Tube Amplifier Basics For Ultra-Dummies
I have been thinking about this quite a bit, and I feel I am ready to expose my ignorance and make a fool of myself. But while I am at it, I was thinking that perhaps smarter people than myself could chime in and nudge my incorrect thinking back into order again.
So...
First, you got your amplifier. It has two types of inputs and one type of output. The inputs are AC power from the wall and an audio signal from another device. If the amplifier does not contain a 'pre-amplifier' (which generally contains some form of input-switching and various controls like volume, tone, and balance in the case of stereo), then it gets its inputs from an external pre-amp of some sort. Otherwise, it might get its audio input from various devices like a turntable, CD player, tuner, etc.
Now, the audio signal in question is too low to be heard directly over speakers, so we need to 'amplify' it (hence, the name 'amplifier'). And certain types of tubes are very good at making a small signal into a much larger signal. The goal, of course, is to amplify that signal without excessive noise, coloration, or other forms of unhappy degradation (some types are less objectionable to the human ear than others, as it turns out).
But tubes by themselves cannot amplify the signal. Like a car, they need an engine to drive them. Tubes require two types of power, plate and filament. These are specified by voltage and current.
That brings us to the section of the amplifier that works with the AC power from the wall. The story begins with a power transformer (PT) (leaving out finer details like fuses and switches). The transformer is an electromagnetic device that uses the principles of 'inductive coupling' to cause electricity to flow on one side of the transformer (the primary) to the other side (the secondary). The ratio of windings 1:1, or 1:2, etc, and the number of secondary windings, allows a variety of different voltages to be taken from the power transformer's secondary windings. This is good, because as mentioned, the tubes generally require different voltages for the plate and the filament.
Now, once we have taken our power from the power transformer secondary windings, we need to turn it from AC (alternating current) to DC (direct current) power. We do this with a rectifier. Rectifiers can be either half-wave or full-wave, meaning that they 'smooth out' half of the AC sine wave (the 'alternating' part of AC power looks like a sine wave) or they smooth out the entire wave. Either way, you end up with DC power, which is what you want for the plate and filaments of the tubes.
You can use a tube or tubes to do the rectification. This tube or tubes would not be part of the amplifier circuit per se, but are rather part of the power circuit, since they do not have the audio portion of the incoming signal passing through them. However, they can impart coloration on the resulting music you hear, so some people design this portion of the amplifier quite carefully, and when you hear people talking about 'tube rolling', they are often talking about experimenting with different types, brands or even instances of various tubes to perform rectification.
You can also use a solid-state rectifier, which means that instead of a vacuum tube, you use either a diode (half-wave) or a set of four diodes (full wave). If you use a full-wave diode rectifier, that is commonly referred to as a 'bridge' rectifier.
The rectifier is not simply made up of tubes or diodes, however. Typically, adjustments are required to drop or raise voltage using capacitors and/or resisters, and in the case of tube rectifiers (and sometimes for solid-state rectifiers) a type of transformer known as a 'choke' is used to further smooth out the DC voltage and make it all flat-line and buttery smooth.
Now that we have our voltages and current all sorted out in the power section, we can apply that power to the pre-amp and amplifier tubes in the audio section. Once those tubes are powered up, they're ready to do what they were intended to do - amplify the audio signal. The tubes in question have a number of pins, which connect to plate, filament, and inputs and outputs for the audio signal, so all the action is taking place inside the tubes.
The incoming audio signal first goes to a tube or tubes that are designed to pop up the signal a little bit, with very high precision. The output of those tubes are then routed to the input of the 'output' tubes, which is where the major amplification takes place.
You need the pre-amp tubes because the output tubes are designed to amplify the signal that comes in by a certain number of times. So if a very small signal comes in an gets multiplied by say 100 times, then the output is much lower than if a larger signal comes in and gets multiplied by 100 times. But at the same time, the output tube is designed to work best amplifying a signal that comes within a certain set of parameters. So pre-amp tubes and amplifier tubes are designed to work together in that way. You need them both.
Once the audio signal is routed into the power (output) tubes, it gets amplified and sent out on a different set of pins on the tubes. In a 'single-ended' design, that's it for the amplification. This may result in not much output power, but it may be a more accurate or nicer-sounding amplification (to some, tastes vary widely and there is no one 'right' way). If more power is desired, the output of one set of power tubes can be sent into the input of another pair of input tubes (as long as it is within the design specs of the tubes) and it will be amplified yet again. Thus, a 2 watt output signal might become a 20 watt output signal, which basically means louder volume at the speakers. This is called a 'Push-Pull' amplifier (PP) instead of single-ended (SE). Typically, the tubes used in PP amplifiers are all the same tube number, but I guess it doesn't have to be that way. And yes, you could have a PPP or a PPPP amplifier for even more power, and there are such amplifiers, but the tube types that can handle the larger inputs can be expensive and rare, and distortion increases with each successive amplification.
Just as with the power section, the audio section is not only made up of tubes. There are also capacitors and/or resisters used through the audio signal. I'm a little fuzzy on what they do or why, but they're in there.
Finally, after the last tube has had it's wicked way with the audio signal, it it taken from the output pins on the last set of power tubes and put into the primary side of another transformer or set of transformers (for stereo), which are known as the output transformers (OPT). The audio output signal, by the way, is AC, since it looks like a sine wave. These OPTs take the output signal and use induction again (remember the PT) to turn it into a different voltage and current, which is then sent to the speakers. Typically, an OPT might have several secondary windings, each representing the appropriate nominal resistance of the speakers being used (4 ohms, 8 ohms, or less commonly these days, 16 ohms).
OK, so the basics - the ULTRA basics of tube amplifiers are these...
1) Power Transformer (PT). Turns the AC power into voltages the tubes need.
2) Rectifier. Tube or solid-state. Turns the AC into DC.
3) Choke (optional). A transformer-looking thing that makes the power even more creamy and good.
4) Preamplifier tube(s). Turns the incoming audio signal into a slightly larger audio signal, but does so with good precision.
5) Output or Power tube(s). Turns the audio signal from the pre-amp tubes into a much larger signal, which it may feed directly into the output transformers, or may feed into another stage of amplification (SE or PP).
6) Output transformer(s) (OPT). Turns the output signal from the last stage of amplification into something speakers can use to make sound.
If the amplifier is an 'integrated' amplifier, it may also have switches for selecting among different input signals, and it may have various controls for things like volume, balance, and tone. Sometimes there are tubes used in those signals as well; commonly on older designs, a 'phase inverter' tube that flips the signal on one side of a stereo pair to the opposite polarity. Sometimes fancy things like reverb and such were also seen on older amps.
If the amplifier is a 'power amplifier', it may lack much of anything except a power switch, AC in, audio in, and speaker outputs.
How's that? Feel free to tear me up here.
. It hasn't left the bathroom since I bought it.
