How a Tube Amp Works, Part 2
Potentiometers, single-ended output circuits, & more!
Welcome to Part 2 of How a Tube Amp Works! In our last installment, we explained how a triode gain stage works (or, in other words, a typical 12AX7 preamp tube circuit). Today, we will explain how the rest of an amplifier’s signal path works.
In future newsletters, we will discuss specific circuits, how they work, and how you could mod them so that they work better. If this article seems a little abstract, please stick with us until then! The practical examples are more fun. Also, if you have any questions, feel free to reply to this email or get in touch here.
If you did not read Part 1 yet, you may want to look through it before reading Part 2. But, the same caveat applies: this is not a step-by-step guide, because the entire tube circuit works together as a whole. So, this guide doesn’t necessarily need to be read in order. If something isn’t clicking, feel free to skip it and come back later. It might make more sense with more context.
Before we begin, let’s start with some definitions that will help when discussing the rest of the circuit.
SERIES AND PARALLEL RESISTORS
Resistors are connected in series when they are connected one after the other, in a line. The same current flows through every resistor, because the current has no alternative paths to follow. However, the voltage dropped across each resistor could be different, depending on the value of the resistors in the chain. For this reason, resistors in series are voltage dividers.
To find the total resistance of resistors in series, add the resistance of each individual resistor together: R = R1 + R2 + R3 + …
When a resistor is connected to the same two spots as another resistor, they are said to be in parallel. In this case, current can travel down any resistor in a parallel configuration, so the amount of current across any particular resistor depends on the value of the resistor. However, each resistor in a parallel configuration drops the same amount of voltage. For this reason, resistors in parallel are current dividers.
To find the total resistance of resistors in parallel, add the reciprocal of each resistor, then take the reciprocal of the result. As in: 1/R = 1/R1 + 1/R2 + R/R3 + … The result is always lower than the lowest resistance in the parallel circuit.
SERIES RESISTORS AS VOLTAGE DIVIDERS
One extremely useful application for resistors in series is a voltage divider. By arranging resistors as a voltage divider, you can produce an output voltage that is a fraction of the input voltage. A classic two-resistor voltage divider is pictured in the diagram above.