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Sunday, March 21, 2010

Tube Amp Basics 2 -- How a Tube Amplifies

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Comments: 5

In my last post, I gave a brief overview of a standard 9-pin preamp tube (12AX7) which was intended as a basic familiarization. In this post, I’ll delve further into detail about what is happening inside the tube when it is configured as a typical gain stage found in instrument amplifiers. I’ll be touching on amplification AND distortion, which are the two primary uses of the triode in guitar amps.

This post will get a bit more technical than the last, but does not involve complex math or physics. Like the last post, it’s intended to be understood by anyone new to the technical side of amplifiers. If you are not familiar with the basic differences between AC and DC voltage, you may want to get familiar with that first for a better understanding of this material.


The Triode Amplifier

Have you ever wondered how exactly a tube amplifies your guitar signal? It’s really not that complex in all reality. As you already know if you read the last article, our 12AX7 is comprised of two triodes. Each triode can act as its own independent amplification stage. So we’re only dealing with three elements in a standard preamp gain stage.

As a review, a triode consists of a plate (or anode), a grid, and a cathode. Their physical layout is in that exact order as well. The grid sits between the cathode and the plate. Remember our schematic symbol representation of the triode?

In a triode gain stage, a positive DC voltage from our power supply is applied to the plate and the cathode is connected to ground. Mind you, there are other components at play in a real circuit, but let’s keep this simple for a moment.

So how does this triode work? Well, remember how I mentioned in the last post that the heater element heats up the cathode to the point where electrons boil off of its coating? Those negatively charged electrons begin to flow towards the positively charged plate, creating a steady flow of current. However, before they can make it to the plate, they must flow through the grid. The grid is between the cathode and plate, remember?

But let’s forget about the grid for a moment. Let’s just picture this phenomenon happening inside of our tube. Electrons are boiling off of the cathode and flowing to the plate. Pretty simple, huh? Almost a little boring.

But if we bring our grid back into the picture, and do some interesting things with it, we can actually begin to control this flow of electricity. More specifically, by applying a negative charge to the grid with respect to the cathode, we can slow down the flow of electrons.

Now if you’re new to the concepts of electricity, here’s an important point: negatively charged electrons will be attracted to positive charges. This is why the negatively charged electrons boiling off of our cathode flow to the plate, which has a positive charge applied to it. Conversely, negatively charged electrons will be repelled from negative charges. This is why applying a negative charge to our grid can slow down our flow of electrons.

The more negative the charge on our grid, the more electrons repel off of it. In other words, the more negative our grid charge is, the less electrons actually make it through to the plate. Eventually, the electron flow from cathode to plate can be completely cut off once the grid charge becomes negative enough. The grid becomes something of a brick wall and does not allow electrons through.

As our grid charge becomes more positive—or in more appropriate terms, less negative—more electrons are able to pass through to the plate.

So the grid serves as a valve of sorts (hmmm, I’ve heard that term used before), which regulates the flow of electric current from cathode to plate. If you’d like a simple visualization, think of a water faucet. When you turn the faucet valve completely off, no water flows from the water pipes in your house to the sink basin. Likewise, when you apply a negative enough charge to your triode’s grid, no electrons flow from the cathode (water pipes) to the plate (sink). When you begin to open the faucet valve, water from the pipes is allowed to start flowing past the valve to the sink. The more you open the valve, the more water flows. Likewise, when you make the charge on the grid less negative (or more positive), electrons begin to flow past it from the cathode to the plate. Making the grid charge less negative is like opening the water faucet valve more.

So where does your guitar signal fit in to all of this? In a standard triode gain stage, the signal voltage from your guitar is sent to the grid of the triode. In essence, the guitar’s voltage serves as the regulator of current flow in the triode.

If you didn’t already know, your guitar creates an AC sine wave. A picture of a basic sine wave is shown below:

As you can see, this AC (alternating current) voltage swings in both the positive and negative directions. So when it is applied to the grid of our triode, we have an ever-changing voltage swing that’s letting varying amounts of electrons flow from cathode to plate at any given moment. As your guitar signal swings more negative, less current flows through the tube. As your guitar signal swings more positive, more current flows through the tube.

The result? An amplified version of your guitar signal appears at the plate of the triode. Pretty cool, huh?

But notice how I said it was an amplified VERSION of the guitar signal. The amplified signal is not identical to the signal that was input to the grid—not exactly identical at least. Due to the operating characteristics of a tube not being perfectly linear, the amplified version of the signal will be at least slightly distorted from the original. In most cases, it’s MORE THAN slightly distorted.

Now you may or may not be confused by that. Some tube amps are obviously known for perfect clean tones, so what is meant by distortion?

There is a difference between distortion and clipping. Clipping is what is happening when we think of a typical “distortion” tone. Distortion in this context does not necessarily mean clipping. In reality, distortion as a concept simply means that the amplified waveform in this case is slightly different than the waveform that was input to the grid. It may or may not be clipped.

Let’s pretend our guitar’s signal is a perfect sine wave represented by the picture below. (The guitar signal is not perfect in reality, but we’ll pretend that it is for this example). As you can see, the signal swings from 1 volt positive to 1 volt negative. We would call this a 2 volt, peak-to-peak AC signal as it actually covers the span of 2 volts. This is actually immense for a guitar signal as a typical, hard-hit power chord with a passive pickup guitar would be more on the order of 0.1 to 0.2 volts peak-to-peak. At any rate, this is purely an example and the picture below was conveniently available with a quick Google search so that’s what we’ll use!

Let’s pretend now that this perfect sine wave is input to the grid of our triode amplifier. Let’s say just for example purposes, that the triode is configured to amplify this signal 5 times (this is an extremely low amplification factor, but again, this is just an example). Let’s also say that it does this without “clipping” the waveform. In other words, it is a “clean” amplification.

That would give us a 10 volt peak-to-peak “amplified version” of our source signal. So you might figure that our amplified version would simply be 5 volts on the positive side and 5 volts on the negative side. That would be the result if our amplifier had perfectly linear operational characteristics. But tubes don’t, and even though we would have a 10 volt peak-to-peak amplified version of our source signal, it would not be a perfectly symmetrical 5v+ to 5v- waveform.

Let’s say for our purposes that the operational characteristics of our tube create an amplified version of our input signal that is 4.2 volts on the negative swing and 5.8 volts on the positive swing. We still have a 10 volt peak-to-peak signal, but it’s now asymmetrical. The negative side of the waveform is smaller than the positive side. Our waveform is now “distorted” even though it is not clipped.

This nonlinear characteristic and the resultant distortion of the waveform are what make tube amplification so pleasing to our ears. The example given above would actually result in the addition of 2nd order harmonic overtones in the amplified version of our signal. This would make it sound more rich and 3 dimensional than the source signal.

So in other words, it’s GOOD that our signal is distorting! Even if we are playing a clean tone!


So I hope after reading this basic overview that you have more of an understanding of how a preamp tube gain stage amplifies your guitar signal.

5:59 pm - 5 comments - 4 Kudos - Report!
mexican_shred wrote on Mar 24th, 2010 7:54am

Great Article dude. keep up the awesome blogs/articles


CECampsa wrote on Mar 24th, 2010 2:49pm

mexican_shred wrote on Mar 24th, 2010 at 1:54am :
Great Article dude. keep up the awesome blogs/articles

Thanks bud! Glad you enjoyed it.


SwampAshSpecial wrote on Mar 24th, 2010 8:06pm

that was great! you should submit it as an article!


mars 88 wrote on Jul 21st, 2010 5:45pm

Thanks a lot for posting these blogs!!

I have been looking to learn more about this stuff, and this was perfect. Most other info I have found was not as easy to understand as this.


Palkom wrote on Oct 21st, 2010 10:20pm

You, my friend, are a genious!

Please keep these exeptionally good articles comming, I'm looking forward to learning more about amplifiers in this simple, down to earth manner.


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