The Gibson Varitone

[frets]

I don’t think the resistor does much on anything in the circuit.

I am perplexed by your lack of hearing the impact of a 15H inductor/transformer. I can hear the effect with a 1.0H. I have used a 10H transformer and it was too much.

I know I’m not being too helpful but I’m wondering if you have it wired correctly. The reason I say that is because the induction effect is so clear.

I make a pedal Varitone that has a 3-Way induction switch. It has no induction, induction at 1.5H; or, induction at 2.6H. Both induction settings are clearly audible in comparison to no induction.

So, maybe you have something awry in the circuit? Tgat 15H transformer should be very distinctive. Is the transformer good?

[sapristi]

Hi and thanks for getting back !

So in the meantime I've checked the circuit (nothing seemed out of order) and checked the inductor (15H indeed).

I replaced the switch with a pair of alligator clips to swap caps around more easily, and I replaced the 100K resistor with a 1M linear pot (lugs 2 and 3, with 1 going to ground).

I'm very happy to say the results are good!! I started with the pot fully clockwise, then turned it back.
As I did so the filter vowel got more pronounced, but the volume ended up dipping down too, eventually losing all signal when turning the pot all the way down. So I ended up finding a good "compromise spot" about 3/4 clockwise and I'm probably gonna change it to a trimpot when getting it into the guitar.

Either that or keeping the pot on the guitar to control the Varitone's intensity, but gotta keep in mind that it would require switching the lugs around (makes more sense IMO to have CW=more effect) and the knob would have a pretty narrow range of acceptable settings. Not ruling it out, I don't mind it being weird as it's gonna be my own guitar.

Interested to hear your thoughts! I still don't see how the original circuit didn't work with 100K though - I might indeed have had some kind of problem that I accidentally solved when changing it 

[swissfrank]

Sorry to wake up an old old thread but it's a good discussion.

I see JohnH has graphed the frequency response of the Varitone settings.  John, did you include cable length in the calculations?  I'm led to believe there's a substantial effect on resonant frequency with 5' vs 10' vs 20' cable, with longer cables having both lower peaks and at lower frequencies.

I'm also curious if there's a pedal that actually exactly duplicates this.  A glance at the circuit makes me think you could do it equally well externally.  I found one British tech selling a pedal but rather than a six-position switch it seems to have two continuous knobs and a third switch so it's clearly doing far more than the Gibson circuit.

[JohnH]

May 4, 2024 17:02:19 GMT -5 swissfrank said:

Sorry to wake up an old old thread but it's a good discussion.

I see JohnH has graphed the frequency response of the Varitone settings.  John, did you include cable length in the calculations?  I'm led to believe there's a substantial effect on resonant frequency with 5' vs 10' vs 20' cable, with longer cables having both lower peaks and at lower frequencies.

I'm also curious if there's a pedal that actually exactly duplicates this.  A glance at the circuit makes me think you could do it equally well externally.  I found one British tech selling a pedal but rather than a six-position switch it seems to have two continuous knobs and a third switch so it's clearly doing far more than the Gibson circuit.

Hi swissfrank and welcome to GN2

The plots did include the cable, which would be representing an average 10' cord at about 40pF per foot.

Here's a new plot to illustrate:






It's a model of a PAF Pro, with 500k pots. The upper plots which are flat with a moderate treble rise, shown without Varitone and with 5', 10' and 20' cords. The lower ones with the mid dip are the same range, with a fully engaged Varitone using the Gibson diagram with a 15H (which is quite a high value), and a 4.7nF cap.

The plots are made using a spreadsheet (GuitarFreak) which can be downloaded from our reference section.

These are predictions just of the electrical response of the pickups and guitar circuit acting as a filter. On top of that, the strings are vibrating and adding their harmonics, which typically fall away at higher frequencies. A true plot with that is very spiky, but here is an envelope by analysis, representing the same as above, with 10' cord, bridge PU, strumming across the strings at the 2nd fret:






It shows an estimate of how the high peaks would be diminished in reality.