Replacing a 4.7v Zener for P45

[solarplexus]

Hi there,

I know there are some combination that can be done to replace a zener diode :

http://www.diystompboxes.com/smfforum/index.php?topic=76762.msg660170#msg660170

My question is:  can I replace the zener diode in a Phase 45 to have it a 4.7v ?  I just built myself the phase 45 with the layout from tonepad and although it is working, the sound is a little distorted and I figured it was because the voltage on the gray rail http://www.tonepad.com/getFile.asp?id=70 is at 7.9v because I am using a zener diode I had left in my inventory, not knowing what the rating was. 

What kind of combination could I use to make that rail go down to 4.7v ?

Thanks,

Matt

[frequencycentral]

I reckon 2 amber 5mm LEDs and 2 1N4148 will get you there. The way to find out exactly is to breadboard just the biasing arrangement, ie the 10k resistor to +ve and your diode string to ground, just add and take away various combinations of diodes and LEDs until you get what you want, measuring the junction where the 10k resistor meets the diode string.

[solarplexus]

ok well, I don't have any amber LED's in stock.  In fact, all I have left is a zener at 7.9v, some 2n4148, 2n4001, some 3mm red LED's and a green 5mm LED which I need for the switch! haha! 

but the real question is:  What is the mathematical equation to stacking diodes in series?

[frequencycentral]

1n4148 drop about 0.6v, Ge diodes drop half of that. LEDs drop more. There's a chart here that will give you some ideas: http://en.wikipedia.org/wiki/Light-emitting_diode

You could use 8 x 1N4148 = 4.8v

[solarplexus]

Very informative!  Thanks!

[Mark Hammer]

You could simply use the bias trimpot to drop the +9v down to the right bias voltage for the JFETs, but what happens to that bias voltage setting as the battery gets weaker?  It will be "off" and you will have to keep adjusting the bias.

The purpose of the zener is to have a reference voltage that is stable enough for a long enough time that whatever you set the FET bias voltage to will be the "right" bias for a long time.  Chances are pretty good that the pedal won't work to your satisfaction once the supply voltage drops below 7v, but since you want the bias trimpot to be easy to set, the zener value selected is close to Vref to be able to make good use of the entire range of the trimpot and optimize "dialability".

You could, in theory, use an LM7805 regulator too if yuo couldn't score a 5.1V zener.  Same thing, just more parts.

[solarplexus]

Just a small update.

I used three 3mm LED in series to give a voltage of 5.1v to the rail on the P45.  I'm wondering if that voltage is a little too high?  The pedal works fine, although the sweep is not really deep because I am using unmatched J201s (waiting for some 2n5952 in the mail to put in). 

Should I add another diode to drop the voltage lower or keep it as is?

Thanks!

[Mark Hammer]

Three will be fine, and enough.  The goal is to have a stable source to divide down.

[solarplexus]

Thanks Mark, I will leave it as is then.