The Magic of the V1 Preamp Tube

In the modern age of the quest for tone many guitar enthusiasts have taken to modifying their amplifiers, pedals, and guitars in order to attain the ultimate sound down to the tiniest nuance. The modern amplifier builder is starting to build with these thoughts in mind bringing forth a slew of new amplifiers on the market that are “mod friendly.” This way most guitarists, despite their level of technical and electronics experience, can swap parts or add basic circuits to their existing amps to improve tonal qualities and make that amp unique to the player. In this quest, there is one very simple amp mod that is sometimes overlooked by many guitarists, the V1 tube. V1 is the first preamp tube in an amplifier. With some simple know-how and experimentation, it can lead to a dramatic tonal variation of any tube guitar amp without the need for disassembly (most of the time) or soldering.

What is V1?

The first gain stage of a tube amplifier is responsible for a large amount of the overall tone shaping of an amplifier. It is the first point in which the pure guitar signal is amplified and determines the way every other aspects of the amp will respond to the guitar signal. The guitar signal is immediately amplified by V1 then sent to the tone circuit and on to any other circuitry before hitting the power amp tubes. It can usually be found closest to the input of the amp. Most modern tube amps use a 12AX7 for pre-amplification, however, you can swap it for a variety of preamp tubes without the need for re-biasing for amp . Just shut off the power and pull out the tube. This makes V1 the easiest mod you can do to really sculpt the tone of your favorite amplifier to suit your needs.

Let’s talk gain

Before we talk about different tube types, let’s get a basic understanding of gain and tube distortion. Tube amps have retained their popularity with guitarists and bassist throughout the decades of amplified guitar despite the fact that they are “old” technology. Why? Without getting too technical it’s because they distort in a more musical fashion than transistors. When a transistor is pushed over its threshold the signal gets chopped off dramatically distorts in a “square wave” fashion. A tube on the other hand has more “headroom” and distorts along a smoother curve with less harsh artifacts. Even a tube amp played clean has a little of this clipping in the signal that comes through as layers of harmonics that fatten up the tone and give that essential sound to the guitar.

So now let’s discuss gain factors and how they affect the overall tone of the amp. Using a tube with a lower gain value in V1 can have a big effect on how the power amp reacts to the signal. A 12AX7 has a gain factor of 100. A 12AT7 has a gain factor of 60, meaning it has 40% less gain than a 12AX7. Using a 12AT7 in place of a 12AX7 in V1 can make the amplifier quieter and give it more extended clean headroom before breakup. If you are playing humbuckers into a low wattage Fender amp, the 12AT7 is also a great way to achieve a cleaner tone overall. Another commonly overlooked affect of this procedure is that a lower gain V1 will cause the power tubes to work harder. This creates more power amp distortion as opposed to preamp distortion.

So what does that mean? Preamp tube distortion, although “hotter”, is usually fizzy, sometimes thin, and doesn’t respond as well to varying degrees of attack since the sound is much more compressed. Power tube distortion is where the dynamics and “natural” sounding overdrive characteristics come from. You will also notice a fatter thump even though the gain might not sound as intense. If this is the sound you’re going for, try a lower gain preamp tube for less preamp distortion and make that power amp work! Along with a different type of dirt, your speakers will also react and push more air. When all the elements; preamp, power amp and speakers are working together, that’s when the magic of tube amps comes out. In fact, Stevie Ray Vaughan used a 5751 in V1 (30% less gain than a 12AX7) to alter the characteristics of his amp and suit the tone he wanted. Keep in mind, this trick does not work with high gain amplifiers that rely on preamp distortion cascading through many tubes to produce massive amounts of thick distortion. This is also dependent on what your ears hear and various playing styles. Different people will hear different sounds.

Besides the gain factor, different manufacturer’s tubes of the same type will have different tonal qualities. Some will be brighter and crisper while some are warmer and more rounded. Some will be noisier and distort easily while some will be quieter and have more headroom. A fun thing to do on a rainy weekend is to pull out 3 or 4 12AX7’s from different manufacturers, and a couple of 5751’s and 12AT7’s. Mic up your favorite amp and kick on the 4 track (or computer based recording system of your choice), play a tune and record 30 or so seconds of it. Then switch V1, switch tracks, and do it again. After you’ve gone through all the tubes in this manner, take a break for a half hour and let your ears rest. When you come back, a fresh listen to each track individually under headphones or studio monitors will reveal a true difference in tone. Even to a relatively untrained ear.
 
So gather a small collection of preamp tubes to try and go for it. Remember though, just because Stevie used a 5751 or awesome guitarist #1 down the street uses a so-and-so NOS $300 preamp tube, doesn’t mean it is right for you. Use your ears and those of the musicians around (i.e. Bandmates) to help you determine what combination is right for your sound. The process can be long but it’s a whole lot of fun listening to the subtle (and sometimes not so subtle) differences that the magical V1 tube swap can create. Enjoy!

A quick disclaimer from our friendly amp tech, please take the time to read it.

Not all tubes are interchangeable. The preamp tubes listed here all can be interchanged with each other but your amp was designed with a certain tube in mind for V1. Changing this can alter tone but also alter the way the amp’s electronics handle things. It is generally a better idea to go down in gain factor rather than up. If you have ANY doubts or questions, please consult a qualified technician or the manufacturer of your amplifier BEFORE swapping tubes. Swapping tubes will usually void any manufacturer’s warranty as well so do so at your own risk. Subject matter in this article relates to preamp tube V1 only and does NOT relate to power tubes. Always refer power tube maintenance or replacement to a qualified technician as the voltages contained inside your amp are deadly.

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