The "Tube Drive" Pedal In-depth

by Phil Taylor

Early Days

I first looked at building an overdrive pedal using vacuum tubes way back in 1995. At this time the boutique pedal market was hardly in its infancy and the larger effects manufacturing companies had not yet caught on to the advantage of using tubes in pedals. The only tube pedal available then was the Mesa Boogie V-Twin a tube/op-amp hybrid, so the field was wide open for an independent designer with wild ideas of building all-tube effects pedals.

I'd been accumulating experience of the technology by repairing, modifying and "hot-rodding" vintage tube amps and reading everything I could on the physics of vacuum tubes from dusty 1950s & 60s texts, I uncovered from the darkest recesses of backrooms in small, secondhand bookshops. It was now time to take a scientifc approach and start analysing my favorite tube amps using circuit simulation, electronic and audio testing techniques in an attempt to quantify the characteristics that make them sound so great. My efforts initially focused on relatively modern high gain amp designs, building on the work of Randall Smith (Mesa Boogie) and Mike (Soldano). The creamy, super-saturated tones these amps could deliver were highly desriable, however I also wanted to combine this with the ability to achieve more natural lower gain breakup and blues tones. I was developing a flexible gain architecture and progressing the art to the next level, in the same way that Smith and Soldano built on the work of Leo Fender and Jim Marshall.

I completed my first overdrive pedal prototype in December 1996. The design was inspired by the lead channel of the Soldano SP-77 pre-amp with a signal path consisting of 4 cascaded tube gain stages to progressively clip the signal. These were followed by a simple passive Fender "Harvard" style tone control. The power supply was built into the pedal housing and based on a sub-miniature high tension transformer that I wound by hand as none of that miniscule proportion were available at the time - they still aren't. The transformers were ferro-magnetically screened from the tubes using a steel partition plate. The result was an extremely compact pedal capable of achieving some great warm, saturated tones, however the lower gain settings were not as robust and natural as I would have liked them to be. Some further work was required . . .

Zen and the Art of Overdrive Tones

After more experimentation it became apparent that to achieve good saturated hi-gain sounds as well as lo-gain, break-up tones, different equalization contours would need to be applied to the signal. My investigations of both the Boogie and Soldano hi-gain preamp circuits revealed that the low-end frequencies were tailored using a combination of coupling and cathode bypass capacitors across the cascaded clipping stages. The result is a gentle roll-off at lower and high frequencies to create some mid-emphasis in the response. This reduces low frequency content in the signal for a smoother, less "ragged" clipping characteristic and minimizes harsh, higher order harmonics for a more focused sound. This made for great hi-gain, sustaining lead sounds, however sounded a little too thin for blues and mild overdrive, which seemed to require that not as much of the low-end is removed. It's impossible to overstate the importance of frequency equalization, which is essential for obtaining great guitar overdrive tones - it's critical to get the harmonic balance right. Differences of less than a decibel here or there in the frequency response can make a big impact on the subjective sound quality i.e. whether your tone is that of a god or just plain lousy!

So, with a handle on what was required in terms of tone shaping, I then had to decide how implement a pedal with easy access to outstanding lo and hi-gain overdrive/distortion. My solution was to use a combination of two approaches. First, I optimized the clipping and coupling/bypass capacitors across the cascaded tube stages. This means that the clipping and filtering is distributed as evenly as possible throughout the stages - I was effectively tuning the clipping characteristics of the circuit. Next the gain control was designed so that it adjusted gain across all stages whilst simultaneously altering the equalization contour. Finally, I incorporated an active tube Baxandall tone control for much improved flexibility over conventional passive tone-stacks as it allowed harmonic content to be added (boost) as well as subtracted (cut). This unique single knob control is optimized to boost/cut the important mid and high frequencies for guitar. It can drastically enhance high frequency components for extra crunch and presence. Deceptively simple, yet flexible tone contouring with zero insertion loss for minimum loading on the signal. Now we are making progress!

Reaching the Design Pinnacle

The Tube Drive was now sounding musical with easy access to a wide palette of usable tones. In use, this pedal also proved capable of delivering substantial boost to provide extra drive to push vintage tube amps further into overdrive. A perfect partner for amps such as the Twin and Deluxe Reverbs and the older Fender Tweed amps, effectively endowing them with an additional high gain channel. The Tube Drive really is a no-compromise pedal that aims to deliver boutique amp overdrive tones whilst maintaining noise levels close to the theoretical minimum. Operation is pure class-A based on three triode vacuum tubes for a total of six stages with no silicon diodes or transistors in the signal path.

After over twelve years of research and experimentation it was good to see the Tube Drive design finally reaching a level of maturity. Although the principle of overdrive/distortion is simple in comparision to other types of signal processing such as delay or flanging, for example, getting the tone and break-up characteristic just right is challenging - the devil is very much in the details. I took my time with this project and consequently was able to weave many more of those important details into the design:

• Precision grade instrumentation resistors
• Polyester coupling caps
• Active tone control
• Wide gain range from mild blues break-up to heavy saturation
• Extremely hot output for some serious level boost
• Reference plane layout
• D.C. tube heaters
• Tubes operate at true amp plate voltages
• Silent true bypass switching

Some of these details might have been missed if I had completed back in 1996. On a philosophical note, it's almost as if these things have a life of their own and need to develop in their own time.