The Gibson Century two-knob tone circuit... unmasked!

Steve Ahola · From: Concord, California

Some of the people here have been interested in the details of the two knob tone circuit used in the early Gibson Century and Ultratone models and presumably in the early Console Grande's as well.

Nothing really unique here- no Holy Grail! The second tone control is the mid/bass cut circuit that was used later in the Ibanez Howard Robert guitars as presented in the Donald Brosnac book on guitar electronics. What is interesting is that it is wired backwards on an audio taper pot so you are getting a reverse audio taper as you turn up the control. And the mid/bass cut position is fully counterclockwise. So 10 is the normal sound and 0 is the mid cut sound.

Details. All of the pots appear to be the same: 500k with a 20% audio taper (so at 1/2 way up from 0 it is ~100k). The markings are "0.5 MEG E VC1 342-214637". The normal tone cap is a Mallory TP 412 .02uF 600vdc wax molded paper. The "other" cap is a Solar Sealdtite wax molded capacitor .001uF at 600wvdc.

The wire from the Volume pot which would normally go from the wiper to the output jack is routed instead to the wiper of the mid/bass cut control. And the wire going to the output jack is connected to the "hot" CW terminal of this pot. The .001uF cap is connected between these two terminals and nothing is connected to the "cold" CCW terminal.

The .02uF cap connects the normal tone control wiper to the volume control wiper and as usual the "cold" CCW terminal is connected to ground.

I'll see if I can get this drawn up both as a schematic and as a wiring diagram to satisfy both the left brain and right brain types out there...

Steve Ahola

P.S. I was reading how a lot of people didn't like that extra tone control on their Howard Roberts guitar so they had it disconnected. Not really necessary- when set to 10 it is essentially out of the circuit. You may need to change the value of the mid/bass cut cap from .001uF to match the pickup(s) that it is connected to. I think that the trick might be to work both controls together

P.P.S. I was going to post this under Electronics but it only pertains to the Gibson non-pedal steels. And yes it does get a bit technical... Whoa!

Here is what the controls look like (not mine):

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Doug Beaumier · From: Northampton, MA

Thanks for the info Steve.
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Andrea Tazzini · From: Massa, Italy

Thanks for sharing Steve!

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