How do tubes work? How is a preamp tube different from a power tube? And what components/elements of preamp/power tubes affect the tone of the amp? (example: what makes EL34’s sound different than 6L6’s, or makes 12ax7’s sound different than ef86’s)
In: Technology
A tube is a fairly simple device in principle. Imagine a lightbulb with two separate filaments. If you charge one positive (anode) and one negative (cathode), you might get some electrons to leave the negative terminal, fly across the vacuum, and land on the positive terminal. Current is flowing.
If you put a separate heater near the negative terminal this will work better, since it would be easier for electrons to leave the metal and fly across the vacuum that way. That’s why tubes glow. You’re usually seeing the heater.
Now imagine putting a little bit of a metal screen (like from a screen door) in between the two terminals. If you put a negative voltage on it, the electrons won’t want to leave the negative terminal as much. If you put a positive voltage on it, the electrons will be more likely to do so. Thus the grid voltage can control the cathode/anode current. You’ve made an amplifier! A small voltage change on the grid can control a large current change cathode to anode.
There are a lot of things you can do to change the characteristics of a single tube. Size/shape of the anode/cathode/grid, spacing, heater configuration, etc. You can even have multiple cathodes/anodes/grids. All of these things can change how the tube works over different voltage/current/temperature ranges.
Probably one of the biggest difference between tubes is linearity. An ideal amplifier has the same (change in current)/(change in grid voltage) at all points. But that’s not usually the case, and there can be all kinds of response curves. Similarly, you want the amplifier to work the same across all frequencies, but that’s usually not true either, and different tube configurations will give different results. Those two things will give different sounds to the amplifier circuits the tubes are used in.
In a vacuum tube there’s a beam of electrons going from one metal piece to another metal piece. That’s why it has to be a vacuum – otherwise the electrons would bump into the air.
There’s a metal mesh or grid in the middle of the beam. When the grid is electrically charged, it pushes electrons away and they don’t want to go near the grid (not even through the holes) which makes the beam weaker.
So you can use it to amplify a signal, e.g. put the beam in between the power supply and the speakers, and connect the grid to the guitar pickup, and now the small amount of electricity from the guitar pickup is controlling the large amount of electricity going to the speakers (it’s actually more complicated than that). So the small variation in electricity, that represents the sound, turns into a big variation in electricity. i.e. a small signal turns into a big signal.
Your amplifier has two stages – the preamp turns the tiny weak signal from the guitar pickup into a medium strength signal, and then the power amplifier turns the medium strength signal into a really strong signal for the speaker. A preamp stage is designed for accuracy because it deals with tiny weak signals; a power amplifier stage is designed for maximum power. If you have a bigger sound system with mixing consoles and stuff, the preamps go at the earliest possible points in the system (i.e. right after the guitars and microphones) and the power amplifiers go at the last possible point (i.e. right before the speakers). Everything in-between uses the medium strength signals.
I’m not an expert on sound, but different tubes may sound different because of different design – basically the same amount of grid charge repels the electron beam a different amount. Especially the “non-linearities” will change the tone of the sound. Adding a bit of charge when the beam is strong probably weakens it more than it does when the beam is really weak, but how much less? You can plot the beam strength vs grid charge on a graph, and different tubes will have different graphs – they’ll be about the same overall shape, but the details can change. Even different tubes of the same type can be different, but the difference is less than with different tube types.
What they do.
These vacuum tubes are signal amplifiers. Functionally, they are similar to transistors: a high voltage is applied across the tube with a sort of gate in the middle that restricts the current passing through the tube. A small change in the voltage at the gate has a proportional but much larger effect on the amount of current that the tube will let through.
How they work.
A simple tube model has 3 parts. The main power connects to the cathode (emits electrons) and the anode (collects electrons). For current to flow, the electrons have to jump from the cathode to the anode. Since they are well separated in space, this can’t happen without help. Help comes in the form of an electric field in the middle of the tube (the gate). The gate’s electric field sucks the electrons off the cathode where they shoot over to the anode. The higher the gate voltage, the stronger the field, the more electrons get sucked off, the more power allowed to pass through to the amp/speakers. Most tubes are a bit more complicated than that but that’s the gist of it.
How is a preamp and power tube different?
They are basically different sizes (powers). The preamp tubes take instrument (or whatever) signal level and amplifies it to line level. The power amp tubes take line level and amplifies it to speaker level. The pre-amp tubes may (i don’t know for sure) also be more complicated/flexible since you may want to condition the signal (tone control and so forth) while the power tubes just make the signal louder.
How do different tubes affect tone?
Every tube and transistor has a characteristic response curve. For example, so-called “switching” transistors have a very sharp response, going from fully off to fully on over a very small change in input voltage. Signal transistors (and the tubes above) are designed to stretch that range as much as possible to allow fine control of the output power. The difference in the curves is part of the reason that different tubes sound different for the same inputs.
>How do tubes work?
Vacuum tubes exploit a phenomenon called “thermionic emission.”
When certain materials are heated to red or yellow hot, they begin to emit electrons from their surface. The electrons are metaphorically “boiled off” from the heat. This is a complex process and it’s still not fully understood, but it’s related to the energy of collision between individual atoms in a material at high temperature.
This gives the hot surface an overall positive charge. In air and most gases, those lone electrons collide with atoms of the gas and become bound to them, creating negatively charged ions. The ions are then carried back to the positive surface.
However in a tube which has all the air removed these electrons are free to fly around and their travel isn’t inhibited by colliding with nearby gas atoms. Thus a heated surface begins to emit a “gas” of electrons.
In a vacuum tube, usually you have a thin coil of tungsten wire that is heated to orange yellow hot by running a current through it. This coil is the anode. If you place a thin metal strip near the heated coil, electrons will boil off the tungsten coil and collide with the comparatively cold plate. This plate is known as a cold cathode. This will result in a difference in voltage between the plate and the heated anode and a measurable current.
In fact you can use this effect to produce electricity without moving parts, provided you have an appropriate heat source to heat the cathode red hot.( In the late late 1960’s early 70’s the US and Russia created small nuclear reactors that produced electricity in space, for satellites, using this effect. Fission in plutonium was used to heat the anode.)
You can enhance the current flow significantly by adding a negative charge to the anode. That is, artificially driving the current.
This forms the basis of a device called a Vacuum diode or Thermionic Diode, also called a “Fleming Valve.” This was invented by John Ambrose Fleming.
Because the cathode is not heated, thermionic emission cannot occur there, so electrons cannot flow from the cathode to the anode, only from the anode to cathode. This means current can flow through a diode in only one direction. This is important if, for example, you wanted to convert AC current into DC. But there are many other applications of diodes.
In 1906 Lee DeForest discovered that you could add a grid made of a fine woven wire mesh between the anode and the cold cathode. A small negative charge on the grid would have quite a large effect on the flow of current to the cathode, effectively shielding it from the incoming electrons boiling off the hottest anode.
Thus a quite small electrical signal could control a much larger current. The resulting device is called a Triode because it has three electrodes. This acts as an amplifier. That is, taking a very small electrical signal and increasing it’s power dramatically. In modern vacuum tube the “Signal Gain” can be as much as a factor of 1,000 or more.
Amplifiers are crucial to basically all modern electronics.
The first Triode vacuum tubes were critical to transmitting and receiving of radio signals. Moreover a Triode could be used as a simple switch, turning a larger circuit on or off. This could be used to construct digital logic systems.
> How is a preamp tube different from a power tube?
You generally get better signal fidelity at modest levels of *gain*, with vacuum tubes, with regards to their maximum capability. Therefore two amplification steps were found to be desirable from a practical standpoint.
A preamplifier tube is designed to boost a signal from the pickups of potentially less than a milliwatt, up to a few hundred mW or a few watts. So these are low power tubes. This is a useful level where it’s above the level of background electrical noise that’s likely to be picked up by the power and from any number of sources.
The power amp takes a few mW signal and boosts it to tens or hundreds of watts. So these are high power tubes, and tended to be specifically developed for the purpose of driving audio speakers.
I’m assuming you’re familiar with how electric guitars work.
The magnetic pickups in a guitar generate a very small signal and it would be almost impossible to create any audible sound from directly. The signal simply wouldn’t have enough power even to be heard on earbuds.
The preamp tube boosts the signal to a level than can then be controlled in volume by a variable resistor, called a potentiometer. This is the main source of volume control. You could also feed this signal directly to a pair of headphones if you wish. Attempting to boost the signal directly from the pickups to the power amplifier requires a large level of *gain* of about 1000-10,000x. This would almost certainly result in serious distortion and degradation of the signal quality with only a single tube.
Early in the history of the electric guitar, cheap power amplifier tubes tended to have some, shall we say, less than desirable electrical characteristics. If the preamp level was turned all the way up the signal could saturate the power amplifier. That is, drive drive it near it’s maximum current output. This basically meant the power amp tube was undersized for the application.
This tended to “clip” or “truncate” the peaks of the sound signal. This was known as overdriving or distortion. It was common problem in cheap consumer guitar amps.
The power supply to the tubes also tended to be poorly filtered which could cause effects like resonance, reverberation, and feedback. This resulted in a metallic, fuzzy, muddy sound.
Instead of treating these like annoying nuisances, blues musicians in the early 1940’s as electric guitars and amps became cheap and popular, wrote songs where the muddy distorted low fidelity tone became a deliberate feature that enhanced the mood. It’s maybe a little more than accidental that these tended to be black musicians who could only afford the cheapest amplifiers and guitars.
>And what components/elements of preamp/power tubes affect the tone of the amp?
This is difficult to explain simply and depends on both the configuration of the whole amplifier circuit as well as the preamp tubes themselves. In general however the power amplifier is largely a slave to the audio and electrical characteristics of the preamp. The power amp will generally play almost as good of a sound as the preamp will send it.
The preamp circuit will tend to have certain frequency ranges where is has a dampening effect meaning certain components or features of the circuit or the tube will siphon off energy from the signal at that frequency before it can be amplified and fed to the speakers. Other parts of the circuit and the tube itself will resonate and causing reverberation or echoing at specific frequencies. This is similar to singing in the shower. No tube or amplifier is perfect or Linear. Rather tubes will have parasitic capacitance and inductance, as will any real electrical device. So they act as an LRC circuit or bandpass filter. They can also be prone to several other effects such a microphony that introduce unintended noise and signals into their output.
The reason why such effects were common in early amplifiers was that manufacturers lacked sophisticated tools and equipment to accurately test and analyze the output of tubes or amps, and to troubleshoot problems. It was largely a question of trial and error. Plug the prototype circuit in and see if it sounded OK. If so, just produce it and don’t ask too many time consuming questions if it performs poorly at certain settings. Then of course musicians started gaining an appreciation for the musical properties of poorly designed and tuned circuits and started demanding them.
(example: what makes EL34’s sound different than 6L6’s, or makes 12ax7’s sound different than ef86’s)
In vacuum tube there’s generally a compromise between different electrical characteristics such high gain, high fidelity, noise suceptibility, and good frequency response over the whole hearing range. Some tubes may be more effective at amplifying high frequency. This is related to the fact that they contain a lot of small delicate components and are quite difficult to design. 6L6 for example tend to suffer from poor gain of both low bass frequencies and very high treble, though it’s pretty good about producing sound clarity.
This turned out to be more like an eli15, sorry. I hope this is understandable.
TLDR: electrons go brrrr in vacuum grid says no.
**First, how do vacuum tubes work?**
If you take a piece of metal and heat it up until its glowing, electrons will start escaping and flying away atvarious speeds (this is called glow emmission). In a pure vacuum these electrons can fly freely, and you can use a second piece of metal to catch them again. This is what happens in a vacuum tube, there is a heated electrode (piece of metal) at the bottom, and a second (not heated) electrode at the top. When there is a voltage between these electrodes the emmitted electrons fly from the bottom to the top electrode: There is current flowing through the tube.
Now we take a third electrode in form of a metal mesh and put it between the other two electrodes. This is called the control grid. We now apply a second voltage from the control grid to the bottom (heated) electrode, so that it hinders the flow of electrons. You can think of it as rolling marbles up a ramp. The faster the marbles the higher up the ramp they get, and only marbles of a certain minimum speed will reach the top and fall off. And because the electrons emitted from the bottom glowing electrode all have different speeds only some electrons get past the control grid. The amount is dependant on the voltage we apply to the control grid.
**How do we use this to amplify signals?**
The signal from an electric guitar is very weak (only in the range of millivolts), but if we apply this small voltage to the control grid of a vacuum tube it results in a large change in the number of electrons that reach the top and therefore causes a large variation in current. If we force this current through a resistor it results in a voltage much larger than our input voltage. (Voltage is equal to current times resistance)
**Why do vacuum tubes sound different?**
The current that flows through the tube for each grid voltage depends on a lot of factors like the temperature of the bottom electrode, the shape of the tube, the size of the grid etc… This means the response for an input signal is different for each Vacuum tube and this makes them sound different.
I don’t really know how each component affects the sound individually, sorry.
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