Phase 90 fet's, zener and pot questions

[JebemMajke]

Does it matter if bf24b's are matched with bf245c's and/or bf245a's? Or any other n channel fet?
Choice of zener depends on fets or fets mili volts by witch they are matched? And how do i choose one?
I can't find any reversed log. So how to change 1mega lin into 500k reversed log?

[JebemMajke]

And that 15 uF. I can't find any. Can I use 20u or 10u? And which is a better choice?

[armdnrdy]

I would use JFETs of one series for matching. Also, without looking at the data sheet....is the BF245 a good equivalent JFET for a P90?

You can "make" a 15µf capacitor by putting a 10µf and a 4.7µf in parallel. (well..14.7µf close enough!)

The 1M pot can be "changed" to a 500K by adding a 1M resistor across lugs 1 and 3.

The zener depends on what works well with the JFET set that you matched. Save yourself some trouble and purchase a few different voltage ratings above and below the stock 5.1v.

If all goes well....when you fire up the P90 and adjust the bias trimpot you'll know which zener doesn't work when you don't get any phasing or it begins to phase a bit when the trimpot is maxed in one direction

[Govmnt_Lacky]

The 1M pot can be "changed" to a 500K by adding a 1M resistor across lugs 1 and 3.

He needs a REV LOG 500K. Doing what you suggest here would make it a 500KB Linear pot.

I dont believe there is a tried and true way to make a Linear pot into a Rev Log pot 

Have you tried any of the finer online parts stores to see if they will ship to you? Smallbear, Pedal Parts Plus, etc.?

[armdnrdy]

You know I almost wrote that I wasn't sure if it would change the taper!

How about adding a reverse resistor? 

[JebemMajke]

I would use JFETs of one series for matching. Also, without looking at the data sheet....is the BF245 a good equivalent JFET for a P90?

You can "make" a 15µf capacitor by putting a 10µf and a 4.7µf in parallel. (well..14.7µf close enough!)

The 1M pot can be "changed" to a 500K by adding a 1M resistor across lugs 1 and 3.

The zener depends on what works well with the JFET set that you matched. Save yourself some trouble and purchase a few different voltage ratings above and below the stock 5.1v.

If all goes well....when you fire up the P90 and adjust the bias trimpot you'll know which zener doesn't work when you don't get any phasing or it begins to phase a bit when the trimpot is maxed in one direction

I've read that guys used bf245s. Hell i've got a ton of 2sk30s, so no biggie about the fets.
I think that RG wrote something about changing the taper of the pot. But i'm not sure.

[samhay]

Yep - if I understand correctly, according to RG's secret life of pots; When using a pot as a variable resistor, a smaller resistor placed across a pot will turn said pot into a variable resistor with a reverse log taper.
Putting a 1M resistor across a 1M pot will give a 500k pot with a variable resistance that has a reverse taper. it will not be very steep, but it is probably better than a stock B500k pot.

[JebemMajke]

You can "make" a 15µf capacitor by putting a 10µf and a 4.7µf in parallel. (well..14.7µf close enough!)

Like this?

[samhay]

yep - that will give you 14.7u, which is near enough for anything you are likely to need.
Alternatively, you could get slightly closer with a 47u and a 22u in series (14.99u) or spot on with 15 x 1u in parallel, but that would be silly in either case.

[Seljer]

Making that cap bigger makes the LFO slower

You can just use a 22uF for the capacitor making the slow end of the speed control a bit slower and decrease the size of the resistor thats wired in series with the potentiometer to get back the high speed settings.

[jimilee]

I used a 22u in mine,works great no issues at all.as far as the pot goes you can use a 500k log or anti log no issues either.

[duck_arse]

on the question of fets, I'm playing with bf245C and 5V6 zener at the moment, and they are fine, coming up about 1V1. the bf245B would be a safer bet with the 5V6, and should give plenty of trimm.

and the B's are selected by the manufacturer *NOT* to match with either of the other two marks, just so they can sell them, and we can use them.

[JebemMajke]

So what you are saying that i can not mix b,a and c in phaser? Even if they are matched? 

[JebemMajke]

5 x BF245b with 2.55v measurement . I've had 4 with 2.54v and 3 with 2.51v. And i went through a bag of 20 bf245b's
Can't wait till tomorrow to finally finish this thing
Thanks guys for support and answers. Still can't get that rev log.  

[armdnrdy]

Still can't get that rev log.  

Read reply #6 from Samhay

[JebemMajke]

Like this?

[JebemMajke]

Tried it with 5v6 zener and it ok, but a bit subtle. On to the next zener.

[bluebunny]

Like this?

I think the resistor needs to be between 1 and 2, not 1 and 3.  Check out R.G.'s "secret life of pots" at Geofex.

[JebemMajke]

Yep

[Govmnt_Lacky]

Remember folks...

I believe that this trick only works when the pot is set up as a voltage divider. In the Phase 90 circuit, the Speed pot is set up as a variable resistance.... NOT a voltage divider.