The humble vacuum tube, and why is it (not always) better than solid state


vac·u·um tube
/ˈvakyo͞om ˌt(y)o͞ob/
  1. an electron tube containing a near-vacuum that allows the free passage of electric current.

Ah yes, the humble vacuum tube, ancestor of the transistor. Also known as electron tube. Or tube. Or valve (for our British friends). As its name implies, the components are housed in a glass (or sometimes metal) envelope that is at high vacuum, where all the magic happens. Get ready, because we're going to say "tube" a lot in this article.

Tubes: a (very) short history and (very) short explanation of how they work

The simplest of tubes - and also the first practical application of the concept - dates back to 1904 and is called the Fleming Valve, invented by John Ambrose Fleming. It contains a single heated electron-emitting cathode and an anode. The current can only go in one direction: from the cathode to the anode. The current flowing between the cathode and anode can be controlled by adding grids and applying a voltage on them. Most tubes used in audio equipment will amplify a signal in one way or another. That's the gist of how an electron tube works.

These tubes, where the cathode is heated, are called thermionic tubes. Other types of tubes exist that rely on different principles (like phototubes), but we're interested in thermionic ones since that's what is used in audio amplification. I'm not going to go into great details about each different type of tube and their uses, though, that's something Wikipedia is better at than I am.

A small caveat

Most tubes have different names depending on where they are sold or produced, or if they are milspec or some other kind of "special use" tubes. For example, the venerable 12AX7 tube (probably the most common gain tube on the planet) is also known as 7025, ECC83, ECC83-S, and ECC803. They are all the same tube, with the same pinout, and same gain factor. ECC83 is the European designation for this particular tube, the 7025 is a special low-hum, low-microphonics version, and the ECC803 is a high-reliability version.

What's in a tube amp?

Your average tube amp will contain 3 sections of audio tubes (the rectifier tubes are different and are not in the audio path; they just rectify the AC from the mains to DC to power the different components):
  1. The preamp
  2. The phase inverter
  3. The power amp

1. The preamp

This is where the magic starts. The preamp takes the input signal from the instrument, and amplifies it to workable levels. Whether it does that cleanly or not depends on many factors, among which are:
  • How hot the input signal is
  • How many stages of gain there are
  • The number and variety of tubes used
Some amps are built with only one clean "channel" (like the Fender Blues Jr.). These amps have a fixed amount of gain stages. You can get these amps to overdrive by cranking the gain and the volume, but they are built to sound clean. 

Some other amps, like the Fender Hot Rod Deluxe (which I regret selling), will have 2 channels on one input. The second channel usually adds a few more stages of gain or do some other black magic to ensure your signal is always overdriven.

The tone stack is usually inserted after the first gain stage, but it all depends of the circuit's design.

If the amp design has an effects loop, it is located after the preamp but before the phase inverter. Some amps allow you to select if the effects loop should kick in before or after the tone stack.

Common preamp tubes

99% of the time, the preamp tubes will be 12A*7 (where * can be either X, T, Y, V, or U; generally it'll be X). These tubes are 9-pin dual-triodes, require 12 volts on the heater, and provide 2 stages of gain (one stage per triode). The average lifespan of a typical 12A*7 tube is around 10000 hours or about 5 years when used 5 hours per day.

The different letter combination indicates the gain factor of the tube:
  • 12AX7: gain factor of 100
  • 5751: gain factor of 70
  • 12AT7: gain factor of 60
  • 12AY7: gain factor of 45
  • 12AV7: gain factor of 41
  • 12AU7: gain factor of 19
A common replacement for the 12AX7 is the 5751. This tube has a gain factor of 70, which is usually sufficient to tame a particularly hot preamp circuit and give more headroom. You might also see some 7025. These are Fender-specific 12AX7 tubes with strict standards in terms of noise and microphonics. In theory, a 7025 is quieter than a standard 12AX7.

Some other tubes can be found in the preamp position, but new production amplifiers will most likely use 12AX7. If you have a vintage amp, refer to its tube chart and use the same type when retubing. Do not replace a tube with one of a different type as the pinout and required voltages may be different and you'll end up blowing something up. Releasing the Magic Smoke™ is never good.

2. The phase inverter

Unless your amp has a single power tube, like a Fender Champ or the 5W Monoprice Stage Right, you will have one phase inverter tube and it is a very important element of the amplifier circuit for the magic to continue happening. In a nutshell, the phase inverter will split the signal in two and invert the phase of one of the split strands to feed the power tubes. Typically, the phase inverter tube will be the same type as the other preamp tubes, and depending on the amp design, having the triodes balanced and matched can make a difference. In most cases though, the difference will be barely perceptible.

Changing the phase inverter tube will have a slight impact on the overall tone and overdrive. Using a tube with less gain will give more headroom and will crunch slightly less. It depends on the phase inverter circuit. If it uses one triode, then no gain is applied to the output signals

The concept of the phase inverter isn't simple. Uncle Doug made an excellent video explaining what a phase inverter is, and why it is needed in a push/pull amplifier: 

Common preamp tubes

The same tubes as the preamp tubes are often used in the phase inverter section of an amplifier. In most new production amps, it will be some form of 12A*7.

3. The power amp

This is the end of the signal chain. Your guitar/bass noise went through the preamp, got cranked up, went through the tone stack and the effects loop (if any), got split and flipped 180 degrees by the phase inverter, and it's almost at the speaker. It just need to go through these Big Bottles and you'll be annoying your partner, dog, cat, hamster, and possibly your neighbors as well.

The power tubes are the biggest tubes in the amplifier design. They are made to pump raw power to the speaker. For comparison, a typical preamp tube can output between 1.7 to 3 watts of power, while power tubes can output between 5.5 to 20+ watts per tube. The downside is they heat up. A lot. And really fast.

The maximum power output of an amplifier depends on the design, and number and type of power tubes it uses. For example, a 15W amplifier can be powered by a pair of EL84, and a 100W amplifier can be powered by a quartet of 6L6. Combining the power tubes give them more output power than if they were taken separately.

Ideally, you'd want the output tubes to be matched so their characteristics are similar, but having unmatched power tubes can sound pretty good. It's all about getting the tone you want.

Common power amp tubes

There are a few more power tubes than preamp tubes in today's amplifier designs. It all depends on the sound and output power the designers want to give to their amplifier. Among the common tubes you'll find, are:
  • EL84
  • EL34
  • KT66
  • KT88
  • 6L6
  • 6V6
Most of them are not interchangeable and have vastly different characteristics, mainly in terms of output power. The average life expectancy of a power tube depends on many factors, but these are the main culprits causing the dreaded tube decay:
  • How often you play
  • How long you play
  • How loud you play
  • How often the amp is turned on and off
  • How stable is the power line
  • How hot the tubes get
  • How hard the tubes work
On average, a typical power tube will need to be replaced every 2 to 5 years. The factors enumerated above will largely influence how long the tubes will survive. If you play at low gain, low volume, for 2 hours a day, you may be able to keep your tubes for 5+ years. If you play at high gain, high volume, 8 hours a day, on a dirty power line, and power the amp on and off 5 times a day, chances are you might have to change them before you reach to 6 months mark...

What's the difference between toobes and solid state?

Well, if it is not obvious enough, solid state uses silicon integrated circuits instead of Big Glass Bottles™. Those integrated circuits, or IC for short, are designed to do a flawless, noiseless, distortion-less signal amplification with gain factors much, MUCH higher than electron tubes. Yes, ICs will distort, but it takes much more to reach this point than tubes need to overdrive. They also take a lots less space, weight a lot less, consume much less power, and barely emit any heat compared to their glassy cousins.

In terms of sound, solid state will amplify your signal with a very low total harmonic distortion. This is the main difference. Not how loud it can get. Not the tone stack that is on the PCB. Not the slightly different Magic Smoke™ the silicon chips contain. No: it's the harmonics. A solid state amplifier will often sound "bland" and unnaturally clean because it does a perfect amplification of its input. We expect a loud sound to be somewhat distorted, such as when someone yells versus when they speak normally. The electron tubes however, are not perfect and will introduce harmonics and slightly distort the sound as it amplifies the signal, and those harmonics are what makes the sound of a tube amplifier "warm" and distinctive.

The type of distortion you get from solid state as the amplifier is pushed to its limits is also completely different than the mellow crunch you can get on a tube amplifier. A solid state distortion does not sound natural - or at least much less natural than the overdrive of a tube. Sure, some pedals exist to reproduce the smooth breakup of a tube pushed really hard, but they lack a little je-ne-sais-quoi. They sound great, but not quite like the real thing.

Which is better? Tubes, or solid state?

There are no good or bad answer to this question. It depends of what you play, of what you expect your instrument to sound like, of what YOU want your tone to be.

Solid state has made tremendous advancements in the last 20 years with modeling engines that can reproduce the warmth and tone of a more conventional tube amp (and much more), and their versatility is great. Solid state's main advantages though, are its durability and its price point. A good solid state amp will probably outlive you without needing major repairs and will cost around 600$.

Tube amps are great for clean, natural tone. Their responsiveness is preferred by many, and it's something you will not find on a solid state amplifier. The tube amplifier will react to how you play, how you accentuate notes, and how hot the guitar signal is, cleaning-up and breaking-up when you roll the volume knob on the instrument. These amps will - like solid state amps - most likely outlive you but WILL need to be retubed every 5 years or so. A full retubing for 3 12AX7 and 2 EL84 will cost you between 200$ and 300$ as of August 30th, 2022. It's not that expensive, but it's not cheap either. The starting price point for a good tube amp is much higher than its equivalent solid state, and it's something that will drive some people away from tube amplifiers. A 15W tube amp is between 700$ and 1000$, much more than the 150$ solid state 15W amp...

I have both tube and solid state amps, they both sound great, and I use both regularly. It's a matter of preference and using the right tool for the job.

Header images: FreeImages


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