DISCLAIMER : VACUUM TUBES HANDLE VERY HIGH VOLTAGE (>300VDC) SO BE CAREFUL.
I've been in this forum for quite sometime now and considering that the amount of non automotive stuff is almost as high as the automotive stuff, I thought I would add something here. First up, if you are not a big fan of electronics, don't fret. I've tried to keep my narration as much non technical as possible.
Introduction:
I am from the solid state school of electronics. I'm familiar with terms like IC's, BJT (Bi Junction Transistor) and diodes. The vacuum tube era, typically between years 1900 - 1970, belonged to my grandfather. The invention of the BJT which technically started the digital era was in 1947 and as usual we were behind the technology curve. The first introduction to solid state electronics in many Indian households was in the form of "transistor" radios (60s and 70s). It was specifically called "Transistor" radios or simply "Transistor" purely to distinguish it from those old cumbersome wooden boxes called "Radios" which used vacuum tubes. This was followed by the B/W era of the television, which contrary to popular opinion, was in fact powered by solid state electronics. By the 90s, vacuum tubes were already history. So Kids these days have practically no idea of a vacuum tube or so I thought.
What is a Vacuum Tube?
A Vacuum Tube or simple a Valve (Called VT For short) is the precursor to the present day Transistor (the BJT). It was invented somewhere in the beginning of 1900's when the first computers took shape. Though invented for calculations, it soon morphed into two main roles, Amplification and radio. A VT is glass tube with structured metallic sheets, filaments and few terminals which are used for control. It is also called as Thermionic tube since heat was the main source of its operation.
I'm not going into the technical details but here's a good video on how a tube works.
Why Vacuum Tube?
Connoisseurs choice. Period. Audiophiles swear that the sound produced by the Vacuum tubes are much truer to the original sound. Well, I'm not going to judge. The choice is up to you. The novelty factor is however very hard to miss. Tube amps are like vintage cars, few love them, few just don't care. A glowing tube which produces music in your drawing room is definitely going to grab those eye balls. My reason for getting into this whole "Tube Amplification" business was due entirely to my son who for various unknown and adolescent concerns, got into it. Peer pressure not being one of them.
Types of Vacuum Tubes
Well, lets see.
1.
Diode - Only two terminals (Anode / Cathode ) used primarily to Bias.
2.
Triode - Three terminals ( Anode/ Cathode/ Grid) used as an amplifier. It gets scarier from here.
3.
Tetrode - Four terminals ( Anode/ Cathode / Grid / Screen). Wait its not over.
4.
Pentode - Five terminals ( Anode / Cathode / Grid / Screen / Suppressor ).
Now you know why I prefer solid states over VT.
Terminologies associated with a Vacuum Tube (A little theory is not going to kill you)
1.
Anode : The positive end of the VT. This is where the peak voltage is applied. Its also called Plate sometimes.
2.
Cathode : The negative end of the VT. This is where the reference (Negative) voltage is applied. This is usually tied to the ground. All voltage measurements are wrt the cathode which is the source of electrons.
3.
Filament : The element which produces the electrons in a cathode. The filament is wound around the cathode and heats it up to produce the electrons. European tubes use 6.3VAC as filament voltage while American ones use 12VAC. Talk of protectionism in pre war years.
5.
Grid : Control part of the tube. The signal is applied here. With respect to the cathode, grid should be negative for proper Bias.
6.
Bias : This is tricky. In order to get the VT into running mode, certain voltages needs to be applied to it. You can think of this as some kind of preparation of the VT. When properly biased, the VT is in conduction mode and any signal applied to the grid is amplified.
What is an amplifier? (You seem to be OK after the last section so a little more theory will not kill you)
Almost everyone here is familiar how an amplifier looks and what it does, I mean musically. Only now we will see the technical aspects of it. For easy understanding, we will use this block diagram.
Theoretically, an amplifier (Vacuum tube or Transistor) will take a signal given to it and amplify the amplitude of the signal while maintaining its signal integrity. If the input signal is a sine wave then output will also be a sine wave of SAME FREQUENCY except with A PEAK VOLTAGE OF ITS COLLECTOR OR ANODE. Energy conservation holds in amplifiers too. You just can't expect a signal with a peak of 1.2V to be amplified to a 120V signal by just connecting it to a Vacuum tube. You need to provide it with that much power EXTERNALLY. All the amplifier does is change the amplitude of the incoming signal. Further, unlike what is shown in the block diagram, the output is 180 deg out of phase with the input signal.
If you see the above graph, the blue one is the input signal while the red one is the amplified form. Notice two things,
1. The blue and red signals are identical except for the voltages.
2. They both have the same frequency.
This is in essence the function of an amplifier.
Types of Amplifiers
There are various classifications of amplifiers.
Class A : Only one amplifier (VT or BJT) does the job. Simple but not effective especially when amplifying both positive and negative parts of the signal but a good starting point.
Push Pull : Consists of two amplifiers ( VT or BJT) one each for positive and negative parts. Very effective.
There are other configurations like class AB, C , D which are high power amps. For simplicity sake, we are going to build only a Class A VT Amplifier.
Audio Amp, Guitar Amp, What's the difference? (Tata and Benz what's the difference?)
Theoretically not much. Only Guitar amps need more "rock" distortion and the circuits and tubes are also different. They use devices like the "Wah Wah" pedal to achieve that. Ask that punk guitarist more since I'm not one of them.