The Tube Gain Stage

Let’s start with a tube gain stage. There are two triodes, or you can say gain stages, in each preamp tube like a 12ax7, 12at7, 12au7 or a 5751. Pins 1 and 6 are the plates, pins 2 and 7 are the grids and pins 3 and 8 are the cathodes.

tube gain stageThe Plate

The plate is usually connected to a plate resistor, which is usually connected to the B+ or power supply voltage. Typical plate resistor values are 100K, 150K, and 220K. Larger values equal more gain.

The Grid

The grid is where the signal enters the tube.

The Cathode

The cathode is usually connected to a cathode resistor which usually goes to ground. The cathode resistor, along with the plate resistor, controls the gain of the tube stage. The cathode resistor will be used more often than the plate resistor to dial in the amount of gain/distortion you want. Typical values are anywhere from 820 ohms to 10k ohms. Smaller values = more gain.

The Cathode Bypass Capacitor

The cathode bypass capacitor is connected in parallel with the cathode resistor. A typical value is a 25uf capacitor. This is a good starting point.  Raise the value and you get more bass frequencies and lower the value and get less bass frequencies.
The cathode resistor and plate resistor control the biasing of the triode. In other words they set the gain, signal amplitude and distortion/clipping.

Sometimes you will see a capacitor in parallel with the plate resistor, much like the cathode resistor bypass cap. It is usually a small value (i.e. .001uF silver mica cap) and it rolls off high frequencies in that gain stage. Sometimes you will see a high frequency roll off cap going from the plate pin to the cathode pin on the tube socket, 350pf to 500pf in value, which also rolls off high frequencies.

preamp tube gain stageThe Coupling Capacitor

The coupling capacitor goes in between gain stages.  The coupling capacitor can add or remove bass frequencies. Higher values equal more bass and lower values equal less bass.  It also blocks DC current from entering the next gain stage.  This helps to prevent throwing off the bias point of the next gain stage.

The Grid Resistor, also called the Grid Stopper Resistor

The grid resistor can also control gain between stages and also interacts with the tube to roll off highs. Values can be from 1.5K to 100K. Larger values roll off more highs and reduce gain between stages. The grid stopper resistor works best when mounted directly, or as close as possible, to the grid pin.  In almost all tube amps you will find a 68K resistor on the very first grid pin.  This resistor can help block radio frequency interference.

The Grid Leak Resistor

The grid leak resistor typically sets the impedance of the stage and biasing. It is interesting because it, and the previous stage's plate resistor, form a voltage divider on the signal. What this means to you is that the grid leak resistor can be used to control the level into the stage. Low grid leak values will attenuate the signal into the tube stage. If you look at different tube amp schematics, you can see where they control the level into the stage by using different values for the grid leak resistor. There is a maximum value that you need to adhere to. Check the datasheet for the tube you are using to see the typical value of the grid leak resistor.

Summary:

The plate and cathode resistors go hand-in-hand. If you change the cathode resistor, the plate resistor should ideally be changed in order to keep the preamp tube bias near the center of the plate voltage swing. Preamp tubes need to be biased, just as power amp tubes do, and there are ranges that are more optimum than others.

The cathode resistor, plate resistor and grid resistor, determine the biasing of the tube stage. The cathode bypass cap controls the amount of bass frequencies. 25uF are commonly used in Fender amps, .68uF are used in Marshall amps. A capacitor can be placed in parallel with the plate resistor to roll off highs and you can see this in bass amps sometimes. The plate receives the voltage from the power supply through a plate resistor, the grid receives the AC signal as input, and the cathode is grounded through a cathode resistor.

Once you have your amp built, here are some basic tips for fine tuning the sound. These are very simplistic mods (modifications) you can do to your amp.  Let your ears be your guide.

The term "bias" refers to a negative voltage that is applied to a power tube's control grid. This sets the amount of idle current the tube draws. It is important to bias a tube to stay within its rated dissipation for good tone and a reasonable service life.