THE TONE CONTROL CIRCUIT
I’ve added a bass, midrange, and treble tone control circuit that uses three potentiometers to adjust the audio spectrum. This circuit is necessary to “sculpt” the tone that you desire while playing a guitar.
However, adding this circuit reduces the gain and quality of the amps sound. As a solution, I’ve added a 12AX7 tube and cathode follower circuit to add back the lost gain. This circuit works well and sounds superb. Without this cathode follower circuit, the tone control addition would have reduced both gain and sound quality and become a negative factor for this amp design.
My goal is to leave the gain (or volume) pretty much untouched while the bass, mid, and treble frequencies are able to be adjusted. That’s why I included this circuit along with the three tone potentiometers that allow adjustment.
The adjustment of these 3 key frequency spectrums was really a necessity and not something that I could easily eliminate or compromise. Every guitar is different in sound quality and an end user’s tonal expectations are different depending upon what kind of tone he or she expects. This 3 band EQ allows for excellent adjustability and provides the flexibility that this type of amplifier really needs.
The cathode follower and tone circuit turret board components allow this inclusion in the best way while providing the end user with the ability to create an EQ style needed for whatever type of music and/or guitar is used.
What’s worth noting is that this tone control circuit can only remove frequencies and not boost them and is made up of several low and high pass audio filters that work together. Its a relatively low impedance circuit (high load) so it works best when fed by a low impedance source such as the cathode follower 12AX7 circuit that I’ve used. It seems simple but in reality is surprisingly complex due to the interactive nature of its four audio filters and how they work.
In this circuit, the audio signal from the guitar enters the treble capacitor and tone slope resistor. The tone slope resistor works as a frequency divider with high frequencies passing around it to the treble capacitor and low and mid frequencies passing through it to the bass and mid capacitors. A larger value tone stack resistor will pass fewer bass and mid frequencies so the value chosen changes the balance or slope of the tone stacks frequency response. This resistor is set to 100K and this value works really well in this amp.
The treble high pass filter consists of the treble cap (the 300pF Silver Mica capacitor) plus the treble pot. This blocks frequencies below approximately 2400 Hz and allows higher frequencies to pass. The high frequencies are sent to the top terminal of the treble pot. The bass and mid frequencies that pass through the bass and mid filters flow to the treble pots lower terminal so the treble pot then becomes a balance control that allows you to select between the high and low frequencies at its outside terminals.
The Tone Slope Resistor + Mid Cap low pass filter works with the bass and mid high pass filters formed by the Bass Cap + Bass Pot and the Mid Cap + Mid Pot to form band pass filters.
Band pass filters remove frequencies above and below the desired band of frequencies. The bass and mid band pass filters are made up of a low pass filter that removes high frequencies and a high pass filter that removes bass & mid frequencies to leave a band of frequencies that pass through the tone stack.
The Tone Slope + Mid Cap low pass filter’s cutoff frequency is very low (around142Hz) so how do any mid frequencies pass on to the Treble Pot? The Mid Pot’s resistance (below the wiper) comes after the low pass filter, which reduces the low pass filter’s effectiveness so some mid & high frequencies pass through the filter and on to the Treble Pot.
The bass control’s high pass filter cutoff frequency varies between approximately 8Hz and 318Hz. With the Bass Pot full up the bass high pass filter only blocks frequencies below 8Hz so all guitar and bass frequencies get through. With the Bass Pot full down the filter blocks frequencies below 318Hz so most of the bass is filtered out.
The mid control’s high pass filter cutoff frequency varies between 318Hz to infinitely high. With the Mid Pot full up the mid high pass filter only blocks frequencies below 318Hz. With the Mid Pot full down the mid filter blocks all frequencies.
This TMB (treble, middle, bass) tone stack interacts really well with the high impedance driving stage provided by the plate of the 12AX7 (similar to the tone stack used in Fender blackface amps). The TMB tone stack’s high load (low impedance) loads down the guitar signal coming off the tube plate causing attenuation and this is why the 12AX7 cathode follower circuit has been added – to put back the gain lost by inclusion of these 3 control potentiometers.
In other words the TMB stack’s low impedance is in parallel with the driving tube’s plate load resistor which lowers gain. The 12AX7 driver tube increases gain and thus compensates for the loss of gain that is experienced by adding the tone control circuit.
I first experienced this volume loss situation using a very simple guitar amp design, the Emery Superbaby guitar amp. This amp had a tone control that could be switched off and removed from the circuit through a front panel push pull switching knob. Whenever the tone control circuit was engaged, the volume of the amplifier dropped substantially and because this was a single knob tone control (not a 3 way like the TMB that I’m using), the attenuation provided was poor and the sound quality poor. That amp only sounded good (in my opinion as well as many other users) when the tone control was totally removed from the circuit. Unfortunately, that amp also had other problems that I won’t go into here. I sold it and moved on but learned an important lesson about how a poor tone control circuit sounds when not well designed.
The 12AX7 driver tube and circuit that I’m using is definitely required (to make up lost gain and sound quality) and it creates a beautiful tone. I now have the ability to adjust the bass, mid and treble as is desired. So the inclusion of the tone control turret board I’ve used and the accompanying 12AX7 tube and three frequency potentiometers (TMB) satisfy one of my main goals for this amplifier. It now has excellent tonal adjustability without the penalty of losing gain (volume) while not impacting the quality of the guitar signal. It works well and I’m very pleased with it.