"Feel the Gain" Part 2: 12ax7 gain stage

Are you strapped in? Let’s look at a common 12ax7 tube gain stage. This is found in many guitar amp circuits. Let’s continue where we left off, as the signal path from the previous section enters

pin 2 (the control grid) of the V1A 12ax7 tube.

Looking at the image to the left, the role of the 12ax7 type vacuum tube in this circuit is to take your tiny signal going into pin 2 and have it come out of pin 1 much much much bigger. Much bigger. Bigger than you think. 100 times bigger. We call that gain!

Pin 1 is the plate. Pin 3 is the cathode. Pins 4, 5, and 9 are the filaments and filament tap. More on those much later. The letter X on the top of this example is a connection point to the power supply. More on that in a later post.

What happens here is that high voltage is applied to pin 1, the plate. We want the plate to pull electrons from ground (those triangles at the bottom) through pin 3, the cathode, by way of pin 2, the control grid. Go ahead and read that one more time and come back. Go ahead, read it a third time.

R5 1.5K sets the amount of electrons that can be pulled through. C1 22μF acts like a little reservoir to allow even MORE electrons to flow through. Like putting creatine in your protein shake. These values are pretty typical for guitar frequencies and clean gain.

R4 100k provides high voltage to the plate. Don’t worry too much about this but it also serves to vary the voltage as a signal is applied and converts AC guitar signal current to AC voltage to push forward to the next stage.

Until Next time...