Phase 90 LFO potentiometer value

[Myampgoesto12]

Hello all!

I've been looking into getting a P90 (either a kit or an older model with through hole components), and building it in a wah enclosure that I have. I know there are other phasers with the following mods stock, but I love the P90 tone.  I'd like to be able to control both the rate of the phase AND the phase position (like a manual phase sweep), with the treadle. I'll have a switch to select which perameter is controlled by the treadle.

For manual control I could easily emplament the design found here

https://www.diystompboxes.com/smfforum/index.php?topic=92129.0

with the 100k hot potz I already have.

However as most of you may know, the stock circuit uses a 500k Rev. Log pot for the rate control.

Is there a means of converting the LFO to function with a 100k with the same results? I know just swapping it in won't work hah, but my research hasn't yielded much info at all on analog LFO design, or the reason this LFO uses a 500k. So I turn to my favorite place for help!

The only other option I've thought of is to physically add a 470k High Gain pot somehow into the existing rack and pinion system. Either by cutting the shafts down and using one pinion, or by meshing the two pinions to be turned by the rack, both requiring a new bracket to hold the 470k pot.

Originally I though about mounting the rate pot on the side of the enclosure just for the sake of making it easy, but the idea of a way better "Variphase" makes the effort worth it I believe.

Thanks checking this out! Sorry for the long post

[Myampgoesto12]

Sticking my foot in my mouth! I may have found a couple of workarounds so having two pots won't be necessary.

Easiest - buying the 470k and using switched parallel resistance across it for the "manual mode". In a perfect condition scenario adding 140k in parallel with the 470k ≈ 108k. But this changes the taper. May try to find a B500k long life with the D shaft for this, may not.

Hardest - finding a dual resistance (~100k/~500k)2 gang long life pot, finding/making a pinion that would fit it, and fitting in the enclosure.

I'm gunna go with easiest first and deal with the odd taper.

Any input would still be appreciated!! Thanks!

[duck_arse]

you might think about using the stock pot to control a led in a vactrol, w/ the ldr doing the lfo rate. look to geofex, RG has a wah conversion page will show the overall idea.

[Myampgoesto12]

That's a good idea.. I'll look into it. I don't currently have all this stuff anyway so I could order the parts for the vactrol along with the P90 circuit, (I am planning on building a kit more so than finding an older model to tear into) for experience and minimal price differences.

Thanks a lot for that!

[ElectricDruid]

My angle of attack would be that if you reduce the pot in the LFO by a factor of five, you need to increase the capacitor in the LFO by the same factor to keep the same range.

[Myampgoesto12]

That seems like a great solution. Then all I'd have to have on a switch is what the pot is connected to. It'll still have the ICAR taper, but that at least wont be altered by parallel resistance.

Both suggestions require few parts, but I have spare caps and the stock pot already. May try the cap/pot swap first.

I've also read on here about other mods, such as a blend pot. But that has the down side of being hard to dial it in at unity. I may do this in addition to installing an LPB1 on a little board in the pedal, post phase circuit.

Thanks again.

[slashandburn]

Are those Hot Pots totally sealed? If not, another option might be to rip it open and swap in a wafer from a pot of the desired value and taper.

I've never tried it but I'm sure it's mentioned in one of the AMZ articles (the secret Life of pots, maybe?). 

[Myampgoesto12]

if i ever had to tear into a pot i would, I've repaired a rotary spdt switch in a 10k pot before, but I'd rather buy pots and try them. If nothing else I'll have spares hah..

I'm not sure what the desired taper for the manual control would be, not sure if there's a standard hah, nor if phase is perceived linearly or not 

I may be wrong but i think for the rate control, the Variphase just has a 470k hot potz (audio taper?) in place of the 500k Rev. Log pot that this circuit calls for. I've never ripped into one, its LFO may be different. But the pot in a wah is turned DOWN when in the toe-down position. So I assume its wired backwards to achieve the proper effect, right?

Enough tinkering may yield a way around it, or just playing with it long enough should get me accustomed to the taper, like the kink in my Crybaby's sweep.

Thanks!

[Myampgoesto12]

Switching the pot out of the LFO won't harm the LFO will it?

[ElectricDruid]

Switching the pot out of the LFO won't harm the LFO will it?

No, I'd be very surprised if it did. If you went right down to some extremely low pot value (<100R maybe) then perhaps you could get enough current flowing that you might fry something, but otherwise I don't see how you'd have a problem.

[Myampgoesto12]

Its going to be switched out of the LFO to be used as the manual control pot, or switched back into place. So with a 4pdt I can swap the pot into the LFO as a variable resistor while throwing the bias voltage towards the gates, or swap the pot out of the lfo, into the manual control 3-pot in series deal from the post above, as a voltage divider while throwing those three pots to the gates.

So with that set up the LFO sees nothing but the cap between the square wave and the gates.. Just didn't know if there'd be a problem.

Soon I'll post a schem of what I'm doing. But progress has halted due to a burnt diode. I either fried it while installing or put the wrong one where the 5.1v Zenger does. Upon power up the diode in that place gets super hot, and that causes the battery to heat up as well. Now that diode has no marking, all i can see is the different materials inside of it haha. Gotta order some more diodes I guess. This kit (from GGG) has a 1n914 (will probably get a 1n4148 replacement) and a 5.1v diode whose part # was never given.

[ElectricDruid]

Ok, I didn't understand you correctly. I'll need to see a schematic or I'm not going to get it, sorry.

T.

[Myampgoesto12]

Ok, here is a schem I reworked with the control switch and a couple of other mods added. Alterations to the circuit are identified by orange dots.




This includes changing the 0.01 CAP in the LFO as suggested above, for having a lower value pot to control rate.

The switch at the bottom, SW2 is what will allow the 100K pot to control the LFO or to control the voltage to the gates directly.

Any thoughts or suggestions welcome!! I'm sorry about not getting this across easily with words alone. Hope this helps!

[Myampgoesto12]

Got the new diodes!!!!

This thing works like a charm. While still testing the circuit I have it wired for LFO control only, I will add the manual control when I have time. I replaced the .01 cap (C8) in the LFO with a .056, so far this seems like a suitable substitution considering the Hot Potz's limited rotation. Its a bit faster than I'd LOVE it to be at its slowest range, but this is tolerable.

Since I have the board unmounted I did add the depth and mix mods, I find the mix control to have limited use. Not a horrible problem but its scratchy when manipulated (better pot may help) since its in the audio path, and using a linear taper between two audio signals puts most of the perceived volume differences really close to the ends of the pots rotation. So I'll either make that section stock or use a trim pot on a mini board to find the  "sweet spot" in the mix. Another way is to use an actual mixer pot (dual gang, single shaft) with opposing A tapers.

The depth control is killer (good). Its as simple as that 

I still have to clean it up, check voltages, add mods etc, so there's more to experience from this little jewel. I ordered several 4pdt toggles so I may get around to that uni mod as well. That might be cool to control manually 

I'll update as I get through this thing, thanks for the input so far, its the biggest reason I'm even trying this.

[pinkjimiphoton]

My angle of attack would be that if you reduce the pot in the LFO by a factor of five, you need to increase the capacitor in the LFO by the same factor to keep the same range.

hey tom,
would that work with a univibe, too? the reason i ask is cuz...

Switching the pot out of the LFO won't harm the LFO will it?

No, I'd be very surprised if it did. If you went right down to some extremely low pot value (<100R maybe) then perhaps you could get enough current flowing that you might fry something, but otherwise I don't see how you'd have a problem.

i actually lit my univibe on fire the other nite monkeying with the pot values, sooo.... rg's helping me debug it, but i'd reccomend being careful... a univibe is just a phaser too, really.

hey 12, check the 45/90 mods. basically ya can bypass two stages with a switch, and have both to choose from.

peace!

[Myampgoesto12]

Hah yeah, my concern is burning up my opamps, though 741s are cheap and available, and i have a few spares, I'd rather not lose what I've got for now.

I just can't find any info on how these things (LFOs) act enough to find out if an open lead where a pot would go could cause issues. Or, for the matter, if switching the pot out or back into an operating LFO is hazardous. Though it did survive a burnt and burning hot diode right next to the LFO IC, I mean it hurt when I touched it hah.

[Myampgoesto12]

I said "Screw it, I'm gunna just see what happens when the LFO sees no pot at all" and snipped on of the leads. Audibly I just doesn't produce a phase shift, I guess because the Gates aren't seeing a voltage shift.

I left it like that for a bit while powered up and then touched that lead back onto the pot. Took off on an auto sweep like normal. So until proven wrong I'm guessing it'll be ok to remove the pot from the LFO with the switch.

Thanks again for the help everyone. I'll get this thing together at some point and post some gut shots. Right now as it is, it is a treadle controlled phase 90, soon to be a good bit more!

[ElectricDruid]

Ok, I'm going to stick my neck out for a bit of educated guesswork. No promises of total accuracy!

I would guess without having tried it that if you take the pot out a of the typical Schmitt trigger LFO (something like this one below) that the integrator triangle output will finish up at one of its two extremes, and probably more extreme than it manages when it's running as a triangle - basically it becomes a comparator with one input tied to ground and the other input open. We can assume that open input won't be perfectly at ground, so it'll fly off to one rail or the other.
That's my best guess, anyway.



You might get "a phase shift" without the pot (since the output is still at *some* voltage level, but you won't get a *moving* phase shift, so it won't be much you can hear.

Jimi, the Univibe LFO is a completely different beast. It's a phase-shift oscillator based on transistors, rather than a Schmitt trigger/integrator oscillator. I don't know a lot about it, and I'm afraid I have no idea how you managed to set the thing on fire. Are you sure you didn't squirt lighter fluid on it?

T.

[Myampgoesto12]

That makes sense to me, because of what the LFO is there to do, oscillate. And since the tie point from bias voltage and LFO- where the 1m and 3m9 resistors connect to the gates of the transistors- will be switched out for the manual source of voltage, the LFO will not impact the circuit at all while in the "manual mode" anyway.

I just hope it doesn't have some crazy runaway train effect without a pot. Like.. that burns a cap or IC or something 

[pinkjimiphoton]

Jimi, the Univibe LFO is a completely different beast. It's a phase-shift oscillator based on transistors, rather than a Schmitt trigger/integrator oscillator. I don't know a lot about it, and I'm afraid I have no idea how you managed to set the thing on fire. Are you sure you didn't squirt lighter fluid on it?

T.

ummmm..... no, not yet, but i may have to if i keep blowing it up. i was playing with tapering resistors on the dual 100k to make a more log pot than a linear one. so i figured i'd try different sized resistors,  i seemed to recall about 20% is a decent value and tried a few different values between 2.2 and 47k, i liked the 22k about the best.
but it was still all bunched up on one end of the pot, i too mounted the whole shebang in an old dearmond volume pedal. anyways, i was being a dumbass and tacking different resistors along the outside lugs of the dual pot to try and get a more useful sweep, and as i connected the second resistor (100k, i thought i could make the pot look more like a 50k log) it freaking started smoking, it burnt the resistor to the b of the lamp driver to a crisp along with most of the electros on the board. a few of them were smoldering. in a split second. i think my iron "leaked" enough voltage to add to the 22 volts or so in that part of the circuit and smoked it. so i've been debugging since.
sorry for the momentary hijack, but hopefully someone else will learn from my stupidity. power down, discharge the circuit, don't be like pink jimi at 4 am out of his mind on caffiene trying to beat the birds chirping lol.....