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
>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.
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