How a Tube Amp Works
Part 1: How a triode gain stage works
Welcome to How a Tube Amp Works! This article is first in a series about vintage amplifiers and how they work. Join me next Friday for Part II: How a Power Amp Works!
Why learn about tube amps? Tube amps are important to understanding all vintage circuits, both tube and solid state. In the early years of solid state amplifiers—the 1960s, into the 1970s—solid state and tube circuits were manufactured side by side. The choice to manufacture a solid state amplifier when tubes were plentiful can tell you a lot about a vintage product, the consumers it was trying to attract, and what the manufacturer believed it should be used for. If you understand tubes, you’ll have a better appreciation of all these points.
In the first half of the article, I’ll define some terms. Then, I’ll explain how a triode gain stage works. Triodes include tubes such as 12AX7s and 12AU7s (both of which are actually two triodes in one glass envelope). A gain stage is a circuit that amplifies signal. The preamp portion of a tube amp usually has at least two triode gain stages.
Think of this article, not as a step-by-step explanation, but as a whole made up of parts. If a section isn’t clicking, feel free to skip it and circle back later. If you have questions, reach out. I’m happy to clarify any points.
VOLTAGE AND CURRENT
Electricity has two characteristics that we care a lot about in an amplifier: voltage and current. To oversimplify, think of a hose. Voltage is the water pressure, and current is the volume of water that is flowing. You can have a lot of water flowing at low pressure; a little bit of water flowing at high pressure; etc. If you change the size of the hose (or, in an electrical circuit, add a resistor), you change the flow and the pressure.
Electricity doesn’t literally “flow,” but that’s okay. Metaphors help us establish that electricity is there and it’s doing something, so we can move on and talk about how we can manipulate that something to our purposes. If we didn’t use metaphors, the discussion would be impenetrably abstract. Just remember that, whenever you see electricity described as “flowing” or “moving” through something, you shouldn’t take the verb too literally.
The relationship between voltage and current is expressed by Ohm’s Law, I = V/R, where I is current, V is voltage, and R is resistance. (Current is I because the first person to use the symbol was André-Marie Ampere, a French physicist. He used it as shorthand for intensité du courant, or current intensity.)