Biasing transistors

[knight_yyz]

hey guys, can someone point me in the right direction for setting the voltage on transistors that use a trim pot?  I know how to adjust the trim with the little brass screw, or the big middle screw depending on the design.  Just now sure what to touch with the meter to read the voltage.  Working on a phaser board and I have ordered the matched set.  But when the board is soldered up, i need to adjust that bias.  

If there is a web page or document showing this I'd appreciate the input.  


thanks

[Beaker]

Can you post up a link, or let us know exactly what phaser it is you are working on - it would be a big help.

If you are trying to bias JFETs, the general rule is to set the trimpot voltage to 4.5 Volts, when tested between the JFET drain leg and ground.

This voltage can vary though, depending on the circuit.

[rocket88]

if you're working on the mxr phase-90, you can do it by ear. when it's not set right you will not get any phase. what i do is i play a note and start turning the trimmer until i start to hear the phase happening, for me i hear a "thunk" when at the end of the phase. once you hit that point, it takes very, very, very little to get it in the right spot, so it's just a little at a time.

now, for biasing transistors you put the positive lead of you're meter on the collectors of the transistors and the negative on ground, i just use one of the screw holes in the enclosure, and turn the trimmer. most transistors will be biases around 4.5V, but as beaker already said that's not a definite. i usually start around 4.5V and then tune it by ear. so for instance, my Ge fuzzfaces i like closer to 5.5-6V as it's a little hotter and smoother sounding to my ears.

[knight_yyz]

It's the MXR phase 90 clone.  Says to set the voltage at 4 to 5 volts until you get the sound you want.  But how do you measure the 4-5 volts?  just need to know where to hold the multimeter leads.

Sorry I said transistors but I guess you're supposed to call them jfets in this case.  Sorry all the names are confusing to me.  

Soldering these components is like paint by numbers to me,  but when you have to tweak that's where i need a little guidance.  

[Chris60601]

Typically, you attach the negative lead of the MM to ground and the positive lead to the Drain of the fet.
Turning the trim that effect the fet will either increase or decrease the voltage reading.

To determine what leg is the drain, consult the datasheet of the part you are testing. They can be found on the net. Not all pin outs are the same, this is why datasheets are your friend.

[knight_yyz]

Crap.   I have to learn to read.  Just wasted money on the wrong FETs.  Woohoo!!

Supposed to set up the 4 matching 2n5457 FETs and I bought 5 of 2N5087, so that isn;t going to work.  



Where can i get a matched set of 2n5457's?  I bought the other matched set from pcbshop.com but he doesn't have these.  Assuming I have to buy a bunch to get a matched set, how do you measure them to be a matched set?

[Chris60601]

http://www.geofex.com/article_folders/fetmatch/fetmatch.htm

[Chris60601]

Then, have a look here:  http://tagboardeffects.blogspot.com/2012/07/greatly-improved-jfet-matcher.html

[Frank_NH]

Here's a post is did a while ago on this subject...

http://guitar-fx-layouts.42897.x6.nabble.com/JFETS-td14064.html

It's pretty straightforward to test a bunch of JFETs and select the ones which match Idss and Vp relatively closely.  I did this again recently for a Dr. Boogie build - no problems biasing the JFETs.

[motterpaul]

You can get 2n5457s a lot of places. Send me your address by email and I will send you two.

Pls include instructions on what needs to match for the two for that build. (I have never made it) Thx.

EDIT: sorry, I see you need 4 of them. Maybe you could paypal me when you get them like $2.50 total or so.