Volume and tone controls, in their simplest form, use potentiometers and capacitors.
Potentiometers, more commonly referred to as pots, are variable resistors with three terminals – a circular or linear resistive carbon track with a terminal at each end and a third terminal connected to a ‘wiping’ contact which travels along the resistive track. The three terminals are positioned so that the centre terminal is connected to the wiping contact.
Pots are manufactured in a wide range of power ratings but as the power generated by a typical passive pickup is so low (i.e. millivolts generating milliwatts), pots rated at 0.25 Watts are quite acceptable for this situation.
The pot shown above has a 1/4”(6.35mm.) dia. split shaft and solderable terminals
The drawing above shows a CTS manufacture pot (from a current catalog) as viewed from above. With the knob rotated fully CCW, the wiper is directly connected to Terminal 1 and when the knob rotated fully CW, the wiper is directly connected to Terminal 3.
The drawing above shows a side view of the same pot. The shaft is shown as 0.25″ dia., plain. The bushing referred to is available in a choice of lengths and in this case, it’s thread is shown as 3/8-32NEF. A Les Paul guitar requires a longer shaft than a Stratocaster, hence the options. Wiper rotation is 300 degrees, common to most pots of this type.
The resistive Carbon track in a Tone pot is uniform along its length, but the resistive track in a Volume pot is not. The resistance slope is as D below, so at 50% of the wiper’s travel, just 10% of the resistance is in circuit. If a linear pot is used for volume control, a small clockwise turn of the wiper will quickly give a large increase in volume. This is perceived due to the response of the human ear.
(CTS Technical data above by kind permission of CTS Corporation).
Pot manufacturers supplying DeArmond included Allen-Bradley, CTS (Chicago Telephone Supply), Centralab and Stackpole.
Allen-Bradley pots were used on some DeArmond volume controllers such as Models 600 and 602. These pots were built to withstand the mechanical stress involved in such an application.
Stackpole pots were fitted in the Model 600 pedal.
Because of their compact size, Centralab pots were fitted in the pickup Models 700, 750-C and 800 (Spanish guitar transducer).
Two extracts from Centralab catalog above.
Two views above of a 1K pot, 1st version, showing the three terminals and two fixing studs (B0541).
Two views above of the 2nd version.
The above chart by Centralab from the 1940s shows the wide range of tapers available. Curve No. 3 or No. 6 would be suitable for a volume controller, with a slow initial rise.
One definition of a Capacitor is:
‘A passive semiconductor with two plates, each connected to a terminal. When charged, the plates can store energy in an electric field’.
The SI unit of electrical capacitance is the farad. The value of the capacitor in the controls box for the DeArmond Model 40 pickup shown is 0.1 microfarads, generally written as 0.1 μF +/- 10%. It is also rated at 100 Volts
If a DC voltage is applied to a capacitor, current will flow for a short period only, dropping gradually to zero, as the capacitor reaches full charge. If it’s two terminals are shorted, the capacitor will discharge immediately.
If an alternating voltage is applied to a capacitor, a current will flow. The Impedance (or resistance in an AC circuit) is not the same as resistance because as the frequency in an AC voltage changes, so the impedance changes also. The impedance is inversely proportional to the frequency, thus allowing high frequencies to pass through more easily.
V & T controls box for DeArmond Model 40 pickup. The volume controller is on the RHS. The pickup’s Ground conductor (screen) is soldered to the lower of the three terminals and the pot case. The pickup’s Hot input conductor (black) is soldered to the top terminal. The Hot output conductor from the from the wiper (white) is soldered to the minijack socket’s Hot terminal, and looped to the bottom terminal.
The capacitor is connected between the Volume controller’s top terminal (Hot input conductor and the middle (wiper) terminal on the tone pot (LHS). The tone pot’s top terminal is soldered to the pot’s case, earthing it.
Both pots are the same, being numbered 1376639 (CTS, year 1966, week 39) and R20. One pot’s resistance measured at 226KΩ (most likely supplied as 250KΩ +/- 10%).
CTS data above by kind permission of CTS Corp.
Allen-Bradley data above by kind permission of Farnell & Allen-Bradley.