Pez90 build Report and questions...

[Samy]

This is my first Phaser, never had an experience before with this kind of effects so, this was somekind of a surprise for me.
Fortunately, when i was finishing my project, a MXR Phase90 EVH arrived for repairs !  Now i can compare them ....
I don't have the 500KC pot for now so, i wired one 50K backwards with one 4,7K resistor in series, i used the Depth mod and one switch to the 22K resistor to emulate the "script/block" versions ... ( i know it's not 100% identical ).... All Fets are matched.
The sound it's quite close to the original, so, I'm Happy with the results, mine it's more versatile, of course.

I just have one question about biasing the Fet´s, i have different voltages in the original one and in the Pez90, here they are:
Original EVH Phase 90:
G= 1,20V  S= 4,35V  G= 4,35V

Pez90:
G= 0.85V  S= 3,9V  G= 3,9V

Like i said, the sound it's close, but i wonder if i can bias my fets a bit better... Tried the 4,7V zener and voltages dropped,  5,6V zener the voltages are identical to the 5,1V zener  .
What is your experience with this build ? Can you tell me what are your voltages in the Fets and how close to the original ones ?

Thanks.

[Samy]

Here's one pic, its not in the enclosure for now ....

[ljkowal]

Sam, I have just finished two Pez and will measure the voltages for you to compare but I am a newbie and not an EE.  I don't understand the G S G you have for the jfets.  If you tell me which voltage is the gate source drain, I can give you an answer this coming weekend(I will try sooner).  My understanding from Mark and RG is that the D and S don't matter, and reading up on them it appears they are just two connections through the medium.  The gate is the all important voltage to the effect because it creates the resistive state in the medium.  My jfets are from Fairchild so they are GSD which means they had to go in backwards to the layout picture.  This puts my source in the middle and drain on top. Keep this in mind when you match your voltages to mine.  Also my problem  with it is using the 500kC all of the effect is at the 75% Resistance and higher.  No effect up until then.

PS  dumb question.  who is the supplier of the sockets you used for the jfets.  I searched mouser and didn't see any. 

[Fp-www.Tonepad.com]

Small bear has those sockets, they come in 20 in a row and you break as many as you need.

D and S are interchangeable, at least in this case. It's the G that you have to pay attention to.

[ljkowal]

Awesome.  I am asuming you are the creator of the Pez 90 and didn't know you were on the forum!  Does it sound right that the effect doesn't kick in until I get to the higher resistances on the 500kC pot? I turn it around 75% before it starts and then it is crammed very tight so that I have to breath on it to edge it up?  Could I use like a 250k ohm resistor and a lower value pot to stretch the effect out across the whole pot?

[Fp-www.Tonepad.com]

apparently you have a potentiometer taper issue. I suggest you read 'the secret life of pots' at www.geofex.com so you can get a better understanding on how tapper affects the feel of a pot , you may also want to get a Linear taper (since you're supposed to be using an antilog taper) pot and try that. But from what you describe I think you are not using an anti-log pot, which is what the "C" in 500kC stands for.

[R O Tiree]

Umm... The Gate voltages should be going up and down in time with the LFO about +/- 0.1V or so. Add that to the fact that there is a fairly wide range of "acceptable" mean Gate voltages that produce a range of phased signals... The LFO on the one I just fixed gives a range at the Gates of 0.8V mean to 1.5V mean and is set for a mean of about 0.85V at the moment for a really subtle phase.

The Sources and Drains on all mine are at 2.82 V. Vref from the (stock) Zener is 3.3V, for what that's worth. This one is a Rev D Dunlop MXR Phase90.

[ljkowal]

Francisco,  as I have nothing but the highest regard for you and your work I went back, turned off the TV, stopped trying to balance the guit on my knees and hooked up the tone generator, the effect is working as you planned it. If it wasn't I knew it was something I did.  It sounds beautiful.  Thank you

For the gate voltages; .998 up to 1.18x

[Fp-www.Tonepad.com]

cool