Phase 90 trim pot adjustment

[Fret Wire]

This may be an OT........ but I've never seen a post on the voltages for the FETs bias when you adjust the trim pot. Usually there's a reference to adjust by ear for even phasing. Does anybody know what the bias voltage should be? If so, where would you take the measurements?

Thanks

[petemoore]

When I adjusted mine, signal would come through at any setting, but the phaze came on and got different sounds depending on where it was tuned [don't know if this is 'right' but it phazes and the speed knob works]
 IIRC it was a 250k pot [perfed trimpot in my case] and about 1/4 or less of it's travel there was a phazing sound but different 'focuses' of it could be fine tuned with the trim.
 I just set it by ear...
 I pulled the 90 out and it has a pleasing tone to it IMO...different than SS and Vibe, very nice indeed...

[Boofhead]

There's no useful bias voltage which can be stated for this, it depends on the JFETs.  Just do it by ear.  You will find a small window of useable adjustment on the pot, and inside that window it's largely a preference thing.

[R.G.]

Reinforcing Boofhead:

There is no "correct voltage". The trimpot is there because the JFETs are inconsistent, and the trimpot allows a wider variety of JFETs to be used, at the expense of having to make an adjustment.

[Fret Wire]

Thanks guys, sounds like I'll do it by ear. One less measurement to remeber! This one takes a 3/32 allen wrench to adjust. There's an access hole in the pc board so you can adjust it without removing the board. Of course, the first time you still have to remove the board to remove the silicon from the pot.  I just finished the script mods on it, and it sounds alot better already. No distortion to the tone, and it's now close to unity gain. I'll adjust the trimpot and that should do it nicely.

Thanks

[Fret Wire]

There is no "correct voltage". The trimpot is there because the JFETs are inconsistent, and the trimpot allows a wider variety of JFETs to be used, at the expense of having to make an adjustment.

I was under the impression that they had to be matched. I remember the phase 90 build at tonepad requires matched JFETs. Circuit difference? Or is it that even matched sets will have different specs from set to set, hence the trimpot.

Thanks, I'm learning slowly but surely.

[Boofhead]

> Or is it that even matched sets will have different specs from set to set, hence the trimpot.

Yes, that's it.

[petemoore]

Phaze 90 I believe does depend on the jfets being matched.
 I went through alot! of Jfets and trim tuning till I got it Phazing.
 I never did get my matcher to work...the prefferred way to do it.