Vintage Wah Analysis

[FloPoeKo]

Folks, I have (in a state of sheer absentmindedness) amassed a collection of Vintage Wah-Pedals (mostly Italian and two Thomas-Organ Sepulveda-Models still to come). I´d like to analyze them in the next weeks and post my findings, starting with the pots. Does anyone have an idea how to adequately measure the pot-taper? Is hooking up a DMM and charting out the resistance-values accurate enough?
I planned on making about 25 test points (Tooth-To-Tooth-Top and Tooth-To-Slot)... How would you plot that stuff out? Engineers, please help...

[ppluis0]

Ho FloPoeKo,

My best guess is that you can fill an Excel worksheet with similar measurements (One value for each tooth written in each cell, for example) and plot the family of curves in the same scale.

I'm intend to say that if yoy have one pot with a value of 100K and other model of pedal that use pots of 250 or 500K you must divide the results by a factor of 2.5 and 5 respectively, to have curves that can be compared at a glance.

Keep us posted about your results !!

Cheers,
Josè

[toneman]

Thought u might mean a "sweep analysis"?
U know, feed a sweep/function generator into wah and look at the frequency curve(s) on a scope.
If the sweep generator has a marker function, u can analyse the center frequency movement and bandwidth.
How much "Q" (resonance) does it take to make it "vocal"?
I've always wanted to experiment with this on wahs and other synth filters.

[ppluis0]

How much "Q" (resonance) does it take to make it "vocal"?

An usual modification for the cry baby wah is to replace one resistor placed in parallel with the inductor. The standard value is 33K and is raised to 100K in order to have higuer Q numbers.

That is called "the vocal mod" IIRC

Cheers,
Jose

[modman]

Sorry for the long silence...
In the meantime, I´ve built a simple measuring device which gives me easy 5°-Steps (depending on the pot about 64 steps) and fits the pinion-gear. This way I can measure the pots of any wha without even desoldering them. The procedure I use is based on the attached pdf and goes as follows:

a) Potentiometer is set full ccw, measuring device is calibrated to 0° (arrow is over 0°-marking)
b) Potentiometer is turned to full cw position to determine 100% mechanical travel (e.g. 314°) for reference-purposes
c) Ohm-Meter measures resistance between Pin 1 (left) and 3 (right), which gives 100% of pot resistance independent of wiper-position for reference-purposes (104kΩ)
d) Ohm-Meter is now hooked to Pin 1 (left) and 2 (middle/wiper), which gives resistance increasing in clockwise motion.
e) Pot is turned back to full ccw, measuring-device is calibrated again so that full ccw (arrow) is 0°
d) Pot is rotated in cw motion in steps of 5°(for good resolution of the curve) over full mechanical angle, resistance is measured at each 5°-Step and written into chart (about 60-63 values).

Now: Does anybody have an idea how to make an excel spreadsheet which displays 0-100% of pot rotation on x-axis and 0-100% of resistance on the Y-Axis as in the pdf and maybe even allows for comparing different potentiometers in the same graph in a normalized way?

[Rjcastro76]

I recently began to mod wahs and also acquired 2 sepulveda crybabys. I did the vocal mod on both, From 33 k to 68k. Also changed the input resistor from 68k to 47k to increase volume, as I also did true bypass mod to both. I would add that in what I have seen, both circuits of these identical pedals had resistor values that varied significantly from the intended values of each respective resistor. I believe this contributed to the different, unique qualities of each pedal. That being said, I now think that pot analysis is important, but pot selection should go hand in hand to the individual pedal....in other words, select a pot to suit each pedal, due to each pedal not being a "perfect/ stock crybaby due to the varying resistor values. The 2 crybabies had different pots, one of them looked like a newer 100k audio pot that measured max 85k. The other was possibly the original centralab pot, measuring up,to 125k. The 125k, for what it's worth, had a wider sweep , more throaty, and a deeper sweep from low to high. The 85k pot sounded mellowed, funkier, bob Marley- Ish. Both cool! Hope this helps...wanted to share my findings and see if this is what you have encountered as well

[FloPoeKo]

The 2 crybabies had different pots, one of them looked like a newer 100k audio pot that measured max 85k. The other was possibly the original centralab pot, measuring up,to 125k. The 125k, for what it's worth, had a wider sweep , more throaty, and a deeper sweep from low to high. The 85k pot sounded mellowed, funkier, bob Marley- Ish. Both cool! Hope this helps...wanted to share my findings and see if this is what you have encountered as well

Can you post pictures of the pot, especially of the coding?

[FloPoeKo]

Just to get things finally off the ground I´d start with a comparison for which I simulated two circuits in LT Spice:
The one is what I named the 1967 Vox Clyde McCoy, the other is the current production Jim Dunlop Standard-Cry-Baby GCB-95.
Both have a pickup/guitar-circuit attached to simulate the effects of the input-impedance on the pickup (only shown with the GCB-95-schematic, but active in both for simulation). I have to thank Bernd Meiser of BSM Finest Treble Booster fame for helping me with the default schematic.
So, there are several distinct properties by which they differ and that I think are worth mentioning:

- The Buffer in the GCB-95 preserves the resonance frequency of the pickup, which results in a enlarged Q at the highest frequency (which is not present in the Clyde).
- The transistors in the Clyde are representative of a lot of vintage wah´s in terms of the Hfe (forward current amplification), which is between 200 and 500, whereas way over 1000 in case of the current cry baby. I used a 2n2222 since there is no difference to be seen in the results:
I somehow can´t get LT-Spice to simulate any changes here (open for suggestions), but it seems that a lower Hfe results in a lower internal resistance of the transistor being a load to the coil and greatly dampening Q (Meaning a sharper filtering with higher Hfe)...
- Re1, the emitter-resistor of transistor 1, is 390Ω in case of the modern cry baby, raising Q a tiny bit and shifting all lower frequencies a bit lower than with the 470Ω in the vintage unit, but not the highest (fmax stays where it is, at least with this parameter).
- The Inductors I could measure all vary around 500mH nominal inductance L (±20%) with the vintage 14x8/18x11 pot-cores and around 600mH in case of the modern Dunlop toroids (black barrel type and red/yellow fasel reissues). I´d opt for 600mH as the nominal value for the current-cry-baby-schematic which is responsible for the whole sweep being lower and a bit narrower in bandwidth.
- As for their dc resistance, the 15+ newer Dunlops i measured all are around 16Ω, whereas the vintage ones come in several distinct ranges by model from 15Ω to 55Ω, over 55Ω up to 140Ω in rare cases.
A higher DCR progressively dampens the Q of the lower frequencies, the lower the frequency, the more dampening. The clyde was simulated with a good representative median of 30Ω and the lower frequencies are a bit more even to the rest of the sweep...
- Other differences: Cbp (4,7/4µF), Rb2 (82/100kΩ)...

[modman]

Another interesting thing is the evolution from the 1966 Del-Casher-Prototype/Vox Grey Wah to the patent circuit and finally to the Clyde McCoy.
Simply put, from what I can make out, the Del-Casher-Prototype is the Clyde without Rq (33k/100k) and a 250mH Coil (RDC unknown to me).
In the later patent, Rq is still missing, but the coil already has the nominal 500mH. This doubling in inductance shifts the sweep down significantly, away from the 60´s beatnik-wah to what we are accustomed to.
Now, both these versions still have a very high Q (around 25dB) which is later tamed in the clyde by Rq parallel to the coil (to around 18dB).
If you want to mimick the response of the Clydes predecessors with your usual wah, remove Rq for higher Q (Patent Circuit) and also lower Cf from 10nF to 4.7 – 5.6 nF shifting the sweep up (Del-Casher/Vox Grey).
That should give you something close enough to get the a groovy 60´s feel of velvet suits, frilly shirts, LSD driven parties of deep thinking and free love...

A word of warning:
Take care of your ears when testing, this is a pesky, loud sound...

[modman]

Hoops, it´s late... Here comes the Sim Vox Grey Vs. Patent...

[FloPoeKo]

So what about impedances? I´ve mentioned it already with the Jim-Dunlop/Vox Clyde Comparison and now visualized a few Input impedances with my trusted GiSi-Simulator (Written by Ulf "Der Onkel" Schaedla from Hamburg/Germany in Excel) on a generic guitar-circuit (Pickup, Volume, Tone, Cable, Input Impedance).
Also note that although the level will go up in case of your series-resistor, it will also go down a bit since the loading of the pickup...
They also contribute to how much high´s will be present.

Blue: 1MΩ, Dunlop Buffer, original Resonance of direct-to-amp is preserved
Green: 100kΩ, Colorsound-Wah
Red: 78kΩ, Input-Resistor Vintage Cry-Baby/Vox, a bit too large due to parts-tolerance of ±20%
Brown: 68kΩ Input-Resistor Vintage Cry-Baby/Vox, on spec
Orange: 60kΩ Input-Resistor Vintage Cry-Baby/Vox, a bit too small due to parts-tolerance of ±20%
Pink: 56kΩ Input Resistor, intentionally lowered to get more level after removing the buffer in your cry-baby for true-bypass and reading further on the stink foot page
Purple1: 47kΩ Input-Resistor, Maestro Boomerang or lowered even more to get even more signal after true bypass mod
Mint: 39kΩ Input Resistor, Fulltone Clyde Wah Standard or lowered to get way more signal after true bypass mod
Purple2: 27kΩ Input Resistor, see Schaller Bow-Wow-Yoy-Yoy-Schematic (first resistor to ground)

[FloPoeKo]

Before going into details, let´s talk wah-pot basics in general:

- In a wah, the potentiometer ideally has to go from zero-resistance to maximum resistance in order to use the whole sweep set by the inductor and the capacitor.
- At zero resistance/full counter clockwise the sweep is at its upper limit or toe down. This means in a traditional sense, the pot is turned down to get to the high frequencies. Vice versa, the pot is turned up or clockwise to get to the maximum bass frequency or heel down. If you take a close look inside your wah you will see that this is the case (exception: In the Maestro Boomerang, the pot is mounted the other way around, so it needs a reverse-taper).
- So one of the main factors is: Is the effective electrical travel (0-100%) of the pot narrow enough (say 270°) to allow to go from 0-100% with the available opening of the pedal-shell (opening only allows for 180° travel). If not, you will only have a certain part of the sweep depending where you set the pinion/gear (you usual "how many teeth back from full tilt"). Also, you will have a smaller change in frequency (say your bandwidth heel-to-toe is now only say 1000Hz difference instead of say 1800Hz).
- The actual value of the pot (how many kΩ maximum/nominal resistance) shifts the lower frequencies down the higher its nominal resistance gets. Not by much, about 40Hz with a 500kΩ compared to a 100kΩ. The highest frequency will remain untouched as long as there is no residual resistance (pot doesn´t go to say 7-10Ω, which is close enough to 0%, but has still say 250Ω left).
- The taper/law/curve of the pot determines the change in resistance between say two teeth of your gear. Rule of thumb: much change in resistance (percentage of nominal resistance, say 0-50% or 0 - 50kΩ) in a given rotational angle ( say two teeth), faster sweep through your available range of frequencies (and vise versa).

All in all, this essentially determines the character of the different pots when used in the actual pedal:

- Which frequencies are generally available?
- How complete will/can they be swept through?
- Where in the complete throw of the rocker are the different frequencies located?
- How fast or slow is the change say "per tooth"?

Btw, It would be very helpful at this point if you have any questions I can work towards to help me direct my posts a bit better...

[modman]

I´ve disassembled an old Thomas Organ Centralab Wah-Pot and made a few pictures... Interesting stuff: carbon wipers, one small on the inner ring to connect the lug to the wiper, one big for the resistive track... I wonder how the baseplate looks from the inside, maybe I will crack it open, the pot is dead as can be...

[FloPoeKo]

Who else wondered what´s inside the ever infamous TDK 5103 Inductor? After half an hour of prison-break-like scratching away glue and plastic casing with an exacto I got to the heart of the matter... Typical 14x8 pot core...