I found was over my head.

[R.G.]

Multimeters have two leads, red for positive and black for negative. They generally have three or more places to plug the leads in. Two of the holes are generally marked "common" and "Volt/ohm". The other one or two are usually "current" and possibly a "10A" hole for higher currents.

You set your meter range to read DC (direct current) V (volts). If there are several settings there, set it to something bigger than the power supply in your pedal. Generally for 9V powered pedals, set it to 10V or 20V depending on which your meter has.

The meter will then tell you the DC voltage between the leads. The normal way to hook this up is to plug the black lead into the common hole, and the red lead into the V/ohm hole, and connect the black lead to whatever place you're using as ground. For almost all effects, any wire connected to the input jack ground will do fine. Clip it there and leave it there. Now probe places with the red lead to read the voltage wherever you want to read voltage.

You're going to need a lot of practice and reading. Don't get discouraged. Just keep trying. It's the work of learning that raises your head so things are not over it.

[Al Heeley]

So the collector of the transistor should normally be getting the main power fed in from the battery - normally 7.5 to 8.5 volt range, I think.
Does that mean you measure the bias (4.5v range) on the BASE or on the emitter leg?

[OSeanEC]

Thanks for the reply R.G.

So the collector of the transistor should normally be getting the main power fed in from the battery - normally 7.5 to 8.5 volt range, I think.
Does that mean you measure the bias (4.5v range) on the BASE or on the emitter leg?

That's part of what I am trying to find out.The pinout is Gate,Source,Drain( from what I found through searches,sorry if that's wrong ).If I put the positive from the multimeter to Gate I get nothing.To Source I get a reading that does not adjust at all using the 50k trimmer.On Drain I get a reading that I can adjust but not as low as 4.5v.If I put the positive across both the Gate and Source I get a reading that will adjust to 4.5v,but is this right?

Basically trying to find out the correct way to bias a 2n5457.

Thanks for the replies also.

[GibsonGM]

Typically, you are measuring bias at the drain.  Having  4.5v through the device allows the signal to go - 4V or so, and + 4V or so at the gate.  Remember, we're amplifying sinusoidal waveforms, which go above AND below zero.
   
If the JFET was sitting at 9V, you could amplify only negative-going signals, while clipping the positive ones  (nowhere to go "above 9V").  This is the principle of the tube diode, in fact.   It is called 'rectification' rather than amplification.
A common trimmer value is 100k, so maybe 50k is too low?


Here is 1 example, 1/2 way down the page:  http://www.runoffgroove.com/fetzervalve.html

After playing with the Fetzer, it will probably become more intuitive...

[PRR]

> So the collector of the transistor should normally be getting the main power ....

THROUGH the collector resistor.

Conceptually, at idle bias the transistor and resistor "split" the supply voltage. 9V battery, 4.5V across resistor, 4.5V across transistor. Now as you diddle the transistor with signal, this point can rise 4V and fall 4V, giving the maximum almost-9V swing.

It's not that simple. External load limits up-swing, and an emitter resistor limits down-swing. And sometimes you don't want anything as big as almost-9V (which is awful big for guitar-level systems).

> the correct way to bias a 2n5457

There's infinite ways to "bias a 2n5457". You want "how to bias the 5457 in this specific circuit". Yes, many popular plans use an adjustable Drain resistor and ask you to trim to 4.5V.

> On Drain I get a reading that I can adjust but not as low as 4.5v

> To Source I get a reading that does not adjust at all

What IS that reading?

It is entirely in-spec for a 2N5457 to bias-up with Source above 5V. Then Drain can not possibly be pulled down below 5V. This is the problem using loose-spec JFETs with a mere 9V battery; some just won't work as expected.

Anyway.... I assume you can adjust up to 9V and down to something like 6V. Find those two points, then split the difference. 7-something volts. That should "play". Listen, and trim, and listen. Near 6V or 9V it will be distorted and weak, somewhere in the middle will be "best".

[OSeanEC]

I am trying to bias two 2n5457's.

The 1st one-Source=1.34.
The 2nd-Source=.88.


The Drain reading on the 1st-14.71 down to 7.20.
The 2nd-13.94 down to 9.19.

I apologize,I thought that there would be a simple answer to this,something like, put multimeter negative here, put multimeter positive there,adjust to 4.5v,done.Hopeful thinking on my part.Maybe I just need to step away from trying to learn/understand this till I learn more.

Thanks for all the responses.

[Wild E]

What voltage are you using?

[OSeanEC]

Not sure if you mean pedal or multimeter.Noob here.

Pedal 9v

Multimeter set to DCV 20v.

Hope one of those was the correct answer.

Thanks.

[Wild E]

Then you shouldn't be getting any readings over 9V. is it a battery or wall wart?

[OSeanEC]

Wall wart Boss adapter.

[Wild E]

Check the output of the adapter, I have an adapter that was for charging FRS radios and is labeled 9V out but actually puts out 14.5V.

[R.G.]

You'll get more out of one hour with a live teacher than 20 hours on the internet. Just an observation.

Find a friend, possibly a new one, who knows at least something about how to use a meter. It'll save you a lot of time.

A lot.

[PRR]

> I thought that there would be a simple answer to this

Actually, you are doing the right stuff, but there's some confusion.

I strongly suggest you read DEBUGGING - What to do when it doesn't work, R.G.'s well-tested guide to diagnosis and reporting.

> I am trying to bias two 2n5457's.

""You want "how to bias the 5457 in this specific circuit"."" It sure would be good for you to say WHAT specific circuit you are working on.

There are a bunch of circuits which take 9 Volts and you set a collector or source to half of that, 4.5V. I'll assume this is one of those.

However it appears that your "9 Volts" is really close to 14 Volts. This too is mentioned in the DEBUGGING essays: read and report your battery (or wart) voltage.

> The 1st one-Source=1.34.
> The 2nd-Source=.88.

OK.

> The Drain reading on the 1st-14.71 down to 7.20.
> The 2nd-13.94 down to 9.19.

That's how we guessed the "9 Volt" power is more like 14V.

Half of 14V is 7V.

This is not too fussy... just get near center.

Your 7.2V is so spot-on that you should quit here.

The 9.2V is a bit high of 7V, but well below the 14V limit.... let this be 9.2V for now.

The box should work, as far as bias goes.

DOES it work?

[OSeanEC]

Thanks PRR for the response.I will read DEBUGGING.Thanks for that.

The circuit is one I found in the Layouts Gallery above in MarkM's gallery called the Subdecay Liquid Sunshine.
The box works fine as far as I can tell.It sounds like a light OD.No buzzing or humming or anything obvious wrong with it.

So why do I care or want to know how to do the bias if all is fine? (just in case someone has this thought)

Because that's what it says to do in the instructions under the layout.I figure if it says to do it,maybe I should learn how to do it.I built this because I wanted to know how to make my own PCB's.It was the layout,among a couple others,that I chose.My first PCB came out good and I had fun making it.I am just trying to learn as I go.

I have built a few pedals that has said to do the Bias to 4.5v thing,but have always just tuned it to taste.This time I would just like to learn a little more.

Sorry if this is more information than you asked for.Just trying to explain why I built this circuit which lead me to the Bias question.

Thanks again for the reply.I do appreciate all the help,and knowledge I can get.

[zombiwoof]

Is that an older Boss ACA adapter?.  Those are unregulated and put out in excess of 13 volts without a load.  That might be in line with what others are saying about the adapter voltage.  Later Boss ACA adapters and PSA adapters are regulated and put out around 9 volts.

Al

[OSeanEC]

Is that an older Boss ACA adapter?.  Those are unregulated and put out in excess of 13 volts without a load.  That might be in line with what others are saying about the adapter voltage.  Later Boss ACA adapters and PSA adapters are regulated and put out around 9 volts.

Al

Yes it is.I didn't know that.Thanks for that info.I'll have to look into another adapter.

Thanks again.

[zombiwoof]

If you can get a Danelectro DA-1 "Zero-Hum" adaptor, they are inexpensive (around $8-9), regulated, and very good.

Al