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Can anyone explain what a power tube does inside an amplifier, eg kt88.

I know a tube is cool looking, and looks like a small lightbulb with many pins on one side and when it's turned on filaments glow inside a vacuum enclosed see-through curvy glass enclosure.  I guess current flows in, goes on a journey, and then flows out.  
 

 

emergingsoul
erik_squires
16,056 posts
 

All tubes and transistors can be simplified by thinking of them as 3 pin devices:

 

Input (music), Output (Music), power.

 

Between the input and the output a device increases voltage, or current, or both.

Amps tend to have multiple stages, up front there’s a voltage amplification, at very low currents. The standard is about 20x from input to output, then at the end is a current stage, which may have no voltage amplification but can deliver the current the speakers need. That’s usually what a power tube or transistor does.

This  is why tube amps will usually have at least 2 different types of tubes.  The smaller one's doing voltage related work, the larger one's current. 

While voltage goes up by around 20x, current must go up by say 40,000 : 8 or 5000:1 and this is very difficult to do with single stage amplifiers.

tablejockey
3,890 posts
 

It's all here...

taste_of_tubes.pdf (ken-gilbert.com)

Best understood with some electronics 101 reading.

assetmgrsc
58 posts
 

@emergingsoul   Someone mentioned using CHTgpt, so I decided to access it and a pretty good and simple explanation comes out. 

Here is what it says: 

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provide a simple explanation of how an electronic tube, such as a KT-88 amplifies a signal
 
 
ChatGPT

An electronic tube, like the KT-88, amplifies a signal by using a process called electron flow control. Here's a simple explanation of how it works:

  1. Cathode Emission: Inside the tube, there's a cathode, which is heated. When the cathode gets hot, it emits electrons into the surrounding space. This emission of electrons is called thermionic emission.

  2. Controlled Electron Flow: These emitted electrons are negatively charged, and they naturally want to move towards a positively charged electrode called the anode (or plate). However, there's another electrode called the grid placed between the cathode and the anode. The grid can be negatively or positively charged, and it controls the flow of electrons.

  3. Amplification: When a small input signal is applied to the grid, it either attracts or repels the emitted electrons. If the grid is negatively charged, it repels some of the electrons, reducing the flow from the cathode to the anode. Conversely, if it's positively charged, it attracts more electrons, increasing the flow. This control of electron flow at the grid allows the tube to amplify the input signal.

  4. Output Signal: As the electron flow from the cathode to the anode is controlled by the grid, the varying input signal gets "amplified." This amplified signal appears at the anode as an output signal, but it's now a larger version of the original input signal.

In summary, an electronic tube like the KT-88 amplifies a signal by using controlled electron flow. The input signal applied to the grid modulates the flow of electrons between the cathode and anode, resulting in an amplified output signal that reproduces the input signal but at a higher level of strength. This process is used in vacuum tube amplifiers to boost the power of audio signals for applications like music amplification.

 
 
 
 
 
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tonydennison's avatar
tonydennison
408 posts
 

I have often wondered this.

The tube makes a larger copy of a signal, yes.

But how?

I imagine they are called valves because they act as gates, only opening to release the larger copy when it is large enough based on specs.

Again, how? How does it increase the size and  know when it has accomplished its goal? How does the gate open?

 

I can understand and explain how a gas engine does what it does, but not a tube.

Ive read some technical explanations but they always seem to avoid the actual answer to the actual question.

 

T

emergingsoul OP
1,132 posts
 

@tonydennison 

Chat GBT did a great job above. And your comment asking how it actually controls the valve activity is very key.

Also there was a document above from @tablejockey that looks very promising.

Ultimately the end answer, is likely very simple, amidst all the complicated fog.

When you ask a child, even in adult, how something works, like how things work when you turn on a sink faucet you'll get a good answer. But if you start going into the actual inner workings of a faucet, the child quickly gets very frustrated. Human nature is a challenge.

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