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!