Not all 12AX7 preamp tubes are created equal.

We get asked a lot of preamp tube questions such as, “Which preamp tubes should I use?” or “Do you care for this brand of tube?”. This article covers some of the questions and provides some info on reasons for changing preamp tubes.

The most common preamp tube for tube guitar amplifiers is the 12AX7. They have been in use since 1947 – so the Fender Tweed’s, Vox’s, Supro’s, and many other amps up through the modern Marshall DSL100 amps use 12AX7 preamp tubes.

The thing to remember when picking one for your amplifier is… the right tube is relative.

Unfortunately, depending on the amplifier and application there will be several good options but nothing will be perfect every time.

Testing Tubes

We test preamp tubes before we install them so we have some idea of what kind of result to expect. Straight from the supplier, preamp tubes range from 60-100% output. This includes JJ Electronic, Electro-Harmonix, Shuguang, or other manufacturers. Companies such as Ruby, Mesa-Boogie, or Groove Tubes only test, sort, and re-label the tubes.

The Amplitrex AT1000 tube tester (one of the tube testers we use) gives a percentage for the emission rate and mutual trans-conductance (g_m) of the tube rather than saying “Good” or “Bad” like some of the older testers.

What Should I Use?

There are an infinite number of factors (power supply capacitors, transformers, pick attack) that contribute to the sound of an amplifier and changing the preamp tubes is one of them. Here are a couple reasons to change things up:

Take for instance, a guitarist that plugs straight into an amp and one who uses effects pedals – each would benefit from different types of 12AX7 preamp tubes. The player with the pedals usually needs a more even frequency response to achieve better clarity, while the other typically needs the amp to add more character and body to the sound.
Another choice of tubes may depend on the tone and feel you are looking to achieve. Swapping the first preamp tube is an easy way to change the tone and feel of an amp. Try a lower/mid gain tube such as a Chinese one by Shuguang or a higher gain tube from Russian by Electro-Harmonix to determine what fits your style. You may notice more midrange from a higher gain tube. Now, depending on your guitar pickups, the higher gain tube may help them out or create more mud in the mid-range. Or if you only play two or three notes at a time, this may be perfect.
Another important preamp tube is the phase inverter. This takes the guitar signal from the preamp and outputs it into the power tubes (while inverting a copy of it for class A/B). An evenly matched dual-triode (12AX7 tubes have two triode sections or two signal amplifiers) will sound better than one that is uneven and weak on one side. This is one of the reasons why we test our preamp tubes first.

Look into the effects of plate resistance (r_p) of various preamp tubes on frequency for more information. A lower gain preamp tube like the 12AU7 in the phase inverter will help lower the power of an amplifier and even out the frequency response for use with pedals.

Does it matter?

We don’t recommend getting too picky… after all, who will hear the difference besides you. However, educating yourself a little if you plan to use tube amplifiers will definitely pay off. There is a lot of info online, however, keep in mind not all tube amplifier theory applies to guitar amplifiers.

Here is a great tube comparison chart on TubesandMore.com.