TubeWorks Pure Tube TW303 SMOOTH

A glorious day, friends! This one has accomplished some work in hopes of earning some coins with which to line his pockets. What came into my possession lately was a rarely-seen TubeWorks Pure Tube TW303 SMOOTH guitar overdrive. It was in dire need of serious repairs, so eagerly took the opportunity to ply my skill and bring it back to life. Unfortunately, a schematic could not be found, and I could only discover only the vaguest information about the device on the ‘net. So, I’ve taken it upon myself to dissect the device for the good of the community, and I now present to you the schematic I have traced.

Here are a couple photos of the interior of the unit, before and after the repairs were made.

See the little tube shield? That is an “extra” I added to help with the noise; it was originally in an ancient Norelco tape machine which I have been salvaging parts from. The main problems with the unit were a missing bypass switch, faulty/missing ground connexion between the board and the signal jacks, and a failing tube socket. The footswitch and socket were replaced and the ground connexion was restored. The device now functions perfectly and has a great overdrive sound. It’s “SMOOTH” moniker is very much appropriate!
This is an interesting circuit overall; a derivative of the the classic Tube Driver designed and patented by B.K. Butler. The board had the label “RT-901/903” screened on it, which leads me to know that it is the same board which was used for the TubeWorks Real Tube RT-901 and Blue Tube RT903. So, it follows logically that the schematic which I have traced should give us the basic topology which was used in those pedals. Though we can be sure that the RT-901 and RT-903 used different values of certain components (at least in relationship to one another), we at least have a good starting point if one wishes to analyze either of those circuits. The Tube Driver itself is a brilliantly-conceived concept; it uses what they call a “starved plate” tube circuit – a tube running at drastically reduced plate voltage. Normally, a tube does not function very well with the 12 volts the Tube Driver uses, but Mr. Butler ingeniously thought to put a negative 12 volts on the cathodes. This, along with the usual negative voltage biasing the grids, helps propel the electrons from the cathode with adequate velocity to reach the positively-charged plate and achieve performance much closer to what is possible at proper tensions. Still, it doesn’t behave quite the same as normal and starved plate circuits inherently distort more easily and in a more exaggerated fashion than what normally happens in a high-voltage circuit. This exaggerated distortion is what makes these circuits ideal for guitar overdrive effects. Working on this project has given me some ideas to experiment with, so be on the lookout for a starved plate tube overdrive DIY project on these pages in the near future. There are a couple good DIY Tube Driver projects currently up at Freestompboxes by Bajaman and Dirk Hendrik, so I will try to come up with something different so it’s not just a re-hash of what has already been done.

-M.C.