tube preamp

When the electric signal of the guitar reaches the tube preamp, the  tubes will amplify this signal into a larger electric signal and affect your tone.

The preamp is needed because the signal of the guitar is so weak that it can not be amplified in the power amp directly. 

An amp contains one or more preamp tubes. The most common tube used in the preamp is 12AX7, but there are other types also (I will discuss that later).

tube preamp and tone

Plexi 50 with effectsloop and master volume


In the plexi 50 on the picture V1, V2, V3 and V4 are the preamp tubes. V5 and V6 are the power amp tubes. V1 and V4  are most important for your tone.

  • V1 is the first tube that amplifies your guitar signal, that is why it is important. The characteristics of the V1 tube will affect the tone the most.
  • V2 powers the signal to the tone knobs
  • V3 is the tube that drives the effectsloop
  • V4 is the phase inverter tube. It is the tube that splits the signal and send it to the power tubes

the phase inverter

As mentioned above, the phase inverter can be an important tube for your tone. There is a lot of discussion about that fact. Some say it does, some say that it affects the tone a little and others say that it does not affect the tone at all.

But first let's see what a phase inverter (PI) does:

The PI splits the guitar signal in two signals. One in-phase and one out-of-phase.

signal 1 in-phase
signal 2 out-of-phase (exact mirror of signal 1)

Signal one is send to one power tube, signal 2 is send to the other power tube (if there is one pair of power tubes). They amplify the guitar signal into a strong signal that can be send to the speakers.

In some amps 12AX7 tubes are used as PI tube. The 12AX7 is a tube with a  gain factor 100. 

There are also amps (like some Fenders) that use a 12AT7 as PI tube. The 12AT7 is a tube with gain factor 60

A tube with a higher gain factor wil send a stronger signal to the power tubes. They will distort more easy and give a higher output than a tube with a lower gain factor.

So changing 12AX7 PI tubes into a lower gain tube like the 12AT7 will certainly affects your tone and respons, but keep in mind that a PI circuit of an amp is based on a specific tube and will perform best with that kind of tube.

In a Marshall the PI is designed for 12AX7 tubes. In other amps they are designed for 12AT7 tubes. It won't harm your amp if you change them, so you can experiment with it yourself.

Last but not least, tubes from different branches can sound different to. Let your ears be the judge!


different types of tubes with gain factor

preamp tube

gain factor

12AX7

100

5751

70

12AT7

60

12AY7

45

12AV7

41

12AU7

19


This small movie explains the gain factor very well (thanks to the tubestore.com)



If you want more information about tubes, or perhaps you want to know wich tube fits your amp the best, you can check their website

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