The Mystery of the Gibson Century tone circuit... revealed!

[Steve A.]

I am posting this here because some of the winders were interested in the two knob tone circuit used in the early Gibson Century and Ultratone models and presumably the early Console Grande's as well.

Nothing really unique here- the second tone control is the mid/bass cut circuit from the Ibanez Howard Robert guitars as presented in the Donald Brosnac book. 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. It might be a good idea to change the value of the mid/bass cut cap from .001uF to match the pickup(s) that it is connected to.

EDIT 09/11/2017: Drawing of control circuit was lost over the years so in the spirit of John Wayne Bobbit I am reattaching it. I'm not sure why I called the bottom control "Treble Bypass" as it is more like a "Bass Cut"...

[Mike Sulzer]

Steve, let me see if I understand this. The cap of the second volume control is in series with the output of the guitar. Assume for the moment that the pot across it is set to infinity. Then the cap and the input resistance of the amplifier form a high pass filter. With .001 microfarad and and input resistance of 1 meg, the bass is three db down at 159 Hz, if I have all this and the math right. I suspect that the guitar was intended to operate into an amp with a lot less than 1 meg so the cutoff would be a lot higher.

The tone pot goes across the cap. It limits the amount of bass cut, and as you turn it to have a lower value the bass cut decreases. This is another reason why I think the guitar was intended to operate into a lot less than 1 a meg load. You would need that for the .5 meg tone pot to allow the circuit to have very much of an effect.

[David Schwab]

I had an old Carvin circuit diagram sheet that included a bass roll off control like this, and G&L has also used it.

[jason lollar]

thanks steve- good job!

[Steve A.]

I had an old Carvin circuit diagram sheet that included a bass roll off control like this, and G&L has also used it.

Thanks for the drawing! I had read about this circuit 30 years ago in the Brosnan book but never did try it out. Nobody I knew had been particularly impressed with the circuit- a big meh! So I figured it had to be something different in my Century.

What I like about it is the subtle shadings you can get working both controls. And the not-so-subtle tone shaping when that is what is needed.

Steve Ahola

[Steve A.]

thanks steve- good job!

Jason:

I got your Supro Steel Guitar pickup installed in my Chandler RH-2 over the weekend and I am at a loss for words how to describe it. Amazing? Incredible?

Clean, raunchy or heavily distorted it is unreal how responsive it is to all of the subtleties of your playing. (This is the strap-over-strings design used by Supro from the 30's through the 50's, folks.) Did I mention that it is hum-canceling?

I suggest that readers check out your steel guitar pickup page and listen to the sound samples recorded by Rick Vito.

Now I just need one for my tele!

Steve Ahola

P.S. There have been many pickups that I have liked and a few that I have loved, but this is the first one that really knocked me on my ass. How it can sound so good and still be hum-canceling is quite an accomplishment. I need to see if I can find the original patent application for this design.

BTW this post is relevant to the Century thread because I will be installing the Century two knob tone control in this guitar to see how it works.