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 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.