Phase 90 fets and zener question

[elflicho87]

Hi everybody,
I'm planning to build a phase 90 and I know you must use matched fets (2N5952) if you want it to sound properly; also, the zener diode value has to be choosen depending on the fets you're using. My question(s) is: should I buy the matched set my online store is offering for 4,46€ or buy 26 fets (or less) for the same money and select them by myself? Is their hfe range very wide or they're pretty close one to each  other? Are those useful for other projects? and then how should I choose the zener value, by ear or what?
Thank you, any help will be appreciated!

[kvandekrol]

In my opinion, buy the matched set. Saves you the trouble of having to build a matcher, and the variance is high enough that you may not be able to get a very closely matched set of 4 if you only buy 26 of them.

For the zener, I haven't seen a formula that can help in selecting it, but would be curious if there was something for this. I've always heard just to do it by ear until you find the one that sounds good.

[elflicho87]

Ok thank you for the tips, i'll buy the set
and for the zener, well they're cheap I'll buy a few of them around 5.1V and just try! (the rated wattage does matter? I can choose between 0,5W 1,3W and 5W)
thank you again!

[DavenPaget]

Mostly i believe 0.5W is efficient , but i buy 1W's all the time 

[Mark Hammer]

The zener is for providing a stable/predictable bias to the JFETs.  Remember that the battery voltage is going to change over time/use.  If you set the bias using a simple voltage divider from the supply, that divided-down voltage is also going to change, requiring you to reset the bias constantly.  The zener sets a reasonable stable maximum voltage, and the bias trimpot divides that down, so that you set the bias once and you're done.  It probably doesn't have to be 5.1v, but it certainly has to be lower than 8v or so, and more than 3v, so that you know you'll always meet the bias requirements for the JFETs, and so it will be easy to dial in.

[DougH]

When I built my phase90, I built it up on the breadboard first before populating the PCB. This allowed me make sure the JFET/Zener combination I picked worked correctly. I don't remember what value Zener I used but I don't think it was "stock". I matched the FETs myself but if you can get a pre-matched set go for it. It will save you a lot of time/hassle that way.

[elflicho87]

Well, thank you all for your help
cheers
Nicola

[deadastronaut]

The zener is for providing a stable/predictable bias to the JFETs.  Remember that the battery voltage is going to change over time/use.  If you set the bias using a simple voltage divider from the supply, that divided-down voltage is also going to change, requiring you to reset the bias constantly.  The zener sets a reasonable stable maximum voltage, and the bias trimpot divides that down, so that you set the bias once and you're done.  It probably doesn't have to be 5.1v, but it certainly has to be lower than 8v or so, and more than 3v, so that you know you'll always meet the bias requirements for the JFETs, and so it will be easy to dial in.

hmmm, if i haven't got a zener, can i use a voltage divider to get 4.7v/5.1v for now then?...i'm not using batteries. psu only (re tweaking my modded phase 45 on breadboard.)

[GibsonGM]

If you have a regulator in there (and of course, a GOOD PSU will be regulated), you would probably be OK with a divider.  You can see how this works, where any changes in your PS will affect your bias....

[deadastronaut]

brilliant, cheers mike.

i have some on order, but just wanted to get it up n running for breadboard fun.  

so a straight forward voltage divider that gives 4.5 should be ok then..?

[Mark Hammer]

That's what the trimpot is on the board for.  The circuit would work just fine without the zener in place.  The only problem would be that as the battery ages, you'd have to open the unit up and rebias the JFETs.  But if you set the bias using the same stable power source you will be normally using, it should be just fine.

[deadastronaut]

excellent...cheers guys.

[R.G.]

Most NPN silicon devices have base-emitter junctions that go into zener breakdown at about 5.x to 7.x volts. You might be able to use one of these. You'd have to pick from several.

Note that even ONE reverse breakover makes a silicon device noisier, so even the ones you tested may not be as good for amplifiers later.

[deadastronaut]

cheers R G,  i have it working ok t the moment without one...using my trusty pedalboard PSU.

i always meant to breadboard this one day, to have a proper tweak about...and try the univibe mods etc..

it would be nice to squeeze a tremolo out of it too...