Help! Blue Magic build question...

[Russ]

I just finished building Jake's Blue Magic. The circuit works fine, but I can't get the JFET drain voltage down to 4.7v as recommended with the 50K trimmer. The lowest I can get is about 5.6v. Any suggestions?

Also, any new suggestions as to capacitor values for the mid-boost switch? As with most threads I've read, the 530pf value doesn't seem to provide enough boost to hear any difference.

Any help would be appreciated.
Thanks, Russ

[Alpha579]

Which JFET did u use?

[RDV]

You could try a 100K trimmer?

Regards

RDV

[Russ]

The JFET is a J201, just as specified. In fact, I built it to GGG's parts specification exactly. That's why I can't understand why it won't bias correctly in its stock configuration.
Russ

Thank goodness you guys stay up as late at night as I do!

[RDV]

You could try a different J-Fet as there is a great deal of variance in them, or a 100K trimmer with the one you've got(assuming you didn't do any subs or make a mistake in parts placement). You could try a 1000pF cap & work your way up for the mid boost. It's good to socket certain components in these homebrew circuits, because the designer's tastes may differ greatly from your own.

HTH

RDV

[Russ]

Thanks Ricky Don,
I changed my J201 JFET out for another one, and with the bias pot at one extreme the voltage is down to 4.7v now.
Russ

[Paul Marossy]

It's amazing just how variable JFETs are. They are all over the map! That's why I almost always use trimpots on the drains on them things.