July 2013

Vacuum Tubes Part 1
Article By Grey Rollins

  Passive components are all well and fine, but for audio purposes we need something that will amplify a signal. All sensors, whether they read digital bits or analog recordings, are deficient in both voltage and current when it comes to driving speakers. High fidelity reproduction of music is impossible without amplifying devices. For that matter, electric guitars, keyboards, and microphones also require amplification; so much of current popular music is dependent on amplification, as well.

There are scads of devices that can amplify a signal. A full breakdown of the possibilities begins to resemble a fairly dense tree, with branches going this way and that. For most consumer applications, this decision is simple: solid-state. Only. However, music reproduction (and production — just try to tell owners of tubed Ampegs, Marshalls, Fenders, and Mesa Boogies that you're going to take away their amps...you'll have a pitched battle on your hands) is one of the few holdouts where vacuum tubes are not only desired, but demanded by some.

Tubes are also a good place to start because they all work according to the same principle (as opposed to the operational differences between FETs and bipolars, for instance) and that principle is simple and intuitive. There's only one thing you need to know to understand tubes and that's static electricity. No quantum mechanics required. If you've ever combed your hair on a dry winter morning and had your hair rise to meet the comb after the first stroke or two, you're already there. Opposite charges attract. Like charges repel. That's it.

So... Let Us Build A Vacuum Tube
Unfortunately, this will be a figurative tube. While you can build your own passive components using common materials, tubes require a hard vacuum to work properly, and that's not something found in your average kitchen or even in most workshops. Then there are the glassblowing techniques and metallurgical wizardry. There are details you never dreamed of worrying about, like the fact that the coefficient of expansion of the metal in the pins needs to be matched to that of the glass envelope so as to prevent air leaks. It's fascinating stuff, but not within the realm of possibility for us to do for ourselves.

The fundamental source of electrons in a tube is a hot wire filament. When you heat metal to red heat in a vacuum, electrons begin to drift away from the surface of the metal, creating a cloud of negative charge. This leaves the metal with a net positive charge and as you might expect, the electrons will eventually find their way back to the metal, attracted by the positive charge. Equilibrium is reached in a short period of time and the whole thing would remain a laboratory curiosity if not for the fact that you can put those electrons to work. (In fact, Thomas Edison noticed the phenomenon during his attempts to produce a more durable light bulb, but did nothing with it. However, he did get his name attached to it — the Edison Effect — for being the discoverer.) The trick is to induce the electrons to leave the vicinity of the heat source. For that we introduce the plate — or more formally, the anode — which has a positive charge. The electrons succumb to the siren call of the positive charge and rush headlong to their destiny. This tube cannot yet amplify, but it can do useful work. It's a diode. The "di-" part of the name is Greek and tells you that this tube has two operational parts; two electrodes. A diode can rectify alternating current because electrons can travel from the heater to the plate, but not from the plate to the heater. This allows you to create a DC power supply or to begin unraveling an AM radio transmission.

To make a practical amplifier we need to add something called a grid, positioned between the cathode and the plate. While we're at it, we'll improve the heater by wrapping it in a metal sheath called a cathode, which allows us to separate the heater and cathode functions; it will have its own pin. This makes the tube much more versatile, because the heater can now be run from either AC or DC and the signal circuit can be isolated from the heater supply.

The grid has a pin, too, and at this point we can start talking about actual amplification. The grid is the control element. A signal at the grid allows you to control a much larger flow of electrons by acting as a gate keeper. We now have three operational elements in the tube as far as the signal is concerned: the cathode, the grid, and the plate (or anode, if you like). The filament no longer counts, since it has been separated from the signal path. The tube is now a triode, from the Greek prefix tri-, meaning three.

Incidentally, one of the common questions posed by those who are new to tubes is, "Why can't I see the filament glow?" The answer is that in some designs of tube, the cathode covers the filament to such an extent that no light escapes. Other designs leave the ends of the filament exposed, allowing a warm glow to fill the envelope. Don't worry, either way the tube will work just fine.

The cathode works in conjunction with the filament (also known as the heater) to create a pool of available electrons hovering in space. The plate, which has a strong positive charge, attracts the electrons before they can fall back to the heater. The grid controls the flow of electrons by introducing a variable negative static charge that repels unneeded electrons.

The question arises: If there's a grid in between the cathode and the plate, how do the electrons make it through?

The cathode and plate are sheets of metal, but the grid is a spiral of fine, stiff wire, precision wound around thicker, more substantial wires that stand off to the sides, out of the way. The thin grid wires are suspended directly in the path of the electrons, but cover so little area that they are close to invisible from any given electron's point of view. Now, yes, electrons do sometimes hit the wires and, yes, they feed through into the grid circuit. However, in normal operation, it's a comparatively rare thing as a long as the grid is not driven positive by a particularly strong signal. As you might expect, if the grid goes positive, opposites attract, and the electron flow is diverted from the plate to the grid. This is called grid current and is considered a bad thing because under extreme circumstances it can degrade the tube. There are rare circumstances in which you might want to amplify a really weak signal, such as the output from a moving coil cartridge, where you might consider biasing a tube such that the grid might go ever so slightly positive, but it's usually considered a problem, either from the design standpoint or perhaps a careless user who put too strong a signal into the circuit.

The term bias means the average, steady state voltage applied to the grid, assumed to be negative to some greater or lesser degree. This, in turn, determines the average current of electrons that flow from the cathode to the plate, known as the idle current. Note that there is a problem in nomenclature here. People who are used to solid state devices — particularly bipolar transistors — think in terms of bias current, not voltage. They're not wrong. It's just a different way of looking at things. For the moment, just be aware that it's not unusual to talk to a tube person and have them say that they've biased the grid at, say, 1V (you can safely assume that they mean negative 1V, not positive), whereas a solid state person might look at the same circuit and observe that the tube is biased at 3mA, by which they mean that the idle current at the plate is 3mA. Different strokes, as the expression goes, but it does lead to disjointed conversations from time to time.

Tubes used to have metal shells, but those are rarely seen these days. All the tubes commonly used for audio purposes have glass envelopes, although some of the larger tubes have metal or plastic bases. Tubes like the venerable 12AX7 (seen here) and the 6DJ8 are dual triodes, meaning that there are literally two independent triodes in the same envelope. They share the same heater, which accounts for two of the pins (three, in the case of the 12AX7 — whose heater is center tapped and can be run at either 6.3V or 12.6V), then there are three pins for each triode. It's not unusual for tubes to have pins that do not connect to anything. Don't fret about it. If the standardized package design calls for 9 pins and you use six for two triodes and two for the heater, then the remaining pin may be used for a center tap for the heater (12AX7), or a shield between the two triodes (6DJ8), or attached to nothing at all.

Something that has caused a great deal of misunderstanding over the years is the silvery-gray reflective spot inside the tube. That's called the getter. It's a combination of chemically reactive metals that act to remove atoms or molecules of gas that may be inside the tube. During manufacture, the tube is pumped as nearly empty as possible, but even that isn't good enough. It's baked at high temperatures to drive off gas molecules that are hiding in the metal and glass, then sealed. At that point a small dish or ring inside the tube is heated, which causes the metals it holds to vaporize. They then deposit on the inside of the glass as the little mirror that you can see from the outside. Being metals, they are electrically conductive, so you don't want them inside the operational parts of the tube. They'll be at the top, sides, or bottom of the envelope.

After deposition, the getter chemically binds stray gas molecules left over from the manufacturing process that would otherwise interfere with the operation of the tube. It then quietly scavenges any gas molecules that manage to sneak in around the pins or that outgas from the internal portions of the tube. If you see a getter that's going white or reddish-brown around the edges, that tube has a gas leak and should be replaced. The color of the getter, whether shiny silver or nearly black, isn't so important. That depends on the precise combination of metals and the method of deposition employed when the tube was made. Likewise, the color the getter turns once it reacts with gas will depend on the composition of the getter and what gas/sees it is reacting with. The important thing to note is whether the getter is changing color around the borders. Rainbow hues at the edge follow pretty much the same physics as that of iridescent soap bubbles; the thickness of the getter is literally down to single digits of molecules, creating diffraction effects.

The metal portions of the tube are mounted in mica spacers that mechanically fix the active positions within the glass envelope. Mica is inexpensive, easily worked, resists heat, and makes a very good insulator. It's the same stuff you find in mica caps, which have a good reputation in audio circles, so you need not worry about the sonic effects of the spacers. What is worth keeping in mind is that tubes, being based on the careful relative positioning of the cathode, grid, and plate, can be microphonic, that is, some tubes respond to mechanical vibration. They can pick up sound. In fact, power tubes sometimes arc destructively if hit — lightning in a bottle, if you will. That's pretty much the end of that tube's useful life, although the partially melted innards make good conversation pieces for your next audiophile get-together.

I remember sitting with a tech from Conrad Johnson one afternoon as he sorted through a case of 6550s to create a "super set" of tubes. One of the tests he employed was to rap the side of the tube sharply with the handle of a screwdriver. Any tubes that arced were discarded. It was brutal, but effective. By the time he was done, he had a virtually bulletproof set to install in a Premier One power amp at one of their larger dealers. Not a process that us mere mortals can afford, given the price of tubes, but fascinating to watch.

Next time out, we'll take a look at tube selection and begin a tube circuit.

Click here for part 2 of vacuum tubes by Grey Rollins.