Quick Trimpot question Mayqueen specific

[Guybrush]

I'm looking to build Runoffgroove's Mayqueen and wondered what the trim pot is for?  It's not mentioned in the documentation. http://www.runoffgroove.com/mayqueen.html

Thanks!

[midwayfair]

It sets the bias on Q3 to 5.5v as noted in the voltages chart.

"The optional cap in parallel with the Q3 bias trim is a high end filter."

This sort of trimmer is common in JFET designs to deal with variance.

[Guybrush]

Ahh... I'd misunderstood what that meant.

Do you set it by ear?  Sorry for the incredibly basic questions.  I've not built a circuit with a trim before.

Thanks!

[midwayfair]

Do you set it by ear?

You can. However, the best way is to use your multimeter to get to the ideal value first; measure the voltage at the drain. That way you can use the designer's recommended voltage as the starting point, mark it (if you like), and then adjust to taste. That's the art part of pedal building ... although JFETs are pretty standard, bipolar transistors aren't, and there are times when deviating from standard biasing creates something unique and interesting or makes an otherwise lackluster transistor sound great or corrects a flaw like harshness or mushiness.

[Guybrush]

You can. However, the best way is to use your multimeter to get to the ideal value first; measure the voltage at the drain.

Hmm... Please could you explain what you mean by 'the drain'?  And (you're going to roll your eyes at this one)  where do I put the DMM probe tips?  One on the drain and the other on ...?

EDIT:  I'm assuming the drain is the voltage marked D and I'll need to check the data sheet to see which leg on T3 it's referring too?

Looks like it wasn't such a quick question after-all.   I think I need to do some reading about voltage reading etc.

Thanks again.

[midwayfair]

You can. However, the best way is to use your multimeter to get to the ideal value first; measure the voltage at the drain.

Hmm... Please could you explain what you mean by 'the drain'?  And (you're going to roll your eyes at this one)  where do I put the DMM probe tips?  One on the drain and the other on ...?

EDIT:  I'm assuming the drain is the voltage marked D and I'll need to check the data sheet to see which leg on T3 it's referring too?

Looks like it wasn't such a quick question after-all.   I think I need to do some reading about voltage reading etc.

Thanks again.

To take voltage in a circuit, set your multimeter to 20v (if it's not auto-ranging). Connect the circuit to the power source (9v DC in our case) and ground. Place one probe [usually the black probe, but it doesn't actually matter] on ground and the other on the point where you want to take the voltage. It might help to just alligator clip the black probe to ground so you can have an extra hand free.

Okay, so you've built your May Queen, and you've adjusted the trimpot, and now it sounds good. Don't put the probe away yet. Experiment! There are other places in the circuit where you get voltage. This is a common method of troubleshooting a circuit, and you can find a huge number of common problems VERY quickly if you have a general idea what the voltages should look like.

As noted in your edit, the drain is the one marked "D". In a schematic, this is the pin at the top of the symbol. Almost always, it will be oriented closest to the voltage source. FETs can sometimes be flipped around and work the same or similarly, but it's best to get used to the orientation of the FETs you work with most often. And yes, you should always check the datasheet for the proper pinout, though googling "[transistor name] pinout" will often get you a picture. I think Mark Hammer says that there are something like over 900 ways for those three pins to be oriented. Well, it feels like that many when it's not standard at least!

[TNblueshawk]

GB, when I Google the datasheet I always put the name of the tranny and then the word datasheet. Then I look for the link that Futurelec usually has. Why them? Dunno. It just always puts me right into the sheet and is the quickest to me.

[Guybrush]

Thanks for the help guys.

Time to learn how to actually use my DMM!

[TNblueshawk]

Thanks for the help guys.

Time to learn how to actually use my DMM!

This is a good tutorial site for you on the DMM edamucation

http://www.ladyada.net/learn/multimeter/continuity.html