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How a Vacuum Tube Amplifies

Back to basics
When I went to electronics college we learned all about the transistor amplifier. We had a lot of formulas and most of us in class could do calculations with these formulas and come up with the right answers for the test. But the basic concept of how the transistor actually amplified or even worked was missing. Nobody of my class could really explain how it really worked. They taught us the physics side of it with electrons being attracted from one side to another and then a lot of high level formulas where you inserted values in to calculate the various resistors and voltages to be used to achieve some sort of gain. I thought that there had to be a simple explanation as to how it really worked rather than a lot of formulas and discussions about electrons moving around.

One day I decided to step a sine wave through the amplifying device to see if I could make some sense of it. What happened to all the voltages as my sine wave went in slow motion into the transistor or vacuum tube? I came up with some simple basics that anyone can follow. Funnily enough I have used these basic rules to debug many circuits over the years and explain basic operation to many kit builders. Very knowledgeable electronics people may scoff at these simple rules but if you are starting from scratch like we all were in electronics college, these simple rules will give you a basic understanding of how the amplifier circuit works.

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The amplifying device
So above we have a generic device which could be either a tube or a transistor, let’s call it a tube and give it the proper pin names. There are basically three pins and this would be considered a TRIODE like a 300B single ended TRIODE.

Now let’s start with the CATHODE of the tube, this is the most important point on the tube for debugging. If you have a correct voltage on this tube you can be close to guaranteed that your tube is ready for operation. In other words the tube is now like a horse about to take off down the track.

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High Tension
Let’s look at some other things in this picture – we have something called HT or High Tension. This is basically the HIGHEST LEVEL DC voltage that we are going to use to run our tube. This is what the power supply will supply to the tube. It sits at this voltage all the time and does not change. At the bottom of the tube you can see GND – this is ground potential or zero volts.

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One of the most important facts about this current flow is that the actually current does not change ‘much’ – it’s called a constant current source. If the tube is set up for 2ma to flow through it, then that is a constant – R=V/I, so if the resistance is fixed then the current is fixed.

The Bias
When a tube has been properly BIASED and is working correctly a big current flow will occur from HT to GROUND – think of it as a river flowing in one direction. BIAS means we are going to set up certain voltages and resistors in order that the tube can actually operate, it’s like getting an aquarium ready of a certain size and a certain water temperature such that a fish of a specific size can comfortably live and thrive in that aquarium.

Preparing the Tube

So how do we start preparing a tube so that it can operate correctly and amplify our signal?

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Let’s start by adding a resistor in the current flow path. This resistor is called the CATHODE resistor because it sits between the cathode of the tube and GROUND. Most of us know the famous equation R=V/I which stands for Resistance equals Voltage divided by current. I can’t tell you HOW many times I used that formula during electronics college. By using this formula we can calculate how much current we want to have flow through the tube – by putting a resistor in here of a certain value we will control the CURRENT flow in the tube.

I have put a 1K resistor (which equals 1000 ohms) which gives me a cathode voltage of 2V which is in the range of our generic tube and a current flow through the tube of 2 ma which is also within spec of our TUBE. Whether it’s a 300B with a cathode voltage of 70V DC or a 6SH7 with a cathode voltage of 1.5V – every tube has a cathode voltage and an associated current flow. By using your voltmeter you can measure the cathode voltage of any tube. If you were to measure a cathode voltage of 0v you would know for a fact that this tube is not conducting any current and is not operating.

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Now by installing an ANODE resistor we can control the voltage drop from the HT to the ANODE of the tube. Our tube spec manual will say that it likes to see no more than say 150v at the ANODE, therefore we would come up with a resistor that would drop 100V ( from 250 to 150) given the amount of current flowing through the tube. In our case it is 2ma, so R = V/I = 100/2ma = 50000 or 50K.

Now we have our two resistors selected along with our HT voltage and cathode resistor and operating current. We are all set – our tube is now operating!

I like to use the analogy of a swimming pool with enough water in it, let’s say 6 feet deep such that a swimmer can now comfortably swim across the pool. The specifications for a 5’10” women to swim in this pool is that we have a minimum of 6” of water in the pool and no more that 8’ of water (this is where the pool overflows).

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Now let’s look at what we do when we input an AC or audio signal of a frequency in the audio spectrum e.g. 1Khz into the tube. Well the voltage at the grid is fixed at our tube – (It would typically be.6v higher than the voltage at the cathode) – So we now input a voltage at the GRID.

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The Wiggle
Now here is the trick to how the whole thing works – think of us ‘wiggling’ the 2V DC at the grid – we wiggle it between 1.8v and 2.2V – so its a little 0.4v wiggle. What is happening here is the little audio signal that is 0.4v peak to peak and looks like a sine wave is ‘wiggling’ the DC voltage at the GRID. NOW the interesting thing about the way a tube or a transistor is constructed is that the voltage at the GRID is directly linked to the voltage at the cathode. So by wiggling the DC voltage at the GRID we are also wiggling DC voltage by the same amount at the CATHODE – so now we see a 0.4v DC voltage swing at the CATHODE which is mirroring the GRID. BUT we are feeding a bigger constant current flow THROUGH the tube path – see next diagram.

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So basically what happens now is that as we adjust the 2V at the cathode by adding 0.2 volts to it we have increased our current flow slightly – this same current flow is now going across the anode resistor but the large resistor at the anode causes a bigger VOLTAGE drop than the 0.2v The drop across the anode resistor now could be 2v, so this is a 10x increase and this would be considered a 10x gain. So what we have is a mirror action where whatever wiggle occurs at the grid gets mirrored to the cathode and then amplified by the bigger voltage drop across the ANODE resistor. So this gives us an idea of how our sine wave is amplified.

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Summary
So by feeding a constant current across a small resistor (cathode) and a bigger resistor (anode) we are able to reproduce the wiggle we saw at the grid. This wiggle is our audio signal which is actually a very complicated signal made up of many sine waves of all sorts of different amplitudes and frequencies which make up the music we listen to.

In an amplifier output stage we actually don’t have an anode resistor but rather the primary or input of a transformer which would have a resistance of say 1K5 – it looks like a resistor to the tube but it is actually a transformer that then transformers the amplified wiggle to the secondary – it’s all quite ingenius. And this is only the very beginning!