How Class A Tube Amplifiers Work – Noisylabs

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Class A tube amplifiers aren't hard to understand. What makes it confusing is all the jargon that goes behind it.

Manufacturers have the tendency to confuse the hell out of us to force our hand and purchase unnecessary equipment.

Sometimes manufacturers say so and so equipment won't perform to its capability unless you purchase another type of equipment.

And guess where you can buy that equipment to make everything better?

The same manufacturer....

It's all about the revenue. And we don't blame them because that's business.

The point we're trying to make is Class A tube amps aren't as confusing as you'd think. Class A tube amps are easy to understand. Let's break it down for you.

How Does A Tube Amplifier Get Its Classification?

It all depends on the way a tube is biased. Tube amp bias is basically the best setup for current in a tube when it's not doing anything.

To make a similar comparison, tube amp bias is like an autonomous thermometer. Whenever it's too cold, the thermometer brings it up to a higher temperature and vice-versa for when it's too hot. It's all about finding the optimal temperature.

If there's too much idleness without bias then the tube's life will decrease. If there's a little bit of bias, then distortion will come. 

To conclude, a tube amp is classified depending on the way it's biased.

How Does Bias Affect Sound?

Too much bias affects sound because there isn't enough voltage coming into the valves. When there isn't enough voltage going into tubes then the sound isn't as colorful as it could be.

Having the opposite effect causes the sound to distort. Sure, your sound won't be as bland as having over-bias, but don't expect your tube amp to last a long time.

If you want to get specific, bias affects tonality. The more current you put through your output tubes, the heavier the tone. This also causes less headroom.

Less headroom means less room for audio input mistakes. The higher you go on the decibel scale means one step closer to clipping. And clipping is when your audio signal is overpowering your amp causing uncomfortable noise. All of which is unpleasant to the listener.

Relating this back to bias, current can either make or break your sound depending on how much oomph you put into it.

When it comes to sound, having the right bias is key. 

Class A Tube Amplifier Fundamentals

Before diving deeper we need to understand tube amps on a better level.

There are 3 parts to any tube amp. The first part is the cathode.

class a tube amps


The cathode is a part of the tube amp which heats up because of current. The whole point of the cathode is to form negative-charged electrons since the current is direct. Having negative electrons helps with alternating current.

The second part to a tube amp is the plate. To counteract the negative electrons of the cathode, the plate produces positive electrodes.

class a tube amps


Another valuable function of the plate is it lets you know what the maximum voltage is. If the maximum voltage says it can handle only 50 watts, then you'll be sure to only put it into something that produces 50 watts or less.

The final part is the grid. This third element is like seasoning on a tasty entree. Its duty is to regulate the current between the cathode and the plate. The grid can be commonly found in instruments such as guitars.

class a tube amps


Class A Tube Amplifier Schematic

If you're looking at a standard schematic with a Class A tube amp you'll notice the 3 elements we talked about are right next to each other. 

You can also find this situation in a triode amp circuit schematic diagram. Most Class A tube amps have similar setups.

Now that we understand more of the fundamentals, let's discuss some of the advantages and disadvantages of the Class A tube amplifier.

Advantages and Disadvantages of Class A Tube Amplifiers

Class A tube amps are known to provide warm rich sounds. This is thanks to the constant stream of current.

Since Class A amps are always streaming current, you get multiple advantages.

A steady stream of current means there's no need for "warming up." Unlike old cars sitting in the garage, Class A amps are always "on."

There's always the case of "smooth compression" too. Remember how bias is all about regulating current? Compression is all about regulating sound.

Whenever sounds get too loud a smooth compressor tones it down. If the sound is too quiet then the compressor adjusts it to make it more audible.

With the right amount of bias set in the tube, your Class A amp is more responsive to different amounts of voltages.

An obvious disadvantage with constant current are the tubes being worn out. Having no breaks means a shorter tube life.

Also, if you wanted to adjust the current to something stronger, you would need another transformer.

Sometimes people confuse Class A tube amps with Class AB tube amps because of their similarities.

Class A vs. Class AB amps

One distinctive difference between a Class A and a Class AB is the lower current. Class AB amps are "idling" with lower plate currency.

The concept of "idling" is somewhat difficult to understand so bare with us in our attempt to explain.

Amps have transistors whose responsibility is to handle output signals.

In a Class A tube amp, output current is transferred in one smooth transition. This is why crossover distortion doesn't happen. 

In a Class AB tube amp, the output current isn't transferred as smoothly because there is a gap in between. This gap transfer is the idling. 

A benefit of having that idleness is you're not wasting any heat unlike Class A amps.

Having a Class A amp means you have a smooth compressor and an amp which is always ready to go. You can't say the same for a Class AB amp.

Knowing Class A Tube Amplifiers

There's one main takeaway from this post:

Class A tube amps aren't complicated. They have pros, and they have cons. Being able to distinguish on both is necessary for audiophiles.

Even then a lot of what manufacturers say about Class A tube amps is a lot of marketing to get you to separate you from your dollars.

What you need to know is bias, and the basics of an amplifier so you're not thrown off by each classification of an amplifier.

Knowing the plate, grid, and cathode are those basics. Remembering the fundamentals helps you navigate schematics better.

Knowing the basics also makes it easier to know what type of amplifier you're working with. 

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