Not all of us are scientists or engineers who can grasp the intricate designs of complex electrical devices. And to the uninitiated, electronic circuits can really look like a maze of difficult clues that one can spend the rest of the life trying to disfigure, and not necessarily with success. Vacuum tube amplifiers are also such devices that have a complex system circuit made up of many small components that can confuse even the best of us while trying to solve the mystery of how they’ve been put together in the first place.
Now you’re probably wondering why you need to know these things in the first place. Well, for a guitar lover, it’s important to understand the workings of the vacuum tube amplifier because that’s the device which gives your musical instrument the perfect synergy of volume and quality of sound. So if you find your guitar sound somewhat jarring to the ears, it’s probably because of the vacuum tube amplifier, which is calling for a replacement.
Let’s try to make things easier for you to understand by explaining the basics of vacuum tube amplifiers.
What is a vacuum tube?
Actually it’s nothing but a triode that works as a transconductor, in which the current is controlled by voltage. Now what happens in a vacuum bulb is that a cathode is heated, leading to a phenomenon called thermionic emission. This may happen due to direct or indirect heating of the cathode. So the cathode needs to be surrounded by a plate or a metal cylinder, which helps in the completion of the circuit.
Simply put, vacuum tubes are made of vacuum-sealed metal or glass container, which contains the plate (anode), the cathode and the heater. If there’s no cathode in the tube it gets headed directly, unlike the cathode tube, which is indirectly heated. The heater is operated by voltage, which also spikes the temperature to trigger the release of electrons from the cathode into the vacuum space where they end up moving to the plate in the form of current.
Basic operational principle
Let’s look at this theory in a little more detail. What really happens is that when you connect a DC power supply or a battery across the cathode and the metal plate in such a manner that the latter is more positive, the electrons around the cathode get pulled towards the plate and collide with it. Through the plate, they flow to the battery and from there to the cathode, leading to the current getting established.
To stop or reduce this current flow, you just need to place a metal mesh between the cathode and the plate and then connect it with a negative voltage across the cathode and the grid. Next, in series with the plate, place a resistor, which helps to vary the plate current through the applied grid-cathode voltage.
As a result of this, even minor voltage fluctuation in the cathode-grid can lead to significant voltage variation across the plate resistor. This is the amplifying effect of the triode.
It’s quite evident that the flow of current will be enabled only if the plate works as an anode, and is positive as against the cathode (positive and negative energy needs to attract for the flow of the current to be made possible). Hence, the plate has to be at a positive voltage to draw the electrons from the vacuum space, into which they’re forced to enter as a result of the heat energy.
How vacuum tube amplifiers differ from BJTs
The principle on which vacuum tube amplifiers work, that is causing the output to vary through voltage supply, is quite different from that of Bipolar Junction Transistors (BJTs) that control output by using a varying current through the base.
The benefit of using vacuum tube amplifiers is that they help create a major voltage change across the load with quite a small change in grid voltage. Since the vacuum tubes can be anything from two triode ones to five grid tubes, they can be easily adapted to a wide variety of audio frequencies, including radio frequencies. In the dual triode variety, there’s a single filament shared by both the triodes, but each half works as a separate entity within a compact device.
Popular vacuum tube amplifier
The most popular dual triodes, usually used in guitar amps, are the 12AX7, which boast of the largest production of amplifiers in the world. In fact, dual triodes were the key component of electronic computers at the time of their invention. However, the type of vacuum tube amplifier you use would depend on the application for which it is required to be effectuated. So you may need to use different varieties for radio signals, hearing aids, and so on.
What’s involved in the operation of a vacuum tube amplifier is a basic principle of Physics –positive and negative are attracted to each other, while two positives or two negatives repel. So as long as you have an anode (positive plate) and a cathode (negative), separated by a vacuum tube, your vacuum tube amp is going to work well, transforming low sounds (input) into high quality and volume sounds (output) by varying the voltage.
The operational principle is so simple, in fact, that you can actually make a good and effective vacuum tube amplifier by yourself, with a whole lot of instructional material available online. As mentioned, the type of vacuum tube amp you use would depend on the device for which you require it.