Part of a blog series Tube Amplifier Circuits Explained
You may have wondered, how does a vacuum tube actually work? What's inside that magic glass envelope that glows and fills my speakers with awesomeness?
This is part of a series of blog posts explaining how a tube amplifier works, using a single-ended circuit as the example. This is the same explanation used in the Analog Ethos AE1 Kit instructions. If you want to build the kit and follow along, check it out here!
Vacuum tubes used in amplifiers are also called “thermionic valves” referring to the way that temperature causes the release and flow of electrons. The basic type we will start with is a triode tube. Inside the glass enclosure of a triode tube are three main components: the cathode, the anode (also called the “plate”), and the control grid. There is also a filament to act as a heater.
The cathode is typically coated with a certain type of metal, and it is heated up to a high temperature. In some types of tubes, the cathode is directly heated by running current through it, but in most modern triodes the cathode is heated using a filament physically close to it, but not connected electrically. The filament is what you see glowing inside of the tube, sort of like a filament inside a light bulb.
As the cathode reaches a high temperature, it begins to emit electrons. They build up in a cloud around the filament and, without any other action, eventually there are so many that the space around the cathode reaches a point where no more will be emitted. Why not? Because electrons have a negative charge and they hate being close to other negative charges, and all those other electrons bumping elbows are making the place pretty dang negative. What do electrons love? Positive stuff. They are attracted like crazy to it. So what would happen if we introduce something positive into the mix here? Yeah, those guys would go for it.
The anode is referred to as a plate because it’s a metal plate surrounding the cathode, and we can put on the anode a juicy and delicious positive voltage with respect to the cathode. You will soon start to see why we need high voltage in a tube amplifier. If you just put a few volts on the anode, the electrons say, yeah man, cool, but I don’t even get out of bed for that kind of voltage. To be sufficiently attractive across the space between anode and cathode, it has to be high.
The glass tube enclosure is sealed and there’s a vacuum inside, remember? So those electrons are free to fly around without colliding into air molecules. So when there’s a nearby high voltage potential on the anode, they are attracted and fly to it at ridiculous velocity, like around a million meters per second or something. Wow, right?
Now let’s take a step back and think what’s happening here. We heat up the cathode and it emits bazillions of electrons that flow at a million meters per second to the positive potential of the anode, continuously. Sounds a bit like…current? Yes. When the cathode is heated, electrons and current flows. (Don’t get tangled up in directivity…the electrons go from cathode to anode, but we sometimes refer to current flow from positive to negative. It’s just the way we measure current as a rate of change of electrical charge.) You’ll note that it flows only one direction (a “diode” at this point). We heat the cathode and electrons can go to the anode. There is no way for electrons to go the other direction.
Alright, so an operating tube is allowing current flow (the “valve is open”). Now, there’s one more component to make this a triode: the control grid. This is a wire mesh in between the cathode and anode that is spaced wide enough to allow the electrons to pass through. But what might happen if you applied a negative charge to it? You son of a turtle, say those electrons! We are not going through that negative fence you set up, we don’t care what’s on the other side. So now we have a way to control those electrons and if we raise or lower the voltage on the grid we can influence how strongly the electrons are repelled or allowed to pass. A very negative charge? No electrons pass and no current is flowing—we refer to this as “cutoff” of the tube. A less negative charge? Electrons and current flows. Maximum flow is referred to as “saturation” reflecting the temperature and physical constraint where the anode is pulling in all of the electrons that the cathode can produce.
By altering the voltage of the grid, we can “open and close the valve,” allowing current to flow more or less. Awesome! This is why a tube amplifier is sometimes referred to as a “valve amplifier.” If we were to put an audio signal on that control grid, then the changing voltage over time of the audio signal will allow current to flow in alignment to the audio signal.
We are getting close! But how does this amplify the input signal? The answer is that the voltage change on the grid has a large influence over the current flow. How much current flow? To really understand, we need to get into a circuit a bit more and discuss load lines. This will be a separate blog post. You will love it. Hang with me.
First, we need one more quick addition to put the tube in context of a circuit. We already said that we would put a high voltage potential on the anode to attract the electrons and allow current to flow. We also will want a load on the anode, in the case of an input stage this could be a simple resistor, so that there is sufficient resistance in our circuit so we don’t have an unreasonable amount of current flowing and also so that we can make use of the voltage change across that resistor. Ohm’s law will tell us there is a known relationship between resistance, voltage, and current, right? And you love Ohm’s law, right? Me, too.
Here’s a simplified circuit using a tube symbol. We won’t worry about how we generated the B+ voltage or how we are setting the grid voltage, but assume the B+ is some high voltage potential, the cathode is at 0V, and the grid could be a few volts negative. We have enough for a working illustration of an operating circuit. (We usually do not show the heater filament in a tube schematic, so just know the heater is operating.)
For the sake of a quick calculation to get started, let’s pretend that we have a 30k Ohm load resistor and 400V supply, and we pick a grid voltage that is allowing some amount of current to flow. How much current is there if we measure the voltage on the anode and it is 250V? Well, 150V must have dropped across the load resistor to get from 400 to the 250, and using Ohm’s Law:
I = V/R
I = 150 / 30,000
I = 0.005 Amps or 5 mA
All we are doing here is using an easy technique to calculate current using a known resistance and measured voltages, using Ohm’s law. But I want you to have the load resistor and Ohm’s law in mind as we get a step further into the operation of the tube..
Up next, Load Lines and Operating Point!