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Story Behind the Tube Drive

by Phil Taylor

The First Ever Effectrode Pedal

Prototype tube overdrive

Inside the first prototype built in 1996

Here’s a snapshot of a tiny piece of history – my first ever tube effect pedal design. This tank-like little box houses a tube distortion circuit based on the lead channel of a Soldano SP-77 rackmount preamplifier which is a really, really nice sounding guitar amp. By 1995 I’d picked up enough knowledge of how tubes work to undertake this kind of project. My experience came from repairing, modifying and “hot-rodding” vintage tube amps and learning everything I could on the physics of vacuum tubes from dusty, old 1950s & 60s books, I uncovered from the gloomy and dingy recesses of backrooms in small, secondhand bookshops – there was nothing about tubes on the electronics syllabus at college.

After considerable trial and error (electronics is painful subject to learn) I’d reached a stage where I could take a more scientific approach and start analysing tube amps using circuit simulation and audio electronic testing techniques – a quest to quantify the characteristics important for good tube amp tone. My initial enquiry focused on relatively modern high gain amp designs such as the work of Randall Smith (Mesa Boogie), Mike (Soldano) and Alexander Dumble. At the time, in the early 1990s, I must admit to being totally addicted to the smooth and creamy tones of these kinds of amps, but I later desired the more natural lower gain breakup and bluesy tones of a ‘pushed’ tube amplifier. This very much dictated my approach to designing this first prototype drive pedal.

I completed the prototype in December 1996. The signal path was based on the SP-77′s four cascaded tube gain 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 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 little pedal capable of achieving some musical 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 . . .

Sculpting Perfect Driven Guitar Tone

Cscaded tube clipping circuit

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

After much more experimentation it became apparent that to achieve good higher gain saturated sounds as well as low gain, break-up tones different equalisation contours would need to be applied to the signal. My investigations of the Boogie and Soldano preamp circuits revealed that the low-end frequencies were curtailed using a combination of specific values for the coupling and cathode bypass capacitors 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) yields in an incremental and gentle roll-off of lower (and higher) 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 minimises harsh, higher order harmonics for a tighter and more focused sound.

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 flexbility 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 distibute 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 far out-perform solid-state devices like transistors, FETS and opamps – they produce this beautifully rich and full-bodied sound. Simply put, they’re musical – everything you could want from an overdrive/distortion. However, this is not a new revelation. Mike Soldano and all good tube amp designers understood this for years. I turned my attention to the tonestack to see if any improvements could be made in that department.

A Much Better Kind of Tonestack

Baxandall Tone Stack

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

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 centered around the important mid and high frequencies essential for sculpting full-bodied drive sounds.

The tone of the Tube Drive now sounded punchier and thicker with the addition of the Baxandall tonetack, even with the control knob in its center ‘flat’ position. This is because the passive Fender tonestack has ‘insertion loss’ which means it attenuates mid frequencies. These frequencies are essential 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 eqs into their amps. There was no such issue here though and this unique single knob control could also enhance high frequency components for extra crunch and presence – a much more dynamic and lively response. The Baxandall tonestack is deceptively simple in use, but in practice allows enough tone contouring for easy access to a wide palette of usable tones – real progress.

And Into Production

Tube Drive Model TD-1A

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

Finally after over twelve years of research and experimentation the Tube Drive had reached a level of maturity – it was time to go into production. The first batch of 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 you used to see 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 vacuum 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 – even most tube amp manufacturers don’t go to these lengths.

The Tube Drive really is my no-compromise overdrive pedal and designed to partner well with simpler, old-style Fender, Hiwatt and Laney amps, effectively endowing them with an additional tube gain channel. It can even be used to record guitar direct and sounds quite respectable considering there’s no speaker/cabinet and output transformer – the tone is that warm and musical. A great deal of time went into this pedal and consequently was able to weave many more of those important details into the fabric of its design, details would have surely been missed if I had completed it back in 1996. The research and developement effort for the Tube Drive had endured for well over a decade of and had just about convinced myself that my work was complete – but I was wrong.

Two Phils Are Better Than One

David Gilmour and the Tube Drive

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

In 2011 I received a phone call from a very nice fellow 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 significant event in history for Effectrode.

Phil was interested in trying out a few Effectrode pedals to see if they could be of benefit to David’s guitar tone. This was to be the beginning of an ongoing dialogue which would stretch over many months. Over this period I worked with Phil to ‘tailor’ and ‘tweak’ the tone of several pedal designs to his tastes. I would post a pedal to Phil at the 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 on his likes and dislikes and suggestions for improvement. His involvement enabled me to more confidently fine-tune the performance of each pedal. He was also good enough to ship several of David’s pedals for reference so I could perform A-B listening tests. I can not overstate how much of a help this was; having access to someone with such depth of knowledge and experience – I consider myself fortunate.

Phil’s independent opinion was also beneficial for my sanity too. Sound quality is a highly subjective and nebulous thing. Just utilising tubes in a design and getting the technicalities right, such as running them within operational parameters, correct voltage, etc does not guarantee that a pedal circuit design will sound amazing. The devil (or god) really is in the details of the circuitry and it can get to a stage where it’s 3 o’clock in the morning and you’ve done so many listening tests that you are no longer in a fit state to make a cup of tea let alone design something magnificient and world-shaking.

Getting back to the Tube Drive. Phil heard something in the pedal he liked, however desired 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 with him describing 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 mailed the pedal back so that I could modify it to reinstate more of the lower register. I increased the cathode bypass and coupling capacitors values to allow the lower frequencies into the tube clipping stages and returned the pedal to Phil again. He still wasn’t happy. The pedal came back and I increased the cathode bypass capacitor values again. This extended low frequency response was beginning to push the pedal into instability or ‘motor-boating’ as old-timers called it. This kind of thing happens in high-gain, wide bandwidth amplifier circuits – they become oscillators – an interesting effect but not what we’re after here. I decreased the capacitor values a little, the circuit came back to planet Earth and I performed signal generator and oscilloscope tests. My test gear was telling me that this pedal would allow the lower registers of a church pipe organ through unharmed and without the slightest hint of attenuation of subsonic frequencies. Surely this would be good enough. I posted it back to Phil, 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 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 Mouldings, 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.