Paul McGowan: This is the first post in our series on Tubes vs. Transistors. I’ll do my best to keep this fun, informative, not uber technical and to the point so we can all have something to look forward to each day.
You may have heard the term “Audion” as there is a UK manufacturer using the name to build tube electronics as well a popular MAC MP3 player software leveraging the name as well. The true origins of the name refer to the first tube amplifier invented by Lee De Forest in 1907. Until the invention of the Audion there was no such thing as an electronic amplifier. De Forest’s invention was not an audio amplifier but rather a radio amplifier – but it started something big for audio.
In the early 1900′s radio, then known as “wireless”, was listened to on crystal radios – basically a piece of crystal with a tiny wire touching the crystal surface and both crystal and wire connected to a set of headphones. De Forest was looking for a way around using crystals – playing with heating a gas to mimic the action of the crystal – a well known but unexplained phenomena at the time. To take advantage of the heated gas’s tendency to conduct electricity, he captured some of the gas in a glass envelope called a tube. Around the glass tube he wrapped some wire and to that wire he attached a set of headphones – he heard radio.
In later experiments he discovered that if he placed a small piece of wire inside the glass tube, formed in a grid pattern, the radio reception was improved. He now had a wire at each end of the glass tube and one in the middle. De Forest had stumbled onto what is known as a Triode: meaning it has 3 parts.
While DeForest’s triode worked better than any crystal radio receiver of the time it was not a linear audio amplifier – it was a non-linear radio receiver. DeForest had incorrectly determined that the glass tube had to be filled with gas in order to work – which was the entire basis of his thought. In fact, so convinced was he of this fact the patent he took out specified the gas requirement and the name “Audion” came two words: Audio and Ion (as in ionized gas). Turns out he was dead wrong.
It wasn’t until 1912 that Irving Langmuir of General Electric figured out the Audion could become a linear audio amplifier by removing all the gas DeForest had so adamantly demanded be included. Once the gas and the air had been removed from the glass tube what was left, of course, was a whole lot of nothing called a vacuum and to this day, most of the world refers to this type of amplification device as a “Vacuum Tube”. Our UK friends refer to it as a valve, which I’ll explain next.
A triode vacuum tube is the most common of all tubes, even today. If you listen to tubes in your high-end system, chances are quite high you’re listening to a triode. Triode means three and the three elements in a triode are:
- The cathode
- The Anode
- The Grid
Picture these three elements as simply pieces of conductive metal with wires attached to them (the wires terminating at the tube’s socket where the pins are). The three elements are housed in a glass “bottle” and all the air has been removed from the bottle.
The cathode, also known as the heater, has power applied to it so it heats up. This causes a couple of things to happen: electrons start to boil off the metal in large numbers and it glows. This glow is what you see when the tube lights up and starts to work and it’s also responsible for much of the heat a tube produces. Think of this as the generator or transmitter.
The anode is the receiver of the electrons that the cathode is generating. If the designer places a positive voltage to this anode from a power supply all those streaming electrons boiling off the heater will be attracted to the higher voltage of the plate and we get a current flowing. In fact, a vacuum tube is always on producing maximum current all the time. The challenge is to turn off the tube – but I jump ahead. Hang in there.
If you want to make an audio amplifier you need a current flowing (which we now have) and then you want to selectively turn that current up and down (selectively off) in synch with a musical signal – so you get more current when the signal is louder and less current when the signal is quieter. To do this, you need some sort of valve. Notice how cleverly I inserted the “Valve” reference here. That’s my tip of the hat to the Brits.
The Grid is the valve. Think of the grid as a piece of metal that looks like a fly swatter. All those boiling electrons go through the holes in the grid. If you apply a voltage to the grid that is the opposite or lower than what you have on the plate, the electrons can’t pass through the grid to the plate anymore.
What’s cool about this setup is you only need a very small amount of voltage going in the opposite direction to stop the electrons from flowing: thus a little change in voltage on the grid results in a huge change in the plate. Voila – the very definition of an amplifier is a little voltage producing a very large voltage (or current).
So now imagine the output of a phono cartridge – it’s really tiny. Place that tiny voltage and put it on the grid of a vacuum tube. Now take a big power supply and place that on the plate of a vacuum tube. At the output of the tube, place a speaker or headphone between the plate and ground. When you play a record, the tiny voltage from the phono cartridge makes a big change in the current passing through the loudspeaker and sound comes out when the loudspeaker starts to move back and forth in perfect synch to what’s on the record grooves.
Houston, we have music.
Remember back to the beginning of this article where Lee DeForest was trying to invent something that did not use a crystal for a radio? Tomorrow we’ll find out how that left turn away from crystals set us back 50 years and how those crystals had to wait their turn to spark an entirely new type of device called a transistor.
Tomorrow From crystal radios to supercomputers.