How many parts does an amp need to work?
That question was the beginning of the 32. Years of playing, repairing and designing many different amps showed some common traits. The simplest, most direct circuit path most always produced the best tone. Seems obvious, but why is this? Every tube, resistor, capacitor and other component the signal passes through imparts some change upon it. The change can be positive or negative depending on the criteria it is viewed through. There is an absolute minimum signal path required for each voltage amplification stage within a guitar amp and a minimum gain stage count. More components and circuit complexity can be added on to achieve different gain structures, frequency response alterations and more voltage amplification stages can be added to accommodate additional features. The question is at what point do these added components cease to maintain or enhance the tone of the signal source and begin to degrade it? Signal integrity is the term used to describe this.
The main difference between a musical instrument amplifier and most every other type of audio amplifier is adherence to the input signal. All other audio amps are designed for absolute signal integrity, and not to alter the input signal in any way other than making it larger. Now, we have to view a guitar amplifier as part of a tone generating system. In conjunction with the acoustical properties of the guitar and the electrical properties of the pickups, the amplifier's job is to enhance the tone, not to merely reproduce a larger version of the input signal.
If we look at each component the signal passes through or around, we can see that each has several effects . Each voltage amplification stage needs to somehow be coupled to the next one in line. This is typically done in musical instrument amps by what is termed RC coupling; that is resistor/capacitor coupling. Every capacitor used as a DC blocking or AC coupling element has both a frequency and phase component associated with it. The frequency alteration is generally used to "voice" the voltage amplifier stage to suit the frequency range and desired harmonic structure the designer intends. More capacitors can be added to further "shape" the signal in the frequency domain. However, each has a phase shift associated with it that must either be compensated for or dealt with. Suffice to say that there is an effect on the tone either way. The only capacitor that has zero influence is one that isn't there. Resistors also impact the signal. A term we came across with particular relevance to guitar amp circuits is TSR; Total Series Resistance. This is the sum of all resistances by which the signal is impeded from input to output. A fascinating analysis of this and other related design issues can be found in the 1999 book "Inside Fender and Marshall Tube Amps" by J. C. Maillet. We found his work thought-provoking and a reinforcement of our design philosophy.
Back to the Model 32 now. Based on what we have discussed above, experiments were devised to put the theory into practice. Many iterations produced various results; mostly promising, but each time able to be improved upon. Substitution of differing component types and optimization of values continued at each step. In reality, the resultant circuit is far more complex than the theoretical "absolute minimum" component count. However, the underlying ideal was adhered to, and within the determined feature set parameters, a relatively low parts count was arrived at. We feel that there was indeed a great benefit from this experimental series and the resultant amp remains one of our favorites.
The 32 pre-amp is a rather unique design consisting of a single input jack feeding two complimentarily voiced gain stages using the 6072 tube (the mil-spec version of the 12AY7) as the front end for both. One gain stage is voiced bright and clean, while the other is darker and richer with more gain. Each gain stage has an independent level control allowing for a wide variety of blending combinations. The two stages are then passively mixed together (keeping the TSR value low and minimizing active components) before being fed to our uniquely voiced passive eq circuit's low, midrange and high frequency cut controls.
The output section consists of two 5881 tubes in a push-pull, fixed bias configuration using a solid state rectified power supply to produce approximately 40 watts. This design and careful selection of power supply components enables a natural, slightly compressed tone when driven hard, while maintaining a tight, full bottom end.
No provision for reverb, fx loops or other features has been incorporated into this design. It has been optimized for the minimum active circuitry possible, to allow for the purest tone.
Standard features include:
- A presence control in the power amp section to vary high frequency response, allowing for a smoother to more aggressive top end tone.
- A variable line level output (1/4" unbalanced) which is useful for slave amping, direct connection to recording or reinforcement consoles, tuner output, etc. Optionally, we offer an additional low-impedance, transformer-coupled balanced output on a 3 pin (Pin 2 Hot) XLR-3-F connector with ground lift switch.
The Model 32 head is available in the following configurations:
- Standard Enclosure: 1" thick, clear #1 pine, dove-tail joined, covered in black super heavy duty Cudora fabric.
- Upgrade Enclosure: 5/4" thick figured black walnut, dove-tail joined, in a natural, clear oil finish.
- Standard Tubes: The best current production tubes are installed, including the fantastic re-issue Tungsol 5881 and EH 6072.
- Upgrade Tubes: All tubes are NOS USA, including Philips, GE, and RCA.
We are currently evaluating a self bias, tube rectified configuration. This version will produce lower output power, but further enhance the organic tone of the Model 32. Please let us know if this is something of interest to you.