The FM Tube Modulator Jimi Loved

His guitar slung across his back, his dusty boots is his cadillac.

Moderators: VelvetGeorge, BUG

Post Reply
Senior Member
Posts: 589
Joined: Wed Oct 29, 2014 9:36 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by daveweyer » Sun Jan 22, 2017 5:09 pm

Just a side note, the added capacitor under the circuit board on the wah was a .003. You need that to get the right range with a 530mh inductor. I added that cap across the .01 Paktron film cap. Believe it or not, a ceramic cap worked best. The sound was so juicy.

Senior Member
Posts: 189
Joined: Sat Jan 10, 2015 9:43 am
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by Tek465b » Sat Jan 28, 2017 11:55 am

Ok a little bit off topic here but since we are talkinga bout the wah.

I did some math....
(my goal here is to achieve the same resonnant frequancy and Q/bandwidth and damping/attenuation with the 500mH inductor and a 15nF capacitor)
If somebody want to try a close equivalent by changing the capacitor and resistor (instead of inductor/capacitor)

here is my result.
with the stock 500mh / 10n /33k

We get and angular frequency = 14 142
in hz = 2.250 khz
Quality factor = 4.666
angular bandwidth = 3 030
in Hz = 482.287
Damping = 0.107
Attenuation = 1 515

Now with your 530mH inductor and 13n, we get a sharper peak(more Q) and a lower resonant frequency.
530mH / 13n / 33k
angular frq = 12 047
in hz = 1.917 khz
Q = 5.168
Bandwidth in hZ = 370.990hz
Damping = 0.096
Attenuation = 1 165

And now with a setting that yield similar result as your 530/13n
500mH / 15n / 30k
angular frq = 11 547
in Hz = 1.837khz
Q = 5.196
Bandwidth in Hz = 353.677
Damping = 0.096
Attenuation = 1 111

That look darn close to the 13nF and 530mH
Resonant frequency is a bit lower (80hz)
damping is the same, Q and bandwidth and attenuation are darn close.

Now if we compare with the mod everyone usually do
500mH / 15n / 33k
angular FRQ = 11 547
in Hz = 1.837kHz
Q = 5.715
Bandwidth in Hz = 321.525
Damping = 0.087
Attenuation = 1 010

We can see that with the standard mod of 500mH / 15n / 33k
we get the same resonant frequency that my mod and close to your mod (80Hz lower)
But the Q(quality factor) is higher wich in turn make for a narrower Bandwidth, less damping and attenuation = sharp peak.

So replacing the 33k with a 30k and a 15nF/500mH get those last 3 parameter closer to the 530mH / 13n / 33k value
In fact if we use a potentiometer/trimmer instead of the 30k resistor and tune it around 29kOhm we will get pretty much the same exact value for all parameters except for the resonant frequency. -80Hz (if we factor 10% component tolerance that is next to nothing)

Senior Member
Posts: 589
Joined: Wed Oct 29, 2014 9:36 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by daveweyer » Sat Jan 28, 2017 7:15 pm

Tek, nice analysis.
Just keep in mind that components make a huge difference in the sound; if you you use Mullard Tropical fish caps, it won't sound exactly like the circuit with film and foil Paktron caps. The inductor ferrite makes a big difference too. So if you match the Q and bandwidth exactly, you may not have the same pedal.
As I have tried to explain here, Jimi liked a darker band, namely because he played a lot of tunes with the circuit engaged but only moved the pedal to hover around the bandwidth center of the strings he was playing. He didn't want the peak too sharp, obviously, and some wahs have a very sharp peak which makes them sound honky. I first used a Triad 500mh toroid, but its peak was too broad; the cup inductors had higher Q and the TDK was just about right.
Geoffrey Teese, the ultimate wah builder, ended up with almost the identical pedal I made Jimi, all through his own research many years later. For Jimi stuff, those parameters are pretty close.
But you have made a nice analysis for all to see, and I think it helps to explain the sound of the pedal I made Jimi. All those interested should copy and paste these figures into their notebooks for future reference.
The bandwidth and Q of tone shaping circuits was a big part of the engineering at Thomas Organ, because that form of subtractive synthesis was central to the voicing of their organs; the wah circuit originally was lifted out of a Thomas Organ, and then made variable with the feedback circuit devised by Brad Plunkett. Thomas tried all kinds of inductors but found the Q of the cup inductors most suitable for their organ voicing circuits.
In your next analysis, check the load resistance on the collector of Q1. If you make that collector provide much ac current to the load, the variable sweep bandwidth collapses. When I plugged the circuit into the three transistor fuzz, and took out the wah output resistor between the circuits, I could adjust the wah sweep with the input control on the fuzz circuit. If I turned it down to zero ohms, the wah would sweep to its highest frequency and remain there no matter which position the pedal pot was in. (The three transistor fuzz had to be at the low gain setting, i.e. maximum negative feedback.)
That's because the three transistor fuzz has a lot of negative feedback at the base of Q1, meaning the input impedance is very low. The wah circuit does not have the current to drive a low impedance and maintain its sweep range. That is one reason I used an output resistor on the wah.
The greatest sweep range is had by adding a source follower FET on the output of Q1, something that might be done on a revised model, something I would have done if Jimi had lived.
To Thomas it didn't matter because they expected the wah to be plugged into amps, high impedance inputs.

Senior Member
Posts: 189
Joined: Sat Jan 10, 2015 9:43 am
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by Tek465b » Sun Jan 29, 2017 9:34 am

Yes of course the component selection will make a difference.
Just the coil winding resistance alone has a big effect on the Q of the coil itself. If that was a LC circuit (without the 33k R) the Q and attenuation could be heavily affected by just 0.1 ohm difference from the coil. since we use RLC the effect is minimum. There are other parameter, but inductor always been quite a mystery for me. :shrug:

Darker band and less sharp peak totally confirm what i expected and i think its a really good idea :). Now its time to warmup my soldering iron ;).

About the fuzz/wah.
The way i see it is simple, the input impedance of the Fuzz get in parallel with the Wah's Collector resistor.
This turn the Voltage gain wayy down thus cancelling the miller effect (can't make the capacitor look bigger, no more wah sweep).
We still get sound going thru because of the current drive.
This is why i made a Ge input buffer at the input of my fuzz (on a bypass switch of course, with a 12k between Buff and fuzz input to control gain), also it seem counter intuitive, but for BOG sound, it to do the trick for me.. strangelly. (BOG was never my goal, it was accidental, woodstock always been my favority tone)

Senior Member
Posts: 589
Joined: Wed Oct 29, 2014 9:36 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by daveweyer » Sun Jan 29, 2017 6:15 pm

Roger made a fuzz with an extra input transistor, I saw it while working on Jimi's fuzz boxes at West Coast. It's logical considering how these effects interact. Personally I liked the interaction on the base of Q1 of the fuzz, because it made things sound different depending on the amount of negative feedback and the input source. It allowed the guitar to control the gain of the fuzz like a remote volume control.
This only works if the guitar is plugged directly into the fuzz; the current in the pickup coils can overcome the current in the negative feedback loop causing the gain to increase, R1/R2.
Putting an input resistor or device on the fuzz can eliminate the interaction, good if using a wah first.

Senior Member
Posts: 104
Joined: Wed Mar 09, 2011 1:30 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by Eb7+9 » Mon Feb 13, 2017 10:00 pm

I got around the classic WAH/Fuz-Face "boundary" issue by designing an op-amp simulation of the original two-transistor circuit, which works with same value components, incl. the inductor and pot ... this is done simply by using the basic small-signal parameters that can be derived from SPICE sims on the original gain stages (1st year analogue circuit theory stuff)

I avoided making the mistake I saw in a couple of other (DIY) op-amp wah designs ... works perfectly and it can be "tuned" via trimpots, ie., instantly tweak-able with a small screw driver ... moreover, I can tweak parametric circuit values without altering bias (which NFB keeps at mid-rail or ground) ... in the original WAH's altering bias meant altering headroom // it's a juggling act as everybody knows ... my op-amp WAH runs at +/- 7.5 volts, so there's all the headroom in the world (if you like that) ... that aspect is esp. useful on Bass and Keys, or the output of a preamp ... which, in theory, can help obtain better signal/noise

in my Ge FF I use a slight variant of the classic circuit, one that avoids bias issues and blather/gating ... and with op-amps I can take advantage of the high drive capability of this circuit to run the input of a fuzz-face without issue, as long as I come off the op-amp directly - and not the WAH pot

what's interesting in all this for me is that the same mods done to the classic WAH classic circuit can be made in my op-amp "simulation" WAH as well, and obtain analogous results ... :P

I'll have an update on the 4-channel CTP shortly ...
modern VT circuit analysis and modeling:

Senior Member
Posts: 104
Joined: Wed Mar 09, 2011 1:30 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by Eb7+9 » Wed Feb 22, 2017 4:09 am

took the plunge, ... I'm about half way across now

biggest and most expensive PCB I've had made so far

following the Baldwin CTP structure very closely
with a few modernizations:

as it stands,

* 4 independently-variable channels ...
incl. OFFSET, DEPTH and SPEED control on each 3-phase oscillator board
so, four sets of oscillator controls

* each channel also has a tank-cut switch, so the Band-Pass function can be inserted, or not ...
recall, the CTP was designed for synchronous "tone-wheel derived" signals
and so we (guitarists) may want to see what happens when we run each channel full-band instead

* each channel will also have its own GAIN control and "kill" switch

* the whole works has a Master Volume control

same tank frequencies as the Baldwin schematic
less lossy summing at the output (should be interesting ...)

picture shows results after two solid days of soldering

so far, I've got temporary LED's on the under side, to show PWM action
later, these LED's will be moved to the front panel

* each channel will have it's own trio of rotating "LFO" LED's to show status

that side of it now works ... on all four Duino's

checked all the voltages, +/-9 volt everywhere required, etc ...
5volts regulators obviously doing their job

checked out one of the converter circuit without OTA's in their sockets

by adjusting the master bias trimpot I was able to get
a 0.06mA to 0.88mA spread in OTA current
by sweeping the OFFSET control end to end for that LFO
that's roughly 5% to 90% volume variation in the OTA at (close to) full depth

I'll take it ...!!

should provide ample separation between modulated signals A, B, and C


next, I need to snake the A, B and C signals from the lattice to the modulators
same order as they are shown in the Baldwin schematic (ie., orig. patent)

hook up, the kill switches, GAIN pots and MV pot
couple of wires ...

and I should be able to hear it in action
hopefully I can get around to doing that this weekend

although, I have a couple of preamps I have to finish first
should have confirmation soon enough ...

can't wait :wink:


in the mean time if anybody can recommend a source of 1U and 2U rack cases
I'd appreciate it ... I can't seem to find the last guys I used

I'm looking for an outfit that can supply brushed Alu front panels

modern VT circuit analysis and modeling:

User avatar
Senior Member
Posts: 2429
Joined: Sun Apr 19, 2009 5:27 pm
Just the numbers in order: 7

Re: The FM Tube Modulator Jimi Loved

Post by Xplorer » Wed Feb 22, 2017 8:52 am

WOW !!! :o what a work !!! that's some serious pcb here !

Senior Member
Posts: 589
Joined: Wed Oct 29, 2014 9:36 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by daveweyer » Wed Feb 22, 2017 3:53 pm

Just cracks me up; I mean, to what lengths will we go for achieving altered guitar sounds? I love it.
Everybody cheer for JC.

New Member
Posts: 1
Joined: Sun Feb 25, 2018 9:41 pm
Just the numbers in order: 13492

Re: The FM Tube Modulator Jimi Loved

Post by Eb7-9 » Sun Feb 25, 2018 10:49 pm

hey y'all ...

it's been an interesting yr // I had to put down the CTP4 project after finding an un-foreseen bottleneck
in my last go-for-gold PCB ... haha, doesn't always turn out as planned

I tried fixing stuff ...

turns out I hadn't considered my current distribution ckt not working right when it came time to trim OTA's for offsets
yada yada ...

I've learned a few thing since then, and in fact busy applying pwm ideas elsewhere in the mean time ...

in some cases I'm avoiding OTA's in the process ... not that they're a totally poor choice for the job
but for the sake of operating at pro signal levels it's easier to go the op-amp route

op-amps also interfaces well with CMOS bi-directional (analogue) gates ... so do OTA's (just to be clear)
which makes op-amps equally well suited in the mixer section //

this means we can have a +/- 9volt rail pair accommodate up to 15 vpp signals throughout
targetting use in un-buffered FX loops and sends (either mono or stereo settings)

since the last time I've up with a more suitable comparator arrangement
or rather, a simple way of dealing with saturation drop-out
converting 0-5v to +/- 9v at high speed for example

when I get the chance I'm gonna shoe-horn a simple single-channel AVR based unit in a 1590BB box ...
will work the same as a chorus/vibrato, but take 15vpp signals ... so, providing low noise chorusing yada etc

the CTP4 is a wonderful dream ... but I think someone else can tackle that one day

ciao for now
take care y'all ...


Post Reply