It has been said that the 12AX7 tube is the cornerstone of guitar tone. This glowing glass bottle can be found in the heart of practically every guitar amp ever made from the 1950s onwards—it’s what makes electric guitar possible! But did you know the 12AX7 is just one of a whole family of 9-pin double triode tube types, such as the 12AU7 and 12AT7, that share the same physical dimensions and pinout? In principle this means one type can be swapped out for another; for instance, a 12AX7 could be swapped out and replaced with a 12AU7, or vice versa.
Now, as to whether tube substitution yields beneficial results very much depends on what type of circuit the tube is operating in. Let’s take a look at a couple of real world examples. How about swapping the 12AT7 in the reverb tank transformer driver circuit of a Fender ‘Twin Reverb’ amp for a 12AX7? Well, a 12AX7 has little current drive capability, so it wouldn’t be able to supply enough electrons to the primary coil in the transformer for it to work properly. The same goes for a 12AU7 or 12BH7 transformer driver stage in a low wattage tube power amp. The 12AX7 cannot deliver the necessary current to drive these kinds of heavy loads. But this should come as no surprise as the 12AX7 was originally designed to operate as a voltage amplifier of small electrical signals—it’s not a current amplifier.
However, there is much to be gained by going in the other direction, that is swapping out a 12AX7 (gain factor = 100) in an amp’s preamp section with a lower gain tube, such as a 5751 (gain factor = 70). This is exactly what Stevie Ray Vaughan did to tame the distortion in his Fender amps and obtain a less fizzy, bolder overdrive from them. You could go even lower still by substituting the 12AX7 with a 12AY7 (gain factor = 40), the same tube found in the input stage of a Fender 5E3 ‘Tweed’ amp. And in more sophisticated amps with multiple preamp tubes, such as the Soldano, Boogie and the Effectrode ‘Blackbird’ pedal you can use mixed tube combinations to sculpt the tone and feel of the amp.
The middle 12AX7 tube in the Blackbird and Tube Drive pedals can be swapped out for other 9-pin double triode tube types, such as the 12AV7, 12AY7 or 12AU7. These substitutions yield lower gain and widen the sweet-spot for mild overdrive and bluesy tones. You can swap out tubes in other positions on the Tube Drive too. For example, the 12AT7 tube, which forms part of the active tube boost circuitry, can be swapped out for a new old stock 12AV7 or 12AZ7 to add subtle colouration. You can also swap the top tube too. The two 12AX7s form the tube clipping circuitry and you’ll get some very meaty classic rock tones by substituting 5751 and 12AY7 tubes in these positions. And it’s worth experimenting with other new old stock tubes made by various American and European manufacturers such as Sylvania or Brimar. Differences in their electrode geometry result in different tonality. For example, the larger ladder plates of a Brimar 12AX7 possess greater inter-electrode capacitance than a short-plate Sylvania or JJ 12AX7s—this substitution results in a smoother, fuller tone.
Discover how to unlock the true potential of your Effectrode ‘Blackbird’ pedal in Yoel Kreisler’s excellent article on tube rolling.