I found the best components for tone control of my Rickenbacher B8 lap steel

[Mike Bonnice]

I've had the time since retiring to actually understand how tone control and volume control and pickups work together. It's lead me to a combination for Rickenbacher bakelite that I really like. Since I play Hawaiian music, I like a mellow tone. I never play with any twang, and if I wanted it I'd crank the treble control on my amplifier. The combo for the guitar is 250K pot for volume and 250K pot for tone with 100 nF capacitor. Short cable or no cable (wireless) are best.

My pickup is a newly-made version by Jason Lollar, so it's not original. The magnets are stronger and more consistent from string-to-string within the horseshoe. I fiddled with tone and volume components and I liked the sound well enough but I didn't like two things: 1) a weird increase in noise when I increased volume near the level where I like to play (from 70% to 100%), 2) a sudden jump in tone in near the level where I like to play (on the low end of the tone control).

With lots of modeling and analysis I now understand the reasons and have eliminated them. The weird noise increase is due to resonance of the pickup (all pickups have resonance) and the jump in tone is due to the taper of the tone pot. The resonance is eliminated by reducing the volume pot from 500K to 250K. The tone taper is improved by using a 250K tone pot from CTS and selecting the unit that has the most gradual resistance curve.

For tone and volume I have been using dual concentric potentiometers so that I could move the tone control out from under my right hand over to the other side of the instrument. Using dual concentric pots is a big constraint on the available potentiometers, both in value and in taper. If the two pots were independant, like on most guitars, optimization would be easier.

I measured the pickup's inductance, resistance and (calculated) capacitance. I created a model in KiCad (SPICE) that calculates the frequency response of the coil, tone control, volume control and cable. I found the taper curves for CTS and Alpha potentiometers. I also measured some actual CTS pots. I measured capacitance of some guitar cables. With the taper curves I could see how the frequency response changes with pot rotation.

With no tone or volume control the pickup's resonance is heard as a large increase in gain for high frequencies. The tone capacitor reduces the increase but beyond a certain capacitance it doesn't have much effect and resistance needs to be changed. When the volume pot is wide open (maximum resistance) the resonance appears and as the volume pot is reduced the resonance is reduced. One could argue that I should just not use my 500K pot in the noisy position but that wastes 30% of the rotation and I get less control at lower volumes. Lower volume pot means lower output signal level from the guitar, so I use the amplifier to make up for it.

Reducing the tone pot from 500K to 250K reduces the maximum twang I can get with the guitar's tone control. However, it gives more range for low-end resistance change (lower slope). I'm trading maximum twang at the high end of tone for maximum control of mellowness at the low end of tone. I'd like to get an Alpha pot with D taper since it has low slope but they are not availabe as dual concentric. The CTS A taper would be acceptable, but the available sources for CTS pots don't specify which audio taper they use. I measured some CTS pots and they seem to be variable between CTS A and CTS BD (which has higher slope and is more abrupt in tone change).

There don't seem to be any 250K/250K dual concentric pots available for purchase. However, one can be constructed using CTS pots. I got two 250K/500K, which are readily available. I measured the taper curve of the two 250K's. I disassembled both then reassembled by swapping the 500K resistor element for the 250K. For the tone control I picked the resistor with the most gradual curve. Having learned this, I suppose I could have bought one 250K/500K and a single 250K to swap for the 500K.

Alpha pots can't be swapped because the resistor element can't be separated from the shaft and wiper.

Now I can stop goofing around with electronics and get back to playing music.

[Bud Angelotti]

Thanks for your hard work and for sharing this.
Bud