During an exchange on tone controls on the AX84 forum I started to wonder why guitars and bass tube amps have such similar tone-stacks. I reasoned that, if the frequency responses of the instruments are different, the frequencies controlled by the tone-stacks should be different.
The middle frequency is the frequency that is left unchanged when you alter the treble and bass controls and is usually around 1kHz. But why?
From what I can see, the reason why it is around 1kHz is lost in the mists of time but it has been suggested that it is the middle of our hearing range. Listing the octaves in Hertz from the lower to upper thresholds of hearing we have: 20, 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240, 20480. So 640Hz is the middle value which is pretty close to 1kHz (although 1280Hz is closer) so perhaps the suggestion is true. You can go to the Eliot Sound website for a little more information about this but the important question is how does this apply to guitar and bass amps?
I have used the Tone Stack Calculator from Duncans Amps to create PDF files that show the ranges of the standard tone-stacks from Fender, Marshall and Vox. You can see that, although the middle frequencies move around a little, they all agree that it sits in the 500Hz to 1kHz range. It is worth noting that 1kHz is the fundamental frequency that you get when you play the top string of a guitar at the 19th/20th fret so pretty much everything above 1kHz is an harmonic. So the treble control on a guitar amp pretty much controls the amplitude of the harmonics while the bass control controls the amplitude of the fundamentals by raising the output amplitude of the bottom string relative to the top string (the bottom E-string on a guitar is 82Hz while the top E-string is 330Hz).
This makes some sense for a guitar but a four-string bass guitar generates 311Hz at the 20 fret of its top string. The bottom string is 41Hz when played open and the top string 98Hz. Shouldn’t this be reflected in the tone-stack? The Fender Bassman 5F6-A circuit was copied by Jim Marshall in the early 1960s and, from that point on, there appears to have been little distinction made between guitars and bass guitars until semi-conductors made more sophisticated tone controls affordable.
it may be that properties of the human ear make changes to the tone controls unnecessary. The frequency response of the human ear can be seen on this interesting diagram. Our ears are most sensitive at around 2kHz and our hearing tails off below 200Hz. Some would regard 200Hz as the upper end of “bass” and too much bass between 100Hz and 200Hz can sound “boomy” so it would seem to be sensible to be able to compensate for our poorer hearing and the possibility that the bass might sound boomy by being able to control these frequencies. That’s almost what the guitar tone-stack does but it starts around an octave higher.
This is interesting stuff and will be the subject of further investigation. I’ll have to leave space on my Northcourt Mk2 layout to try out different tone controls.