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Vacuum Tube Theory

by Mike Fratus,

{This is a limited-content document that will be used in my upcoming Tube Amp Class and Book.}

Vacuum tubes must seem like magic to most people...

What is a tube?

A vacuum tube is an evacuated tube (all the air is sucked out) with two or more conductive elements in it. One terminal, called the Cathode, is connected to the battery's Negative terminal and provides the electrons which travel through the vacuum (like an astronaut in space) and land on the Anode, which is connected to the Positive terminal of the battery, and is more commonly called the Plate.

During their travel through the circuit, the electrons use the 1st mode of travel as discussed above, until they reach the tube. Then they travel by way of the 2nd mode, through a vacuum. Then they travel again in mode 1 until they reach the battery's other terminal.

The above described tube would be a "cold cathode" tube. You probably have one like that as your laptop backlight, a tiny fluourescent tube.

The kind we usually use in amplifiers has a heated cathode. In it, the cathode is a metal tube with a heated wire inside of it, which makes the electrons more mobile or "anxious to get somewhere." A heated-cathode tube can operate at lower voltages and carry more current than a cold-cathode tube of the same size.

What will tubes do?

Depending on the construction of the tube, they can:

1. Act as a one-way valve by only allowing electrons to travel in one direction.

2. Act as an amplifier of very small signals (voltages) and small currents.

3. Act as an amplifier of large signals and heavy currents.

How do tubes work?

1. The Heater warms up the Cathode, and electrons form a cloud around it, ready to go somewhere. (They are too energetic to settle back down.)

2. The cathode is connected to the negative supply voltage, which provides it with a ready source of electrons.

3. There is a Plate, or Anode, that is connected to a positive voltage, and the electrons REALLY want to get there.

4. In between the cathode and plate can be nothing or several gateways called Grids.

A. If there is no grid, then the tube is a Rectifier or Diode ("Two-electrode").

B. If there is one grid, it is a Triode ("Three-electrode").

C. Then from there on it goes Tetrodes, Pentodes, and so on.

Now, here is the important part that explains how tubes work.

5. The grids control the flow of electrons through the tube. The reason this is true has to do with electric fields and potentials and stuff like that. Some grids act like a throttle, some act like fences, some act like mirrors, depending on how they are charged, which is called Bias.

What tubes are the ones to learn about?

The Triode are the PREAMP tubes in your amp. The Tetrodes and Pentodes are the OUTPUT tubes in your amp. If you only learn those, you can understand your amp.

Normal operation of tubes

(Tube amps designers use the term "Stage" for the building blocks that make up an amplifier.)

There are preamp stages, tone control stages, and output stages.

The way the stages are designed and the order that they occur has a major influence on the way an amp sounds. That is why you can't just put in different tubes and make a Fender sound like a Marshall.

You would have to change the Tone Stack, the output tubes, and the output transformer. Well, that would be a start. The more you change of the design, the further you get away from Fender design.

The way each stage is designed is why you can't just put in a different tube and make one model sound like another, or why a crappy amplifier model is only going to sound like a slightly better crappy amplifier after you put a $150 NOS tube in it. It really isn't lack of appreciation after all.

The most common preamp stages are a design called self-biased/grounded-grid. The grid voltage reads zero volts with no signal, and the cathode voltage is about 1.5Volts with a 12AX7 preamp tube. The plate voltage ranges from 80 Volts or so, up to 280 Volts or more.

Now, in that amp design, the tube is regulating the current flowing through it in a delicate balancing act. When a signal voltage is impressed upon the grid of the preamp tube, for example your guitar signal, it disrupts that balance and this disruption is amplified by the tube.

[The reason is, the grid is close to the cathode and surrounds it, so any small change in the grid voltage results in a large change in the current flow. This appears as a varying plate voltage that is the amplified signal.]

Normal output stages can be self-biased/grounded grid designs, or fixed bias designs. A few amps have both types, or can switch from one to another.

The output tubes act in a similar way, except they are larger and the signal sent to the grid is already quite large. The same thing applies, though. A small variation of voltage on the grid equals a large variation of voltage (and current) at the plate. But in this case we want lots of electrons to flow - lots of current. Then the Output transformer converts this large-voltage/large-current variation to a small-voltage, REALLY large-current signal to drive the speaker.

So, that is how tubes, preamps, output stages and tube amplifiers in general work.