Phaser JFETs and the Zener Diode

[Bucksears]

I was reading in some instructions for the Tonepad Phase 90 that the value of the Zener diode depends on the transistors used.
How is this determined?

Also, is there a 'depth' mod for the Tonepad Phase 90 so you can add a depth control?

Thanks,
Buck

[Mark Hammer]

I was reading in some instructions for the Tonepad Phase 90 that the value of the Zener diode depends on the transistors used.
How is this determined?

Bear in mind that while the JFETs need a specific bias voltage to be "in the zone", the actual voltage source - the battery - will change over time.  The zener acts as a simple regulator to provide a basic voltage source that will be dependable even as the battery drops to 8.5v, 8v, 7.5v, etc..  The trimmer then divides down that stable voltage source.  While it is easy to imagine the existence of JFETs for which that range of possible bias voltages is inappropriate, it is also easy to imagine that there are many JFETs which only require a tweak of the trimmer to work, because their optimal bias voltage is not that far removed from what isoptimal for the 2N5952.

[Bucksears]

Thanks Mark - will any Zener diode do, or does it need to be above a certain voltage?

[frequencycentral]

You don't have to use a zener at all: http://www.diystompboxes.com/smfforum/index.php?topic=76762.msg660170#msg660170

[Mark Hammer]

He's right, of course.  The zener is there simply to stabilize the voltage supplying the bias circuit in the face of an everchanging battery voltage.  If you use an external PS, then all those pesky battery-related issues simply vanish.

[mac]

Check the zener before building. I noticed that some zeners (cheap, chinese, etc) do not provide a steady voltage when a batt goes from 9.5v to 8v.
I had buy lots of zeners until I found one that worked fine. Getting quality parts in my country is a pita.

mac

[Bucksears]

Hmmm....thanks guys.
I use a BBE SupaCharger for power, so no battery worries. It's going to be a JFET measuring weekend......

Thanks again,
Buck

[Bucksears]

Bumping this one:
Is there a 'depth' mod for the Tonepad Phase 90 so you can add a depth control?

[frequencycentral]

http://www.geofex.com/FX_images/p180plus.gif

[flintstoned]

Check out the build reports on the phase 90 at tonepad. They call it an intensity mod there.