According to the Wiki article, ‘Early audio amplifiers were based on vacuum tubes (also known as “valves”).’ A little digging (very little) indicates that the theory behind the working of a vacuum tube/valve – thermionic emission – was described in 1873. Edison filed a patent for a voltage-regulating tube in 1883.
So here’s the question. Suppose a civic-minded individual in 1875 imagined that there must be a better way of addressing an audience. (Or maybe this should be in 1876, when A.G. Bell patented the first loudspeaker.) The circuit for an amplifier comes into his mind in a flash. There it is, exactly what he needs to make his voice heard through The Miracle Of Electricity. Only while he knows what the current has to do to gain the desired effect, tubes haven’t been invented yet. He has no concept of vacuum tubes, and wouldn’t know how to make them. Knowing the anachronistic circuitry, could he duplicate the workings of an electronic circuit using only the electrical circuits he can fabricate using the technology of the time?
You may wonder why I’m asking. Hell if I know. Sometimes these things just come to me.
I think that technologists at that time knew that varying the voltage to the field windings of a commutated DC generator would vary the power delivered at the brushes, which would be electrical amplification. I think this device is called an “amplidyne”. They might have had to do some experimenting to figure out they had to make the magnetic parts out of thin laminations to handle audio frequencies. But they could have approached it this way.
Well by this date, the manufacture of incandescant lamps was established, so you already have some of the parts for a valve amp. The evacuated glass envelope and the heater are in place and understood.
Its not too much of a leap to experiment with having a couple of other electrodes and applying voltages here and there to see what happened to the wires coming from the other electrodes.
By this date we also have electroplating, plenty of cookery utensils were made this way, so we have a means of manufacturing electrodes.
Both these technologies were reasonably mature, even by 1870 so you have the basis for it.
Electrical generation isn’t much of an issue, at least on a small scale, by only 1890 there were some towns with street lighting.
Batteries and accumulators were available too so there isn’t much need to worry about diodes and rectification.
So all we are left with is the manufacture of other components, resistors would be no problem, capacitors moresoe - these would have been physically large for the given capacitance values.
All the parts and technologies to make valves would have been in place, but it would need someone to stitch it altogether.
As an aside, I have known folk make transistors accidently by leaving a plate of silicon on a windowsill, the sun changes the properties and there was a measurable - though hardly dramatic transistor effect when solder pads were stuck on.
Although the Hall effect was not described until 1879, you could theoretically make a hall effect based audio amplifier without transistors or vacuum tube. Now I don’t know if this is actually feasible, but if I only had power sources, magnets, metal, paper and wire but no glass or semiconductor, and I was charged with making an electronic audio amplifier, the hall effect would be first thing I would try. (Disclaimer: I’m not EE)
>An amplidyne is a rather more sophisticated system
Terminology seems a little muddy. There are generators intended for use where their power output is varied by varying the much smaller field magnet power input. Some folks call this an amplidyne and say that an amplidyne system uses this and other components to accomplish a regulation task. Others say that the amplidyne is the system including this generator and other components. Still others use the term both ways, as the Wikipedia article does after starting out by saying “An amplidyne is an electrodynamic amplifier invented during World War II by Ernst Alexanderson.” The Wikipedia article shows an illustration with an amplifier driving the field windings of the generator; I propose that what they label “amplifier” and “generator” are better labeled “electronic amplifier” or “first amplifier stage” and “electrodynamic generator” or “second amplifier stage”. Finally, some use the term “amplidyne” to refer to the generator plus the motor (typically an AC induction motor with nominally constant speed) used to drive it, as a subsystem.
I’ve always looked at an amplidyne as being a closed loop system - that’s really the essential part, its what made them useful in guns, but they were used in anti-sub mortars.
Have a look at Ralph Milne Farley’s books The Radio Man, The Radio Beasts, and The Radio Planet. His hero was Myles Standish Cabot*, in the mold ofg John Carter of Mars, who finds himself on Venus and has to built a radio system from scratch. Farley describes doing it in quite a bit of detail. Pretty impressive for 1924.
Were i stuck in that situation, I’d use whiskers on galena, looking for dislocations to make my rectifiers. Because building a good vacuum pump, with a diffusion pump and all is a bitch.
*Real name: Roger Sherman Hoar
There was a story about a programming guru who was shipwrecked on a desert island and managed to build an entire multithreaded OS using nothing but a couple of Z80’s and a torn copy of Kernighan & Ritchie.