Pent-O-Drive

(photo coming soon)

This is one of those ideas that, when I first thought of it, I couldn't believe that nobody had done it before.

Most guitarists know that the venerable EF-86 small-signal-pentode tube has a certain tonal mojo that nothing else can quite deliver. Complexity of tone and sensitivity to touch are the characteristics with which it is most often associated. Moreover, it is particularly well suited for driving low-impedance loads, like tone stacks.

So why don't we find these wonderous little bottles in more applications? There are several reasons:

  1. They're notoriously difficult to configure (particularly for those who don't really know what they're doing, but simply try to cop circuits from older designs.)
  2. They have a reputation for being microphonic, particularly when used in combo amps, where the proximity of a speaker can exacerbate the problem.
  3. Unlike the more common 12A*7 preamp tubes, which encase two seperate triodes in a single package, the EF-86 is just one unit per bottle. That means bottom-line-driven designers have to use an extra socket, larger filament transformers, and more chassis real estate.

Based on its very high gain capabilities, the earliest builders saw it as a way to build a cheaper product: intead of using multiple, lower-gain triode stages, they use just one pentode, configured for maximum gain. Unfortunately, this brings out the worst characteristics of this tube, and they soon abandoned that approach in favor of the more common multiple-triode preamp topologies. Still, when these venerable amps were cranking, they definitely had a wonderful tone that everyone coveted. As a result, later designers, attempting to capture some of this mojo, simply copied these early designs, flaws and all!

As you'll see elsewhere in this site, I use these tubes extensively in all of my amp designs, and have experienced absolutely none of the issues often (and erroneously) attributed to them. The 'secret'? I configure them conservatively, and don't cut corners by trying to make them take the place of multiple triodes! By keeping the gain at a more moderate level, they retain all their touch sensitivity and tonal complexity,and exhibit no greater tendency toward microphonics than any other tube.

While there are lots of tube overdrive pedals available, they invariably use one or more dual triode tubes. After working with the EF86 in several amp designs, I realized that the overdrive pedal is an absolutely ideal applicaton for this tube.

For one thing, it places the tube early in the signal chain, where its touch sensitivity and complex tonal response is most fully exploited. Moreover, since a little gain goes a long way in this context, the tube operates in a range where it's strengths are maximized, while virtually elliminating its weaknesses. Finally, it is safely isolated from vibrations that might tend to introduce microphonic noise.

A switchable boost circuit is configurable to either precede the EF86, pushing it into overdrive, or follow it, in order to hit the front end of the amp harder.

Overall switching, of course, is full bypass.

Specially voiced bass and treble controls complete the circuit, with everything housed in a sturdy, powder-coated steel chassis.

Projected direct price: $349.00, including external power supply.