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RCA Receiving Tube Manual - Tone Control p. 45

The following is a note from the RCA Receiving Tube Manual, Technical Series RC-27, 1970, page 45. It outlines a tone-control network with separate controls for bass and treble boost and cut. I have not tried this circuit, but I think it might be interesting to add this to an AX84 P1 build between the two gain stages.

The tone-control network shown Fig. 1 has two stages with completely separate bass and treble controls.

Fig. 1

Fig. 2 shows simplified representations of the bass control of this circuit when the potentiometer is turned to its extreme variations (usually labeled "Boost" and "Cut"). In this network, the parallel RC combination is the controlling factor. For bass "boost," the capacitor C2 bypasses resistor R3 so that less impedance is placed across the output to grid B at high frequencies that at low frequencies. For bass "cut," the parallel combination is shifted so that C1 bypasses R3 causing more high-frequency than low-frequency output. Essentially, the network is a variable-frequency voltage-divider. With proper values for the components, it may be made to respond to changes in the R3 potentiometer setting for only low frequencies (below 1000 Hz).

Fig. 2

Fig. 3 shows extreme positions of the treble control. The attenuation of the two circuits is approximately the same at 1000 Hz. The treble "boost" circuit is similar to that used for the equalization of crystal phonograph records [discussed earlier]. In the treble "cut" circuit, the parallel RC elements serve to attenuate the signal voltage further because the capacitor bypasses the resistance across the output. The effect of the capacitor is negligible at low frequencies; beyond 1000 Hz, the signal voltage is attenuated at a maximum rate of 6 dB per octave.

Fig. 3

Radiotron Designers Handbook, 4th Ed. - Paraphase Tone Control p. 665

According to the RDH, the following tone control "can be designed for any cross-over frequency." The circuit shown in the following figure crosses over at 400 Hz. "The design procedure is:"
  1. Choose a cross-over frequency fc.
  2. Choose R10RL where RL == the generator impedence.
  3. Make C == 1/(2πfcR).
  4. Make K == 10; then KR == 10R and C/K == C/10.

Example: Choose fc == 400 Hz. Say generator impedance == 1000 ohms, then select:

Radiotron Designers Handbook, 4th Ed. - Cross-Coupled Input Tone Control p. 663

I really like the idea of the following tone-control circuit. It uses a phase inverter to amplify the difference between the two input triodes. This unit provides simultaneous bass-treble boost or attenuation around a cross-over frequency. The cross-over frequency is governed by the RC network that drives the top half of the 6SN7. Rotating the dual ganged 1M ohm pots changes the cross over from 360 Hz to 4000 Hz. Effectively this is movable notch filter or notch pass circuit. Centering the 250K ohm provides linear response. Rotating the pot to the right, effectively grounding the grid of the bottom half of the 6SN7, attenuates the bass and treble and boosts the range around the cross-over frequency. Turned to the opposite end where the grid voltages of the top and bottom halves of the 6SN7 are approximately equal, boosts bass and treble and attenuates around the cross-over frequency.

The beauty of this circuit is that it drives right into a Push-Pull power stage through the cross-coupled phase inverter formed by the 6SL7 tube. It costs the extra pair of triodes for the differential amplifier. I intend to try this one out on my next project.

An active Baux tone control using an anode-follow circuit

The following circuit comes from an article posted over on Randall Aiken's site: "The Anode Follower" by Charles P. Boegli, Audio, December, 1960. Basically, a Bauxandal tone control is incorporated into an inverting feedback amplifier stage. It features very low output impedance. Note the drawing does not show a finished design. An actual implementation specify an appropriate cathode resistor and bypass capacitor.

Now for an actual implementation of the design.... This is from P. J. Baxandall's original article "Negative Feedback Tone Control, Independent Variation of Bass and Treble Without Switches," Wireless World. October, 1952, p. 402-405.

Those interested in the history of this circuit may find E. J. James' article worth reading: "Simple Tone Control Circuit: Bass and Treble, Cut and Lift," Wireless World February 1948, p. 48-50. (Scans of this article were provided to me by Danny Noordzy. Thanks, Danny!)