The Fender Bassman 5F6-A Tone Stack
The Fender Bassman 5F6-A offers the guitar play a wide range of tone settings from three separate controls: bass, midrange, and treble. The tone stack is driven by a 12AX7 cathode follower circuit with a very low output impedance capable of providing a source voltage that isn't easily dragged down by a varying load. It drives a long-tailed-pair phase inverter with a very high input impedance. These two factors, a low impedance source and a high-impedance load, make the tone stack's frequency response relatively independent of the preceding and following stages.
The passive tone stack manipulates response through frequency-selective attenuation. In the extreme with all of the tone controls set to zero, for example, it induces a whopping 15dB loss at midrange and even more at bass and treble frequencies. The attenuation is mitigated by the combined gain of the Bassman's voltage preamps and twin-triode phase inverter. With what amounts to an additional voltage amplification stage and a zero-gain cathode follower circuit, the 5F6-A tone stack requires the equivalent of a complete 12AX7 dual triode for adequate drive voltage. Such is the technological price of placing an incredibly rich tonal palette in the hands of the guitar player.
The following graphs show the tone stack frequency response for various settings of the treble, midrange, and bass controls.
