Today though, I'm going to talk about triodes and amplifiers
What's going on around us is very serious, but I'm still a curious fella and just discovered something really neat. It's really cool new technology - or rather the merging of an old with a "less old" technology, to create something new.
First, let's discuss transistors and amplification.
Modern audio amplifiers use transistors to amplify a weak input signal. Transistors do a really good job of amplifying the signal, and they do it with high efficiency, low distortion, and high reliability. Because of their efficiency and reliability, transistors have mostly replaced vacuum tubes for signal amplification.
Below is one of the simplest transistor-based amplifier circuits. It uses an NPN transistor in what is called a class A amplifier. A weak input signal is applied at the left, and passes through C1 to the base of the transistor. The input signal on the base causes the collector-emitter (vertical path through the transistor) to pass much more current, which is proportional to the input signal. The power for the amplifier is supplied from the right, externally from +Vcc and Gv. The amplified signal is picked up at the collector of the transistor. The other resistors are there to properly bias the transistor voltages and to limit current through it.
Before solid state electronics were invented, the above circuit would have used a vacuum tube called a Triode. The triode tube worked on a similar principle, but instead of using semiconducting materials, it used a heated filament, a blocking grid, and a collector plate.
Below, a DeForest Audion amplifier tube On the left is the Edison screw-in base to heat the filament. At the center, weaving up and down, is the grid. The grid is suspended between the plate and the filament. The heated filament will emit electrons that will tend to flow toward the plate. However, voltage applied to the grid will cause it to build up a charge, and this charge will affect how much current flows from the filament to the plate. A weak input signal applied to the grid can amplify a much larger current flow from the filament to the plate. Early signal amplification! Clever stuff. It takes *a lot* of DC voltage between the filament and plate for this type of tube to work though.
Below is a basic triode amplifier. Looks very similar to the solid-state circuit up top, doesn't it? The input signal is applied between the filament and the grid. How much the tube conducts bottom to top depends on the signal that is applied to the grid. The amplified output signal is picked up across the plate and the filament.
Vacuum tube Triodes are still pretty common in guitar amplifiers though. There are a couple of reasons for this. Tube amps have a reputation as having a "warmer" sound than transistor amps. More importantly, unlike transistors, tubes deliver a neat distortion sound when they are over-driven. The reason they do this is called "clipping". Clipping is a term for the flattening of the peak of a waveform, and this happens because the tube is at the limit of its ability to pass any more current from the filament to the plate.
Below is an example of a clipped wave-form. The peaks of the sine wave are flattened off.
If you come across a transistor-based guitar amplifier, it will very likely have a "distortion" effect, but that effect is a signal processor that attempts to mimic how vacuum tubes sound when they are clipping. You do hear distortion, but it's not produced by the mechanism of clipping.
Below is a video explaining the hows and whys of vacuum tube clipping, and how it works going from guitar though the amplification chain. You also get to hear the sound of an over-driven pre-amp and output tube.
What I recently learned though is that a small type of vacuum tube that was originally developed for digital displays has been re-purposed for audio amplifiers. How cool is that!!!??? The Vacuum Fluorescent Digital Display that I mentioned briefly in this post, has been modified and used as a miniature low-power, high efficiency triode tube.
This new, modern triode is called a NuTube, and was invented by Korg in 2015. It has mainly been offered in Vox amplifiers. Below is an amplifier circuit, with the NuTube at the right.
Below, a two-channel NuTube powered up.
I like this hybrid idea. We can get old-school sound (and overdrive???) from a more modern, reliable and efficient device. One problem though - they are microphonic. That means that any vibration the tube experiences can become noise in the amplified output signal. Microphonics is not a desirable property for an amplifier tube, and for this reason the NuTube has to be physically isolated from anything that might cause it to vibrate. Dampening vibrations will probably be difficult in an environment where the NuTube will usually have a very loud speaker vibrating nearby.
Below, one of my favorite YouTube guys evaluates a NuTube and amplifier board. I really hope the microphonic issue can be resolved, because this is such a clever re-purposing of digital display technology.
Below is a video advertisement for a few Vox guitar amplifiers that use NuTube technology. They are displaying the amps, to be certain. However we also know that the NuTube is highly microphonic, so that's probably why it's sitting on a stool, well away from the speaker cabinet.
2 comments:
The microphonic issue would be a problem for me. However, with good placement of the unit, it seems to work well. I wonder if the NuTube could be cushioned in some way, to minimize the microphonic problem.
I agree. The microphonic issue will probably be a show-stopper that keeps this cool device from getting into widespread use. It's really kind of discouraging that such clever re-purposing of that technology may not work out. Hopefully they can work the bugs out. I suspect it may just be the cathode filament vibrating. In these devices, the cathode filaments are long strings, so that might be what's making them ring.
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