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.  



"I guess current flows in...." Works for me. :-)

If you want a tutorial or an actual answer I think there are better places to ask this kind of question. 

a tube is connected first to the filament supply that heats up the cathode so it can emit electrons inside vacuum.

A small offset voltage can be applied to the grid and plates of the tube.

KT88 is a penthode tube connected usually as a tetrode in the tube amps with usage of an extra reflection device called plate. There are 2 plates in penthode totally giving 5 electrodes: Cathode, Anode, Grid, Plate1 and Plate2. 

An input signal is usually applied to the grid. That's where you get the MOST voltage gain and the output to the transformer is going to be between anode and ground or Cathode is in this case usually grounded. 

There are hundreds of websites explaining this in dozens of different ways.  Google it and find one that suits your mind.

This OP  is a bit provocative , between a bot and a narrow mind audiophile... He never ask a question by curiosity and sincerity it seems... He already know the answer and laugh in his beard ...

I may be wrong... But it is what i think after his thread about S.S. and tubes amp.

I've asked this question directly to humans and haven't had good answers. I don't think many people really do know. Google reference is not good on this point for some reason. YouTube doesnt offer an explanation of what happens inside a tube that I could find without getting odd vids scarily technical and not helpful.

And then there's this amazing forum which offers incredibly smart people willing to offer their knowledge and be very helpful.

So this is my last hope. 

I’ve asked this question directly to humans and haven’t had good answers. I don’t think many people really do know. Google reference is not good on this point for some reason. YouTube doesnt offer an explanation of what happens inside a tube that I could find without getting odd vids scarily technical and not helpful.

And then there’s this amazing forum which offers incredibly smart people willing to offer their knowledge and be very helpful.

So this is my last hope

The book is free...

This one is free too...

This book is free too :

more basic and free book :

Very good book here , begin here :

Deeper book :

Another one :


I will stop here...

And listen to this short youtube video :

Let's design an build a vacuum tube amplifier from scratch


You are really comical. You started one thread declaring (as fact) that tubes were better than solid state, and now you admit that you don't even know how they work. @mahgister is right, read a book!

Common guys, give him a break. You do not have to know how a technology works to realize it does.

I just wanted to be useful with my list of free books...If he is serious... I dont know that but i am trustful of people ...

No one here will explain him in few line the function of this power tubes...

I advise him to ask for atmasphere help politely; he is one of the best tube amp and class D  designer and he is here very often in audiogon ...


Common guys, give him a break. You do not have to know how a technology works to realize it does.

Simple answer, tubes are valves. Small signal opens the valve and lets the power from your power supply go to your speakers. They are called valves for a reason.

Do a youtube search. Watch some videos if you actually care to know. Lots of info out there.



Ever see or use a pantograph? This is a mechanical device that allows you to draw an enlarged copy on a piece of paper when you trace a small image. A vacuum tube (and also a transistor) are just electronic versions of this. A small signal to a control grid in between cathode (source of electrons) and anode (the output), makes a bigger copy (i.e., more voltage) of the original signal.

That's why they are called "amplifiers" -- they are amplifying a small signal, or again, making a bigger copy of a smaller signal.



Agree, it is making a bigger copy of the small signal. The small signal is just making it possible for a separate bigger signal to flow. Two signals. The small one is controlling the big one, it’s not that the small one is being magnified directly. 

A vacuum tube is a device  that uses a small input voltage to control a larger output fed by the resources of the power supply. Typically, the high voltage low amperage output is then sent through a transformer to  become a lower voltage, higher amperage output for driving a speaker. 

Because of the limits of the technology, usable amplifiers are developed in 2 or more 'stages', each optimized for voltage or current gain, or for specific applications like the pre-pre amps which provides magic loudspeakers in

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


An attempt at Simplification. Perhaps someone else can add clarification while still keeping it simple.


tubes are sometimes called ’valves’.

think of it like a garden hose. open the valve a little, a little more, much more.

low strength signal from component

to/thru controlled volume control, varies signal strength to the tube

sends a little or a little more, or much more signal to the tube

tube is like a water valve that varies the high power circuit that goes to the speakers: high power signal is proportional to tube’s low power signal strength: a little, a little more, much more.

Emerging Soul already knows how they work "current (sic) flows in, goes on a journey and then flows out". What more is there to say...

Simple explanation.  It's magic!  electricity goes in-music comes out!  Lots of people will tell you that tubes are magic.

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.

@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

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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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.




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.

It is a valve that opens and closes in response to the input signal fed to it. Output side, (anode or plate), is connected to a high voltage power supply, that feeds the output transformer connected to your speaker through the tube. The input side, (grid), accepts the tiny input signal from your preamplifier, and turns the large power supply on and off in response to that input. The result is a much larger signal being applied to your speaker , than the tiny input signal being applied to the grid. Basically how amplification of the tiny input signal is achieved. 

Of course there are more parts to the tube that sets the amount of current bias is applied. But the above is a simple as I can attempt to explain it.

So does the plate collect the electrons until the charge is enough to push through the gate? And what prevents loss of  information? Once the tube gets warmed up, I guess it is just continous?

Oh my God, the document above is 760 pages long.

It looks pretty damn awesome and it has pictures.  I wonder if it was ever translated into another language.  Fortunately I only can read English and how lucky I am this document is in English. I failed German when I was in high school. What an awful experience that was. 

Why is it necessary to have eight Power tubes on a mono block amp? Assuming its 300 watts, does this mean each tube is incrementally amplifying the total watt availability? Why not just do a few tubes, or would that means less watts available?

That's a lot of vacuum space for electrons to flow through going through so many tubes so quickly. I recall seeing one amplifier with a single tube about eight times the size of a kt88.

Takes space for all those sweaty little charged particles to do their thang...😏

SS stuff makes the work easier....and less expensive...

Well, originally...anyway...;)

"Why is it necessary to have eight Power tubes on a mono block amp?"

A KT88 tube is rated at about 35 watts per tube. That is why you need 8 tubes to produce 300 watts!

So is each tube individually responsible for incrementally amplifying signal?? I believe all eight tubes would be on the same circuit.

Just like a Christmas tree lighting system?

There's a reason why the Brits refer to vacuum tubes as's a bit more visually comprehensible.  The smaller signal input to the grid modulates the current flow from cathode to plate and through the output transformer.  It's akin to mechanical leverage where a small action can be magnified.  If you want a thorough understanding of how tubes work in various circuits, pick up a copy of Radiotron Designer's Handbook.  It'll keep you busy for a while.