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Swapping Tubes in the Blackbird

The Blackbird preamp pedal ships out of the Effectrode workshop with a set of three JJ 12AX7 (ECC83) tubes fitted as standard. However, its circuitry is specially designed to operate with other kinds of tubes too. You can swap out the 12AX7s for different B9A types, such as 12AT7 (ECC81), 12AY7 or 12AU7 (ECC82) to customise the tone and ‘feel’ of the Blackbird to suit your playing style.

What the Tubes Do

The 12AX7 is a 9-pin miniature dual triode. Contained within its glass envelope are two identical amplifier sections; which means there are six tube amplifier sections within the Blackbird’s circuitry. If we start at the V1 position, one of these amplifier sections makes up the input gain stage for the Clean channel. The Blackbird’s Clean channel is identical to the grounded cathode input stage of a Fender ‘Blackface’ amp. The second section of the 12AX7 in V1 is an output cathode follower buffer, which is common to both the Clean and Gain channels. We recommend to leaving this tube as is.

However V2 and V3 can be swapped out. The amplifier sections of the 12AX7 in position V2 are gain stages ‘1’ and ‘2’ of the Gain channel circuit; and the amplifier sections of the 12AX7 in position V3 are gain stages ‘3’ and ‘4’ of the Gain channel. Swapping out the tubes in V2 and V3 tubes for other 9-pin dual triode types alters the gain, character and feel of the Blackbird. So let’s delve in and discover how we can tweak the Gain channel to the n^th degree, so that it complements your playing style perfectly.

Savage Saturated Sounds

The three stock 12AX7 tubes yield greatest gain and range. With this tube configuration the ‘CREAMY’ (Red LED illuminated) and ‘CLASSIC’ (Blue LED illuminated) modes are voiced quite differently to one another. The Gain channel covers a wide range of guitar sounds from warm, bluesy overdrive to devastatingly saturated distortion. This 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 a guitar amp.

Raunchy Retro Rock

If you don’t like cream (a.k.a. heavy, harmonically saturated distortion) in your cup of tea, then try swapping out the 12AX7 in V3 for a lower gain 12AY7; this will cool the Blackbird down. The amplification factor of the 12AY7 is only 45, whereas the 12AX7 is 100. This lower gain tube substitution transports the tone back to the 1970s, where the ‘CREAMY’ mode effectively becomes a lead channel for soloing, and ‘CLASSIC’ mode into a rhythm channel for chord-work—a rock guitarist’s dream. What’s more, the 12AY7 minimises the volume difference between the ‘CREAMY’ and ‘CLASSIC’ modes. The result is a smoother transition, and less of a jump in distortion clipping levels, when switching between the two modes.

As well as these ‘CLASSIC’ and ‘CREAMY’ modes on the Gain channel, the Blackbird has a Clean channel (Green LED illuminated) which functions as, well, a clean channel. This channel sounds different to when the Blackbird is in bypass—it adds more clarity, definition and punch to the guitar; in a word it sounds ‘glassy’. You’ll notice a pleasant lift in the tone of the guitar when engaging the pedal.

All these channel switching options vastly extend the flexibility of a single channel guitar tube amp, giving the guitarist access to a total of four channels, if you include bypass. Further, 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 eight different gain combinations!

Blistering Blues

The amplification factor of the 5751 is 70, which is 30% lower than that of the 12AX7. Installing a 5751 sheds a little more gain, further opening up that magical sweet-spot where the clean sound of the guitar transitions smoothly and seamlessly into soft-clipping to generate a richer, more full-bodied tone. It’s here where you’ll discover those warm, bluesy Larry Carlton and J.J. Cale tones.

Smooth, purring overdrive is where vacuum tubes really do shine. But it’s not just the tone, it’s the feel and responsiveness too. Transistorised FET and modelling/profiling gear just doesn’t get this right. Tubes leave this modern solid-state gear way behind on the distortion dirt track, face-down choking on digital dust.

This 5751/12AY7 combo is my all-time favorite tube configuration: The two tubes transform the Blackbird into a dedicated blues drive machine. And this combo can be tweaked even further, by replacing the modern JJ tubes with NOS (New Old Stock) equivalents. Try a Tung-Sol 12AY7 (authentic U.S.A. manufacture, not the rebranded Russian Sovtek) and a Sylvania 5751 in there. You might be surprised. But, whether you’re using JJ tubes or vintage American tubes, the sound is addictive and a joy to play through.

If mellow, bluesy tones are your thing then it’s also worth trying a 12AV7. This dual triode has a slightly lower amplification factor than the 12AY7. Not only that, its characteristic curve is different too, so in circuit a 12AV7 sounds distinctly different to a 12AY7.

The 12AV7 works well in position V2 or V3. As with the 12AY7, putting a 12AV7 in V3 will reduce the level difference between the ‘CLASSIC’ and ‘CREAMY’ modes of the Gain channel. Although the 12AV7 is no longer manufactured there are still plenty of NOS deals to be had, in fact this tube can be purchased for less than a new JJ 12AX7.

How Low Can You Go?

The 12AU7 is the tamest of all the B9A double triode types—it’s amplification factor is only 20. But does this tube yield useful results in the Blackbird? Well, yes: with a 12AU7 in position V3, ‘blocking distortion’ and clipping distortion artifacts diminish and dwindle to nothingness. As a result the Blackbird becomes a tool for colouring the tone of the guitar or adding a mild lift to the volume level to enhance the sound. What’s more, vintage 12AU7s with longer plates can make the tone even mellower. For example, Mullard and Brimar manufactured 12AU7s with 15mm and 17mm ladder plates.

Bad to the Bone

But not all tubes sound good in the Blackbird. For instance, take this pair of old Brimar computer tubes pictured below. Now, they measure up just fine on my trusty Tube Imp™ tube tester with good emissions and gain, but in circuit they howl like a mad dog. This is uncontrolled self-oscillation and it makes them unusable for audio applications.

The reality is, computer tubes were designed for longevity, not tone. Their large cathodes give them plenty of electron reserve and their plates are also large to improve cooling (power dissipation). These large electrode structures are prone to picking up externally induced vibration, a.k.a. microphony. The manufacturers weren’t overly concerned about damping or stiffening these electrodes to reduce vibration pickup. It didn’t matter, because these tubes were to be used in ‘on-off’ applications, a.k.a. digital switching.

That’s not to say you can’t use computer tubes, in fact they can yield surprisingly good results. I’ve obtained some excellent tones from the Sylvania Gold Brand 5963—despite its large electrodes. I guess this is down to Sylvania’s stellar quality control and manufacturing. But generally speaking, you will need to cherry-pick the quietest tubes from a bunch of them. It’s a hit and miss process—more miss than hit, in my experience.

Well, I think that just about wraps things up. Did I leave anything out? Perhaps you’ve got some tube swapping ideas of your own? If so, do let me know and I’ll see about including them in this article. Over and out.

For a more in-depth look at clipping distortion and how it affects guitar tone check out the article, ‘Guitar Preamp Tone Explained’

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Swapping Tubes in the Blackbird

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