Unlike that vast majority of guitar amplifiers, the Gretsch Chet Atkins uses the two triodes of a 12AX7 to drive bass and treble controls independently. Each triode forms a traditional common-cathode voltage amplifier, as shown here.
The plate load resistors RL are 270k. The coupling capacitor CG driving the bass control is large enough to be considered a short circuit for all audio frequencies. For the treble driver, however, it is only 0.001uF, which creates significant bass attenuation. The cathode resistor RK for the treble driver is 1.5k and is bypassed by a 35uF capacitor CK. This effectively shorts the cathode to ground for all audio frequencies and creates 38dB gain. The cathode resistor in the bass driver is larger, 2.2k, and unbypassed, creating negative feedback and 32dB gain.
The treble driver's output impedance is 51k. Because the bass driver uses a larger, unbypassed cathode resistor its output impedance is 139k. In most circumstances a high output impedance is a disadvantage, because the stage is more likely to be bogged down by the load it is forced to drive. In this case, however, the output impedance is a desirable artifact. The designer, in fact, places a 150k resistor in series with the output to effectively boost the driver's output impedance to 289k, as we will see next.
Here is the bass control, which consists of a low-pass pi network followed by a volume control.
Ro represents the 139k output impedance of the driving stage. The parts values are
R = 150k
RB = 500k
R3 = 100k
C = 0.005uF
RB and R3 form a 600k volume control that limits the minimum setting to 100k.
The Gretsch Chet Atkins uses 0.005uF capacitors. Here are the bass responses for the more common values of 0.0022uF, 0.0033uF, 0.0047uF, 0.0068uF, and 0.01uF, volume control at maximum.
The graph assumes that the volume control is set to maximum and that its impedance is large enough compared to the other resistor values that it can be ignored. Increasing the capacitor value steepens the slope of the curve near 82 Hertz, which is the lowest note of a guitar with standard tuning. At higher frequencies all the curves eventually transition to 12dB attenuation per octave. This is twice the cut of a simple RC filter and double the maximum bass-to-midrange attenuation rate of the Fender Bassman tone stack, so this is quite an aggressive tone control!
Here is the treble control, which consists of a high-pass T network followed by a volume control.
Ro represents the 51k output impedance of the driving stage. The parts values are
C1 = 0.001uF
C2 = 500pF
R = 220k
RT = 500k
R3 = 100k
Once again RT and R3 form a 600k volume control that limits the minimum setting to 100k.
The Gretsch Chet Atkins uses capacitor values of 0.001uF and 500pF. Here is the treble response for nearby values.
The Chet Atkins tone stacks use a traditional passive mixing circuit formed by the 100k resistors RM shown here.
In contrast to the isolation of the bass and treble driving stages, the passive mixer introduces some interactivity in the controls. Turning down the bass control affects treble response via the mixing resistors and vice versa. Even with isolated drivers, the Chet Atkins controls have substantial interactivity, a characteristic common to many classic amplifiers. The limiting resistors R3 mitigate the effect somewhat by limiting the load each stage presents to the other.
Ironically the Chet Atkins also contains an active mixer that is rarely seen in a guitar amp. It is used to mix preamp/tonestack circuits to create independently adjustable channels. It would be interesting to hear the effect of using the active mixer to combine the bass and treble outputs for a single channel. Tell the sound guy you don't need any equalization for the mix - your amp has a vintage tube mixing board built right in!
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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