Though these “12A” preamp tubes look similar, they offer different levels of gain, which means you can substitute one for another to alter your tube amp’s tone and response. From left: A 12AX7 (gain factor 100), 12AT7 (gain factor 60), and 12AY7 (gain factor 40). Photo by Andy Ellis
From Headroom to Hot Rod
Let’s take a look at what four different compatible, nine-pin, dual-triode preamp tube types can do for the sound and playing feel of your amp when you substitute one for the other. We’ll take these from the lowest gain to the highest, although the latter is the tube that most players will remove to begin the swapping process—the 12AX7 (ECC83 in the U.K.). In most situations these four can be used as direct replacements for each other, but I’ll mention any caveats if and when they apply.
12AY7 = Gain Factor 40
This tube’s gain factor of 40 is the lowest of the bunch, and this might seem dramatic when compared to the 12AX7’s gain factor of 100. As a general point of observation, however, you’ll really only notice a gradual increase in volume and gain as you jump from one tube to the next, while other aspects of your amp’s sound reveal themselves more prominently. Of all the tubes considered here, the 12AY7 (aka 6072) will push your amp’s front end the least, but in many cases that’s exactly what you want.
This was the original tube in the first gain stage of most Fender tweed amps from the mid 1950s to 1960s—which also makes it the V1 in the countless tweed-inspired amps still made today. Most fans of tweed amps agree this is exactly the tube you want in those circuits, even with higher-gain substitutes more readily available. Tweed-style amps can often sound too ragged and unhinged with a 12AX7 in that socket, with a fizziness or buzziness to the breakup that can be unappealing to some players.
Looked at from the reverse perspective—swapping a 12AY7 in place of an original 12AX7—this tube is excellent for taming a harsh preamp stage, tamping down the gain when you just don’t need as much firepower as your amp wants to deliver, or removing strident harmonic artifacts from your tone. Gain aside, a good 12AY7 can still sound very full, rich, and well-balanced, and offers a great way to achieve maximum headroom and minimum early-stage fizz from many preamps in other types of amps.
Contemporary makers occasionally recommend 12AY7s in newer, non-tweed-based amps, when this type of sound and performance are desirable. Brian Gerhard of TopHat suggests it as an alternative for the V1 position in his Super Deluxe, Super 33, and the Top Boost channel of the Supreme 16. And while you might not immediately see this as a fit, I even tried one in the lead channel (channel 2) of a Mesa/Boogie Mark Five:35 head with great results, taking its Xtreme setting from heavy rock and metal down into the classic-rock, blues-rock, and garage-rock zones.
Caveat: Some techs will point out that the 12AY7 has a lower plate resistance than the 12AX7 and others in this group, and will therefore draw more plate current through the resistor that couples this tube to the amp’s power supply (usually a 100k-ohm resistor). For that reason, some recommend that you should upgrade these plate resistors from the usual 1/2-watt types to larger 1-watt types, to handle the heat generated by the extra current. That said, you can almost certainly substitute a 12AY7 for a 12AX7 in the V1 position of standard guitar amps without experiencing the slightest problem.
Historically, a 7025 was a military-spec version of a 12AX7, built to be more rugged and have a lower noise floor. Modern 7025 and 12AX7 preamp tubes are essentially identical, differing only in their names. Both offer a gain factor of 100. Photo by Andy Ellis
It’s worth noting that the Fender tweed amps famous for their use of these tubes had only 1/2-watt plate resistors in that position, but they also had relatively low plate voltages on their V1 tubes to begin with, so there’s generally even less worry about things getting overheated in any amps inspired by the tweed or Marshall JTM45/plexi platform (although these Marshalls substituted an ECC83, the British equivalent of the 12AX7, for the 12AY7 in the first place).
12AT7 = Gain Factor 60
The 12AT7 is probably best known for its use in reverb stages or in the phase inverters of blackface and silverface Fender amps of the ’60s and ’70s, but it can also be used in the V1 (or equivalent) position in many amps to lower that first stage’s gain and thereby achieve more headroom and a tighter overall tone.
Some players find that a 12AT7 yields a slightly dull, cold tone when used in the V1 position, but that conclusion seems to vary from amp to amp and player to player. If you’re looking to tame a first gain stage that’s a little raw and hairy and needs some tightening up, the right 12AT7 might do the trick. Even though its gain factor of 60 makes it appear 50 percent stronger on paper than a 12AY7, you likely won’t hear anything close to “half again as much gain” when substituting an AT for an AY, although you’ll notice a pretty dramatic decrease when popping in one of these in place of a hotter 12AX7.
Caveat: The 12AT7 is often set at a slightly different bias level than the other tubes discussed here, so it might not perform optimally in some circuits. But you’re unlikely to harm the amp or the tube by trying it out, so if you’ve got a good spare 12AT7 handy, the best route is to stick it in for a while, play, and see.