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Story Behind the Tube Drive

The First Effectrode Pedal

Effectrode's first guitar pedal design — Inside the first prototype built in 1996

Here’s a snapshot of a little piece of history—Effectrode’s first tube effect pedal. This tiny, tank-like box houses a tube distortion circuit based on the lead channel of a Soldano^® ‘SP-77’ rackmount guitar preamplifier, an awesome amp! At the time, the early 90s, the SP-77 was the amp to own, the sound that guitarists wanted, but that fabulous tone came at a price⁠—these amps were not cheap. However, I’d picked up enough electronics knowledge to start building my own tube amps. My experience came from repairing, modifying and “hot-rodding” vintage tube amps and learning what I could about vacuum tubes from the yellowed pages of dusty, old physics and electronics books, that I’d come across in the gloomy and dingy recesses of small, secondhand bookshops—there was nothing about tubes on the electronics syllabus at college.

Electronics can be a brain-melting subject to get to grips with. It takes awhile for things to click. But after a time I learned to approach design work more scientifically; analyse tube amps using circuit simulation; implement strict audio electronic test regimes; and slowly, but surely, got a grip on how to control and blend the vital, basic ingredients that constitute good tube amp tone.

My initial inquiries focused on modern high gain amp designs, such as the creations of Randall Smith (Mesa Boogie^®), Alexander Dumble and Mike Soldano’s SP-77. The creamy, saturated tones of these types of amps are addictive, but over the years I came to desire the milder, lower gain bluesy tones of a ‘pushed’ tube amplifier going into breakup. My tastes for these two distinctly different overdrive and distortion characteristics had a big influence on my first prototype drive pedal.

The prototype was completed in December 1996. The signal path was based on the SP-77′s four cascaded grounded cathode stages 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 voltage transformer, which I wound by hand as none of that minuscule proportion were available. The transformers were ferro-magnetically screened from the tubes using a steel partition plate. The result was the chunky little tube pedal with warm, saturated tones, however lower gain settings were lacking robustness. Further work was needed…

Sculpting Perfect Driven Guitar Tone

Further investigation revealed that good higher gain saturated sounds and low gain, break-up guitar sounds required distinctly different equalisation contours. Both the Boogie^® and Soldano^® preamp circuits chisel away at the low-end frequency response using specifically chosen coupling and cathode bypass capacitor values across the four cascaded clipping stages. This in conjunction with simple, but carefully placed R-C networks (sometimes large value grid resistors working with the tube ‘Miller’ capacitance) results in an incremental and gentle roll-off of lower (and higher) frequencies, creating mid-emphasis in the response. The reduction of low frequency signal content yields a smoother, less “ragged” clipping characteristic and a tighter, more focused sound where harsh, higher order harmonics are tempered and restrained.

cascaded_tube_clipping_320px — Schematics for ‘Hot-rodded’ (cascaded tube clipping circuit) Fender Twin Reverb Amplifier. August 1996

This made for great soaring, sustaining lead guitar sounds, however sounded too thin for full-bodied blues and those tube amp pushed, break-up drive tones, which require that the low-end is pretty much left intact. As a side note, it’s impossible to overstate the importance of frequency equalisation when sculpting great guitar overdrive tones—it’s essential to get the harmonic balance right and there’s a real art to it. 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.

These requirements for low and high gain drive tones were totally at odds with each other—yet more head scratching was needed. I had a handle on what was required in terms of tone shaping, but needed to figure out how to satisfy these two seemingly conflicting requirements—retaining all the bottom-end for pushed amp break-up and removing the correct amount of bass for creamy distortion tones—to create my dream drive pedal. My solution to achieving this flexibility was to use a combination of two approaches.

I began by first fine-tuning the clipping and coupling/bypass capacitors across the cascaded tube stages. The aim was to distribute the clipping and filtering as evenly as possible across the tube stages—effectively tuning the clipping characteristics of the circuit. The gain control was designed so that it adjusted gain across the last 3 stages pushing them into clipping to produce a signal that is rich in even and odd harmonics. This is where tubes out-perform solid-state devices (transistors, FETS and opamps) every time, producing a beautifully rich and full-bodied sound. Simply put: they’re musical—everything you could want from an overdrive/distortion. However, this is no astounding, new revelation: Mike Soldano and all good tube amp designers know this. I turned my attention to the tonestack to see if any improvements could be made in that department.

A Better Kind of Tonestack

The second development was to incorporate an active tube ‘Baxandall’ tone control. I adapted a pentode circuit that I’d found in an old 1952 copy of ‘Wireless World’ article by J.P. Baxandall to work with a 12AT7 dual triode tube instead—I prefer triodes. This type of tone control has much improved flexibility over the traditional Fender^® style ‘Bass-Middle-Treble’ passive tone-stack found on pretty much every tube amplifier ever built from the 1950s until today. The passive tonestack can only remove or subtract harmonic content from the guitar signal, whereas the Baxandall can actively add or boost it as well. I then experimented with adjusting the capacitor and resistor values in the circuit until the boost/cut became centred around the important mid and high frequencies essential for sculpting full-bodied drive sounds.

baxandall_tone_stack — Pentode based tone control from J.P. Baxandall’s original article published in ‘Wireless World’ magazine October 1952

The tone of the Tube Drive now sounded punchier, thicker and more powerful with the addition of the Baxandall tonetack, even when the control knob was set in its centre ‘flat’ position. The passive Fender^® tonestack always results in ‘insertion loss’, attenuating the mid-frequencies. These frequencies are of vital importance for obtaining robust and full lead guitar tones. Players often compensate for this lack of body by using a stompbox equaliser and a few amp manufacturers, such as Boogie^®, even build graphic equalisers into their amps. But there was no such lack of ‘oomph’ with the Tube Drive circuitry. Further, this unique single knob tone control could also enhance high frequencies, actively add higher-order harmonics for extra crunch and presence giving the pedal an incredibly dynamic and lively tone. Although the Baxandall tonestack is deceptively simple it allows enough tone contouring for easy access to a wide palette of usable tones—a giant leap for guitarist-kind.

And Into Production

Tube Drive pedal — The finished production Tube Drive model TD-1A released in 2009.

Finally, after twelve years of inspiration and perspiration, all the kinks were ironed out the Tube Drive’s thermionic circuits—it was time to press the button, ramp up the production lines and get ready for the big time. The first batch of two hundred (that’s two hundred!) pedals were released in 2009 and were available in any colour as long as it was old-school, ‘mushy-pea’ green—a nod towards BBC radiophonic composer, Delia Derbyshire. The pedals were fitted with JJ Tubes, authentic ‘Dakaware’ phenolic knobs with those felt washers underneath them, just like on the old tube radios. Internally, precision grade instrumentation resistors and polyester coupling capacitors are utilised throughout the circuitry. Operation is pure class-A, based on three dual triode tubes (a total of six stages) with the tubes operating at true amp plate voltages of 300 volts—there are no diodes or transistors in the signal path. The high voltage power supply is fully regulated and smoothed and the tube heaters operate from D.C. voltages to ensure noise levels are kept as close as possible to the theoretical minimum—most tube amp manufacturers don’t go to these lengths.

The Tube Drive was designed to be a no-compromise overdrive pedal. It partners well with simpler, old-style Fender^®, Hiwatt^® and Laney^® amps, effectively hot-rodding them with a very classy second tube gain channel. It can even be used to record guitar direct and sounds quite convincing considering there’s no speaker/cabinet and output transformer—its tone is that warm and musical. A huge amount of time and passion was lavished on creating the Tube Drive. Because of this it was possible to weave all those important, subtle details into the fabric of the pedal’s circuitry, details that would have surely been missed if a line had been drawn under its development back in 1996. The research and development effort had taken well over a decade to complete and I felt assured there were no more improvements to be made—but I was wrong.

Two Phils Are Better Than One

In 2011 I received a phone call from a chap by the name of Phil Taylor, not me, another Phil Taylor—I haven’t resorted to talking to myself… yet. Phil is David Gilmour’s guitar technician and studio manager since 1974 and consequently understands something of Pink Floyd’s gear and its importance to tone. Hearing from him was a big deal to say the least.

Phil was interested in test driving a few Effectrode pedals to see if they could be integrated into David’s guitar rig. This was to be the beginning of an ongoing dialogue which would stretch over many, many months. Over this period Phil and I worked together to ‘tailor’ the tone of several pedals to his tastes. I would ship a pedal to Phil, usually to David’s Astoria studio in London, for him to evaluate. Sometimes he would involve other Floyd studio engineers as well before getting back to me with positive criticisms, his likes and dislikes and suggestions for improvement. He was also good enough to ship several of David’s pedals to Effectrode as a reference so we could perform A-B listening tests.

David Gilmour with effects pedals — The last of the TD-1A Tube Drive pedals now lives in David Gilmour’s studio. (Photograph taken October 2014)

TD-2A Tube Drive pedal — The current model TD-2A 'Tube Drive' pedal is manufactured with the modifications undertook for David Gilmour's guitar technician, Phil Taylor

Working with Phil was a fantastic opportunity. His vast knowledge, experience and connection with the Floyd from their early days was of big help and kept me on path. Audio testing can be a rabbit hole. You can lose your way. Start questioning what sounds subjectively “good” or “bad”. Before you know it your time has evaporated away and it’s three in the morning—your next stop, the “Twilight Zone”. You can’t even make a cup of tea, let alone create a radical, new effects pedal design. So, having Phil involved was a real stroke of luck, like having a top-notch professional consultant on the team… for free! His involvement enabled me to confidently fine-tune the performance of each pedal and make it better then I could have ever done alone. He’s a real gent too.

Getting back to the Tube Drive, Phil heard something in the pedal he liked, but wanted more bass from it. I hadn’t noticed any particular lack of bottom-end when testing the Tube Drive through my Fender^® ‘Tweed’ amp (fitted with 12″ Jenson speaker) at bedroom levels, however David’s rig consists of Hiwatt^® heads driving combinations 4 x 12″ and 2 x 15″ cabinets at substantially higher volume levels. I recall a relatively recent interview where he described that there was nothing like the experience of leaning back into the sound when playing at concert levels. Phil described it to me as, “a loss of power” when engaging the pedal which was a fair criticism as the cascaded gain stage topology is designed to shed a little bass energy at each stage as described earlier on in this article.

Phil sent the pedal back in to Effectrode for modifications. He wanted more ‘bottom’, improved low frequency response, so we increased the cathode bypass and coupling capacitor values—this allowed more the low frequency content into the tube clipping stages. The pedal was returned to Phil, but he still wasn’t happy with it, so back it came. The cathode bypass capacitor values were increased further, pushing the pedal into instability or ‘motor-boating’. This happens in high-gain, wide bandwidth amplifier circuits—they become oscillators—an interesting effect, but not what we’re after here. The capacitor values were decreased a little, the circuit ceased generating its “electronic tonalities” and returned to planet Earth. My oscilloscope was now telling me that the Tube Drive would allow the lower registers of a church pipe organ through with zero attenuation of subsonic frequencies. Surely this would be good enough for Mr Gilmour. The pedal made its way back to Phil, I kept my fingers crossed and waited to hear back from him. A few days later he called. He was… satisfied. The pedal, the last of the TD-1As, is now residing in David Gilmour’s Studio.

All the model TD-2A Tube Drive pedals are manufactured with the modifications I undertook for Phil. Like all Effectrode pedals they are built entirely in Great Britain—the aluminium boxes are cut and drilled, painted and silkscreened by local companies within a 20 minute drive of my home. The circuit boards are also fabricated and components soldered in place by a local electronic assembly company. The pots and knobs are custom made specially for Effectrode by Omeg, UK and Davies Moldings, USA. All pedals are given a 24 hour burn-in and tested by me, the designer and packed and shipped by my wife, Sam. And that wraps this little story up. The only thing to add is to thank everyone involved—without you I couldn’t possibly make this stuff and life wouldn’t be anywhere near so interesting.

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Story Behind the Tube Drive