With a gain factor of 70, a 5751 is a popular replacement for a 12AX7, as it reduces the gain of the first preamp stage by around 30 percent compared to a 12AX7, the typical V1 tube. Stevie Ray Vaughan was known to use 5751 preamp tubes to coax a firmer crunch from his vintage Fender amps. Photo by Andy Ellis

5751 = Gain Factor 70
This is one of the most popular subs for the common 12AX7. A 5751 will reduce the gain of your first preamp stage by around 30 percent relative to a 12AX7 originally in that position, which can often be enough to tame a fizzy overdrive tone without sacrificing too much of the amp’s overall power and muscularity. Good 5751s are also often just great-sounding tubes in many circuits, delivering so many of the family’s desirable sonic characteristics, but with less of the gritty edge and ragged breakup of higher-gain preamp tubes.

This tube has been a favorite of many professional players for just this reason, and was one of the tricks Stevie Ray Vaughan often used to achieve a bolder, firmer breed of crunch from his vintage Fender amps. Dig into any discussion of preamp tube substitutions for the 12AX7, and you’ll likely see the 5751 mentioned early on, and confirmed time and again as a popular choice.

Lowering the gain to achieve a better overdrive tone might seem counterintuitive, but doing so will often help you induce less distortion in the early stages of the amp, while passing along a less compressed, more frequency-rich signal to later stages. That, in turn, means you can push the output stage harder to induce a bigger, beefier crunch from those power tubes without passing along the fizziness of an already-distorted preamp tube.

Whether you’re playing clean, crunchy, or heavily overdriven sounds, the sonic results of this swap will often yield tighter lows with less farting-out, a somewhat more balanced and less hyped midrange, and highs that refrain from being strident or spikey.

12AX7 = Gain Factor 100
We’ve now worked our way back to the 12AX7—the baseline for our exploration. Because it’s the most common preamp tube type in use, particularly in contemporary guitar amps, the 12AX7 is the little bottle you’re likely most familiar with. With a gain factor of 100, it’s also the hottest tube in this selection, so most amps that carry one in the V1 position are coming to you at their highest potential gain level.

A 5751 will reduce the gain of your first preamp stage by around 30 percent relative to a 12AX7 originally in that position, which can often be enough to tame a fizzy overdrive tone without sacrificing too much of the amp’s overall power and muscularity.

Why would you want less gain in this position? If you’re going for all-out metal or shred, chances are you don’t, and in such cases a good 12AX7 is likely your best bet. But if you haven’t tried such swaps before, you’ll probably be surprised by what a tube with a lesser gain factor will do to your amp’s response, right off the bat. And even many heavy-rock styles can benefit from a less slamming tube in V1. In high-gain preamp designs, there’s usually more than enough gain to be had from lower-gain preamp tubes anyway, and often this comes with the bonus of a sweeter, smoother breakup.

None of which is intended to imply that there’s anything wrong with a good 12AX7, and it’s the standard for good reason. This tube can sound great kicking off your amp’s sonic and gain characteristics, no doubt, but in some circuit designs it can also be associated with a slightly harsh distortion characteristic when pushed hard. In such cases, this can induce a “wasps-in-a-tin-can” lead tone players describe as fizzy or fuzzy. You can ameliorate this effect by tamping down the gain a little with a different type of preamp tube.

Swapping Phase-Inverter Tubes
As mentioned earlier, the phase-inverter (PI) tube doesn’t affect the tone of your guitar signal in a conventional tone-shaping manner, but its gain factor does determine how hard it drives the output stage. In addition, the degree to which the PI tube itself distorts while splitting and transferring the signal to the output tubes will contribute to the amp’s overall distortion content.

With this in mind, it’s not hard to extrapolate from the general preamp information above to guess what these different tube types might do when used in your PI position. Swapping tubes in this stage, however, should generally be done with a little more care and consideration for the intentions of the original design, since amp makers have usually put more thought into phase-inverter design than merely tweaking distortion characteristics. For that reason, it’s probably best to pursue nuanced changes here, if at all.

One of the most obvious lessons in what different preamp tube types will do in your PI can be found by simply examining some of the characteristics of classic amps designed with these tube variations, while considering the designers’ intentions in using these tubes.

Most players are familiar with the easy crunch and sweet overdrive of the Fender tweed and Marshall JTM45/plexi templates, both of which used 12AX7s (or similar 7025s) in their phase inverters. The preamps in front of these PIs are by no means high-gain, but with the volumes up pretty high, hitting this higher-gain PI tube with a pretty hot signal, these stages are able to work together to get the output tubes cooking pretty well. Fender’s early-’60s amps with brown control panels and tan or blonde Tolex covering mostly retained the 12AX7/7025 in the PI, but when Leo and crew further refined these designs to achieve improved headroom and a cleaner overall response, they loaded this position with a more restrained 12AT7 tube instead. That change is an important part of the blackface sound, which is partly characterized by the later onset of distortion—an effect this cooler tube contributes to.

An intriguing alternative for the phase-inverter tube, the 12AU7—aka ECC82 in the U.K. and Europe—has a gain factor of only 19. The 12AU7 is extremely resistant to distortion in the PI stage, which is why, circa 1963, JMI included a 12AU7 in their new 100-watt Vox AC100. The goal was to give the Beatles enough clean power to be heard over thousands of screaming fans. Photo by Andy Ellis

Taking things even further in this direction, when JMI wanted to create the cleanest, most powerful guitar amp imaginable in the early ’60s—specifically to help the Beatles be heard over hordes of screaming fans in larger and larger venues—they used a 12AU7 in the 100-watt AC100. With an amplification factor of only 19, making it a lower-gain tube than any of the four we’ve discussed so far, the 12AU7 was extremely resistant to distortion in the PI stage. It also helped maintain high headroom in the four EL34s in the output stage. (Note that the 12AU7 is not a great substitute in the preamp positions in most guitar amps, even when you really want to tamp things down. It will “work” in most of the same circuits, but this tube prefers some circuit tweaks to optimize its performance, and usually just doesn’t sound very good in V1.)

One challenge regarding PI swaps is that it’s often difficult for the typical guitarist, and even one with some tube-swapping experience, to gauge the tonal and gain-induced sonic effects of any given PI tube. If you’ve already tried a 5751 or a 12AY7 in your V1 position, however, and you feel the output stage is still being driven a little too hard at your desired volume settings—which might be heard as a “splatty” or harsh response from smaller output tubes in particular, like EL84s or 6V6s—it might be worth trying a 5751 or a 12AT7 in place of the 12AX7 that was (most likely) originally there. You can go even further if this doesn’t do it, by popping in a 12AY7. Note that alongside the increased headroom, you’ll experience a drop in output level when swapping PIs down the gain range, so you’ll likely find yourself adjusting volume controls accordingly.

This is a technique that some contemporary amp makers use as part of their overall design. For many years, Dr. Z used a 5751 in the PI in several amps—notably the Carmen Ghia model—to avoid hitting the EL84s too hard and inducing a ragged response. (Although more recently, Dr. Z has sometimes changed up that formula.)

Going in the other direction, if you play a Fender blackface or silverface amp, or one derived from that design, and would like to drive the output stage harder, try replacing its original-spec 12AT7 with a 12AX7 to hear whether that does the trick. If that takes it too far, try a 5751, which should give you something in between the two.

Technically, some of these tubes—even if they are “compatible” in the broader sense—are not designed to work optimally in PIs that were devised with another type in mind. But as with our discussions of the 12AY7 and 12AT7 in the preamp section, in most cases they will work fine, and should at least give you a taste of what such PI swaps might do for your amp’s overall performance.