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
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.
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