Swapping Tubes in the Blackbird
by Phil Taylor
The Blackbird’s all-tube circuitry has been specially designed to function with a variety of different tube types (12AX7, 12AY7, etc) allowing guitarists to tailor the pedal to their own specific playing style and fine-tune their guitar tone to the absolute nth degree. There are three 12AX7 tubes fitted in the Blackbird pedal as standard. The 12AX7 is a 9-pin miniature dual triode, that is, the glass envelope of each tube contains two amplifier sections. In V1 one amplifier section is the ‘Clean’ channel input stage (identical to the input stage of a Fender ‘Blackface’ amp) and the second section is the output cathode follower buffer, which is common to both the ‘Clean’ channel and the ‘Gain’ channel. In V2 both amplifier sections are the first two gain stages of the ‘Gain’ channel and in V3 both amplifier sections are gain stages three and four of the ‘Gain’ channel. This article discusses everything you need to know on swapping out the stock 12AX7 tubes for different 9-pin dual triode types to alter the gain, character, response and feel of the Blackbird pedal for your own playing style and tone.
The three stock 12AX7 tubes yield greatest tonal flexibility. With this tube configuration the ‘Creamy’ (red LED lit) and ‘Classic’ (Blue LED lit) modes are voiced quite differently to one another allowing the Blackbird to effortlessly cover a wide range of guitar sounds from warm, purring bluesy overdrive to devastatingly saturated rich distortion. This huge flexibility is especially useful in a recording studio session situation where you can totally rely on the Blackbird as your one-stop tone tool for creating a wide range of authentic tube guitar sounds and capture them directly to tape without needing to use a guitar amp.
If heavier, saturated distortion isn’t your cup of tea then swapping out the 12AX7 in V3 for a 12AY7 will tame the Blackbird somewhat by lowering the gain opening up the sweet-spot for those smooth, bluesy Larry Carlton and J.J. Cale tones. In this position the 12AY7 also reduces the difference in gain between the ‘Creamy’ and ‘Classic’ modes resulting in a smoother transition, less of a jump in distortion clipping levels when switching between the two modes. The 12AY7 is a great tube substitution for the Blackbird as the ‘Creamy’ mode works devastatingly well as a Lead channel for soloing and ‘Classic’ mode as a Rhythm channel. On top of these Lead and Rhythm channels the Blackbird’s Clean channel (green LED lit) greatly extends the flexbility of a single channel guitar tube amp giving the guitarist access to a total of 4 channels if you include bypass. Incidentally, the Clean and Gain channels respond exceptionally well when pushed, so placing a booster pedal such as the Fire Bottle in front of the Blackbird multiplies channel options two-fold – that’s 8 different gain combinations.
The 5751 has a slightly lower gain factor than the 12AX7 and installing one of these sheds a little more gain to further open up that magical tone territory where clean tones transition into breakup and mild soft-clipping. This is where tubes really shine, there’s nothing that comes close to the depth and richness they can add to the tone of guitar. But is not just the tone, it’s the feel and responsiveness too, which, in my opinion really does leave transistorised FET and modelling gear in the dust. This is my favorite tube configuration for the Blackbird and it transforms it into a dedicated blues drive machine. This tube configuration can be improved on a little further by utilising a new old stock (N.O.S.) Tung-Sol 12AY7 (authentic U.S.A. manufacture, not the Russian rebranded Sovtek) and a N.O.S. Philips 5751 has an edge over the JJs. However, whether your using JJ tubes in there or N.O.S. American tubes, the sound is utterly addictive.
The 12AV7 is another very nice tube substitution that’s worth checking out if you’re into mellower, bluesy tones. The gain factor of the 12AV7 is a little lower than the 12AY7 and its characteristic curve is different too. What this means in practice is that this tube sounds markedly different to the 12AY7. You can use the 12AV7 in position V2 or V3. As with the 12AY7 earlier, utilising the 12AV7 in V3 will reduce the gain difference between the ‘Classic’ and ‘Creamy’ modes of the Gain channel. Incidentally the 12AV7 is no longer manufactured, however there are still plenty of N.O.S. deals to be had, in fact this tube can often be found at lower prices than a modern manufacture 12AX7.
That’s all… for now. I’ll be adding a few more updates to this page after I’ve had time to experiment with a 12AU7 in the Blackbird…
For a more in-depth look at clipping distortion and how it affects guitar tone check out the article, ‘Guitar Preamp Tone Explained’