Vulcan Tone Control

[Schappy]

Ive breadboarded a Vulcan and tried the BMP tone stack and also the Stupidly Wonderful Tone Control.

They both take something away from the sound of the circuit.

Has anybody had success with adding a tone control to the Vulcan?

[WGTP]

Download the Duncan Amp Tonestack, plug in the values for the Vulcan BMP and tweak from there.  Some people have added TMB tonestacks to the Vulcan, Obsidan, Blackfire and liked them.  All 3 have plenty of output so the tonestack doesn't suck up too much output...   

[will]

The BMP tone control will change the character of the sound, usually creates a dip in the midrange depending on values used. Check out the Duncan Tonestack calculator to see the effect of the different values.   I expect your simply wonderful tone control is cutting too much treble for your tastes. It can be designed to be just a subtle cut depending on the values used.

I found the Vulcan circuit to sound good but too much treble and a little too much gain for my tastes. So I used the output tone shaping resistors and caps from the Meteor circuit before the final Volume pot. (10K 3.3nF and 15K 1nF). You can change one of these resistors to a simply wonderful tone control to reduce the treble cut. I feel this improved the overall tone voicing of the circuit. I also cut the volume before the last stage by replacing the 220K resistor before the final Vulcan diode with a 1 M resistor.

Regards,
Will

[Schappy]

How did you reduce the gain?

[will]

I initially put in 1k trim pots between the 22uF cap and the emitters on the 1st 2 transistors to reduce gain for the 1st 2 gain stages. So I could experiment between 0 and 1K ohms. I'm not sure what I left the settings at as I did this 6 years ago. The change I mentioned above replacing the 220K resistor with a 1 Meg before the last stage.

I liked the tone of the effect but the volume control was always set really low otherwise it was too loud so I put in a 200K resister before the volume control. This made the volume control much better for my tastes.

I then moved on to vulcanizing the FET Driver which I think  ended up sounding even better.
http://www.diystompboxes.com/smfforum/index.php?topic=19751.0

Regards,
Will

[WGTP]

Though I'm morally and ethically against it, another way to reduce the gain the of individual stages is to remove the 22uf by-pass caps.  I'd start with the last stage. 

[Mark Hammer]

Ive breadboarded a Vulcan and tried the BMP tone stack and also the Stupidly Wonderful Tone Control.

They both take something away from the sound of the circuit.

Has anybody had success with adding a tone control to the Vulcan?

You do realize that a passive tone control of any type will result in a volume drop to just about any circuit?

Joe D's circuits generally have enough output level, though, that even with a noticeably volume drop through passive loss, there is still plenty of signal left to provide a much louder output than bypass.  The BMP-style tone control will produce volume drop, but ask yourself if there is still enough output level left over for your tastes.  And if there is, then no problem; the overall circuit (Vulcan + tone control) is behaving exactly like its supposed to.

[Schappy]

The output loss isnt too much since this thing started out with a ton of volume.

I like the circuit stock although Id like to tame the highs.

The tone controls Ive tried just changed the sound in a negative way.

I  Put a 15k resistor after the .01 cap and then put a .001cap to ground(twice in a row actually). I like the sound but just want some more control.


If Im taming the highs what kind of values would work in the stupidly wonderful tone control. Im still trying to learn the effects of component change on a tonestack.

[Mark Hammer]

It works like any other RC filter.  The roll-off starts at the frequency dictated by 1 / (2 * pi * R * C).  So, let's say you were to use a 25k pot in conjunction with a 100k volume pot, and run a cap to ground from the wiper of the tone control.  When the wiper of the tone pot is rotated to the side closest to the lug connected to the input of the volume pot, the treble rolloff is set to the lowest frequency in the range of the pot.  A .01uf cap, with 25k of resistance leading up to it would provide a 6db/octave rolloff starting around 637hz.  If the cap was .015uf, the rolloff would be at 424hz.

Rotate the tone pot completely the other way, and now the 25k of the tone pot would be between the cap and the volume pot, with no resistance preceding the tone cap.  We can't have that since the treble cut needs a resistance to work in conjunction with the cap.  If we stick a 2k2 resistor between the stock  .1uf cap and the added 25k tone pot, 2k2 and a .01uf cap to ground gets us a rolloff at 7.2khz.  Using a .015uf cap gets us a rolloff at 4.8khz.  If you feel that doesn't give enough sparkle at maximum treble, then drop the 2k2 down to 1k for a wider range.

A 1k fixed resistor in series with a 25k pot, and a .015uf cap to ground will get us a total range of rolloff points from 10.6khz down to 408hz, which is nice and wide.

Finally, I might point out that one of the major differences between the 2nd and 3rd transistor stages of the Vulcan is the use of .0022uf cap that bypasses the 220k resistor, providing a lower-resistance path for high frequencies.  If a person inserted a 100k-250k pot or trimpot between the .1uf cap and .0022uf cap, they would be able to vary the extent to which the cap provides an express-route for highs.  Higher resistance should, in theory, mellow out the Vulcan.

[Schappy]

I know how to use the equation to get the rolloff pts but dont know what to do with the numbers.

What frequencies are desirable?

[Mark Hammer]

The rolloff is actually pretty shallow: 6db/octave.  Content one octave above the rolloff point is down 6db, and content 2 octaves above the rolloff point is 12db down.  So, even when the estimated rolloff frequency is very low (e.g., 450hz), there is still more than enough treble coming through for a person to know that the signal has added harmonic content, and is not just a pure tone.  I hasten to remind folks that the venerable Tube Screamer applies a similar RC filter after the clipping stage with a rolloff starting at 723hz, and folks have no difficulty whatsoever knowing that it is a distortion pedal.

For all intents and purposes, rolloffs above 8khz are generally only heard as a slight reduction in whatever hiss the pedal produces, and rolloffs as low as 400hz still yield something with audible grind.