Beginner Introduction Guide to Tubes & Valves in Amps & Audio November 19 2018, 0 Comments

Tubes can be a little confusing when you first start looking into them. All you know is that your amp / audio device needs them. If you want to learn more about them, to gain a basic knowledge of the subject, this guide is for you. Most of what is written will be in the musical context of amplification.

What is a Vacuum Tube?

A vacuum tube is a device which is currently used in audio devices, and used to be used in a range of electronic equipment. They come in varying size, from 5cm all the way to 12cm+ and are commonly constructed from glass, plastic and metal materials. They often resemble a glass bottle or jar with metal bits inside.

It essentially controls the flow of current and amplifies signal in audio applications. It has the added benefit of providing a touch of that tube distortion which makes a tube tone more pleasing to the ears. It's for this reason people still use tube amps.

Basic Terminology & Naming

Firstly some terminology to avoid confusion in the future. Are tubes and valves the same? Yes – “tubes” and “valves” are used interchangeably, they refer to exactly the same thing. It’s just personal preference which one you use. We’ll stick to saying tubes in this article since that’s what I’ve always called them.

You’ll notice that most tubes also have a series of letters and numbers after them – this can add to confusion too. This is because a single tube can have two different identifiers. This usually refers to either the European or American designation. Often both terms are included in product descriptions for clarity. Here are some of the most common tube types with the European name followed by the American name. 

ECC83 (EU) is the same as the 12AX7 (US).

EL84 (EU) is the same as the 6BQ5 (US)

ECC82 (EU) is the same as the 12AU7 (US)

But what do those letters and numbers all mean? On some of the tubes, the code simply represents the product code, for example the ECC83. However, the American designation often contains some information about the tube. The 12AU7 for example can be split into several parts:

12 - Filament voltage.

AU – Product designation.

7 – Number of elements.

Any extra letters often refer to an update in the version number, or an adjustment by the factory. They needed to distinguish it from the old version.

What do Vacuum Tubes Do?

Tube used to be used in a range of electronic equipment (computers, radio, TV and general electrics) but was replaced by the transistor which you get in standard solid state guitar amplifiers. However, they’re still used in musical equipment and devices. They’re really good at:

  • Amplifying signals.
  • Inverting the polarity of a signal.
  • Mixing signal together and splitting them.
  • This makes them great in audio applications.

How do Vacuum Tubes Work?

First we need to look at the parts of the tube to understand how they work. Most tubes, but not all of them contain these components. It’s best to think of a tube from bottom to top with each part fulfilling a certain task which allows the next component to do its job. It’s a one-way street.

The Filament

The first component that truly is common to all tube types is the filament.  The filament is the part of a vacuum tube that you see glowing when the amplifier is powered on. 

When an electrical current flows through the filament, it creates both heat and light. It’s actually the purpose of the filament to create heat, and therefore light. Not just something to make it look cool (I’ll admit that I used to think that’s what it was for). The heat is essential for working with the following component – the cathode:

The Cathode

The cathode is an element located next to the filament so they can best work together. It is negatively charged.  It has a coating on it that when heated (by the filament), releases electrons.  That is the whole point of the cathode, to release free electrons into the vacuum tube after being heated. If there wasn’t a vacuum this wouldn’t work.

Electrons work like a magnet -  a positively charged electron is attracted to a negatively charged electron and vice versa.  After being released from the cathode, the electrons flow to the next component – the anode, due to this charged attraction.

The Anode

The next element is called the anode, or plate. It’s positively charged. It's job is simply to attract the free electrons released by the cathode after being heated by the filament. It’s the end point and in a way, it “collects” your guitar signal when it has been inserted through the next element…

The Grid

The grid is a charged piece of metal located between the anode and the cathode. It’s where your guitar signal enters the equation – your guitar signal is connected to the grid. If the grid is negatively charged, then the flow from the anode to the cathode is reduced. This is because a negative (from the cathode) and negative force (from the grid) repels. However, if the grid is positively charged then the flow of electrons in accelerated since positive and negative attract.

This is the amplification in action. The amplification process also adds that tube distortion to the mix.

When Everything Links Up

So to summarise – the filament is the heat generator. If it’s not generating heat, then the tube isn't working. This heat then causes the cathode to release electrons towards the anode due to the negative to positive flow. However, in the middle is your guitar signal and grid which is where the amplification takes place as long as there’s a positive charge. This “boosts” the electrons on the way to the anode and exits the tube on the way to the rest of the amplifier and then speaker.

For beginners I have oversimplified this process in order to aid understanding, if you want specifics check out the further reading at the bottom. Also this video help explains the process very well:

Different Types of Vacuum Tubes

You’ve probably seen loads of different tubes. Some small, some big, some long and thin. Let’s look at the different types:

Rectifier - Diodes

The purpose of a diode / rectifier is to turn alternating current from your house supply into direct current like from a battery.  A diode is a two element tube that does not have a grid mentioned above. Nearly all electronics need to turn AC into DC.  A diode tube also needs the help of some capacitors to make this possible.

The rectifier will usually work in conjunction with the transformer. They aren’t always essential in amps, sometimes the rectification takes places using non tube based technology or the transformer alone.

Examples:

  • 6X4
  • 5AR4 / GZ34
  • 5U4
  • EZ81

Preamp - Triodes

A preamp will amplify the signal and prepare it for further amplification by the power tubes. Or it’ll act as 100% of amplification in smaller tube audio devices. A triode is the tube which usually does this. A triode is the most common sort of tube. It’s a three element tube which has all of the parts mentioned in the previous section.  They can amplify the signal but also create some distortion. 

Triodes can combine signals and split them into two separate signals.  They can also invert the polarity or phase of a signal.

Examples:

Power Tubes - Tetrodes Pentodes

Power tubes further amplify and drive your signal. They are required on larger amps in order to drive the bigger speakers. Tetrodes and Pentodes are usually used in the power amp section, as they are capable of amplifying your input signal much higher than a standard triode. Tetrodes are four-element tubes and pentodes are five-element tubes. 

Typically, these tubes work in a similar way to triodes but have additional elements called screens.  Screens are usually in between the cathode and anode. It “screens” the anode and grid improving performance. 

Examples:

  • EL84 / 6BQ5
  • KT66
  • KT88
  • 6V6
  • EL34 / 6CA7

There are different types of tubes in one of the three groups above which all can all sound different. They can sound better or worse at different points along the sound spectrum which results in different uses. And then alternate brands can sound different when compared to a competitors equivalent.

Tube Brands

Tube brands can be split into modern production tubes, and older brands which aren’t made anymore (often called NOS or new old stock).

Modern Brands

Explaining the different brands of tubes get tricky. There aren’t many factories which still produce tubes, and a lot of the brands you see on the market will be a “rebrand” of a tube already out there – maybe with a minor tweak. There are 3 factories who currently make 95% of all tubes:

  • JJ Electronic based in the Slovak Republic.
  • NewSensor based in Russia.
  • Shuguang based in China.

NewSensor don’t sell their tubes under that name. Rather they have a few different brands which are very similar but with minor changes / design tweaks. These are Electro Harmonix, Sovtek and Tung Sol.

Any other brands you see are rebrands of these three main factories. For example some of the stock Fender, Marshall and Mesa Boogie tubes are rebranded from these factories.

There are subtle differences between the three factories, the below are some generalisations about the different tube brands.

  • JJ Tubes - Tight bottom end and crisp clear tone.
  • NewSensor range – Decent amount of gain and quite thick sounding.
  • Shuguang – More gain than average (probably the most gain-y) and are quite bright.

Everyone has a brand that they like. Searching online or obsessing over reviews isn’t always that helpful. There are numerous posts online saying X brand is terrible, while there are as many posts saying that brand X is the best out there. Always be suspicious of blanket statements like “All tubes from brand X that I’ve had have failed”. This will almost certainly be a badly biased amp, some other fault in the amp, or one tube blew and they’re over exaggerating.

Also you get people saying "I replaced brand X with brand Y and brand Y sound much better!" The reason brand Y sounds better is because they're newer. And new tubes sound better than old worn out tubes. You can't compare old tubes to new tubes. So it's not a fair comparison. The only way to know is to try a certain tube for yourself.

NOS / Vintage Brands

Vintage tubes are very much in demand because they are thought to be incredibly well made and some think sonically superior. The problem is that these tubes are no longer in production and are quite rare. Consequently they’re expensive. It’s certainly not a case of new tubes being bad and old tubes being good, like with many things in the world of tubes, it’s subjective.

These tubes are usually sold as NOS or New Old Stock. They are unused old tubes from brands such as Mullard, General Electric, and Phillips.

In reality there will probably be minimal difference between the tone of an amp with NOS tubes, or one with the modern production brands. I’d be incredibly surprised if someone could consistently tell the difference between the two in a blind test. If you want the very very best, or are reproducing an authentic vintage amp then NOS are a great choice. But for all other purposes the modern brands also sound great and save you quite a bit too.

Many modern brands also have premium ranges which claim to meet and exceed some of the vintage tubes. However these will cost more than their standard range.

Some possible tangible reasons that old NOS tubes could be better are:

  • Materials used which are now heavily reduced, or not used at all, due to safety concerns (such as nickel). But were actually a better material to use musically. 
  • Standards needed to be much higher back then. Now if a tube fails, your amp breaks. Back then, a super computer or a MIG jet failed. So the old suppliers, who needed to compete fo military contracts, needed the BEST possible quality and reliability.
  • The new manufacturers had to start from scratch because the old designs had been lost. This is before the days of digital backup. Additionally, back then was the period of lifetime jobs. People got very good at their jobs over a lifetime and really honed their craft.

However, other advances in technology can compensate for this.

Experiment if you can and see what sounds best to you and your style.

Tube Amps vs. Solid State

There are two types of amplifiers – tube amps and solid state amps. The solid state amps do use "replacements" for tubes – transistors. They are still fulfilling the same function of diode and triodes tubes, just with an alternative technology. They’re much cheaper too.

With sound it always comes down to subjectivity, but a lot of people do prefer the sound of tube amps. Both types can sound very good. Much will depend on the build quality of the amp itself. But let’s take a look at why tube amps could be better from a technical point of view.

It’s best to think of tubes not just as amplifiers, but also means of adding distortion. Distortion and tube amps are practically inseparable, even if you don’t think you can hear any distortion, it’s there. Just light. We’re not talking about distortion pedal levels of distortion here. Without distortion a guitar could sound thin / weak / empty. It’s the distortion which makes it sound better – warmer and well rounded. This type of distortion compared to solid state distortion is called harmonic even ordered. It’s second order distortion which means more of a musical distortion, more pleasing to the human brain.

Tube amps also have softer clipping - so they overload gradually. The more input, the more distortion. But there's no sudden point where they suddenly start to clip. Unlike solid state, there would be no sharp edges if you looked at the waveform. So tube amps should sound warmer and more well rounded because of this. 

Legendary producer Alan Parsons (Beatles, Pink Floyd) summed it up quite well when he said:

"There’s a place for modelling. It’s usually because of budget or convenience. But I’ve always maintained that guitars invariably sound better with a couple of those glass tubes behind them." 

Problems & Changing Tubes

In this part of the guide we’ll look at changing tubes, and when exactly you should do it. Tubes do go bad, or break. The time will come when you’ll need to change your tubes, so it’s best to know what to look out for. Changing them is pretty easy, however diagnosis of a tube problem can be a little bit harder.

Removal and Insertion

First, let’s assume you know that a tube needs to be replaced. You may be replacing a stock tube for an upgrade, or replacing an old one which may have developed a fault similar to below. The most important thing to keep in mind is that you need to replace like for like. So you could replace a stock ECC83 with a JJ ECC83. But don’t pop a ECC81s in there.

Removal:

  1. First make sure your amplifier is unplugged and has been turned off for a good amount of time. Tubes get very hot and you’ll be touching them – you don’t want to get burnt.
  2. Gain access – in some amps you may unscrew various covers protecting the tubes. Obviously these need to be removed.
  3. Firmly grasp the top of the tube as close to the bottom as possible and gently pull. If it’s not coming out I find adding a circular motion into the mix while pulling helps too.
  4. If you’re really having trouble getting it out (which is rare) you may want to visit an amp tech. Remember that they’re made of glass and you don’t want shards lodged in your hand / amp.

Insertion:

  1. As before make sure your amp is unplugged and off.
  2. Line up the pins in the bottom of the tube to the slots in the amp.
  3. Grasp the tube as close to the bottom as possible and gently use the circular / rocking motion to insert the tube. If it's easier, you may need to hold the tube a little further up. 
  4. Your amp should be good to go.

Preamp tubes don't usually need rebiasing, however once you've settled on a new pair of power tubes a rebias may be needed. Even tubes of the same brand aren't always going to be the same, especially when they're from different batches. Your amp may run fine without a rebias - but I'd certainly recommend you get it done. One of the problems with tubes is their inconsistency - an ideal bias will be different in identical models.

Hearing a Problem

It isn’t always easy to tell that there’s a problem with your tubes, look and listen out for these warning signs.

Disclaimer & safety warning - Tube amps operate at very high voltages and should not be opened and modified by an unqualified person. The most you should do, even to an unplugged amp, is change the tubes. If you have any doubts about anything consult an amp tech. This information should serve as general advice only and act to narrow down potential issues. Amp are complex so what you think is an obvious solution may not be the case. 

Keep Your Old Stock Tubes

Many of the problems listed below can can be confirmed simply be placing an old set of tubes you know work into your amp. If you have one of the problems below, and the stock tube solves it, then you’ll likely need a new set. It may even be a good idea to keep tubes which don’t sound great, but you know still work.

For the more serious problems below (amp not turning on) they can still be a useful tool in helping your diagnosis. Also, if it’s a fault with an amp, and your stock / old tubes blow straight away, you’ll not lose a new set.

Your Amp Just Doesn’t Sound Amazing

This is one of the least serious, and perfectly normal signs. Your amp just doesn’t sound as good as it once did. It no longer has that amazing tone you’ve come to expect. If your tubes have had a good run (several years they usually last depending on use) then they may just be past their best and need replacing.

There may be nothing wrong with them from a technical point of view, but if it drops below what you think is an acceptable level, then a change is needed. The old tubes are good to keep as a test pair for diagnosing potential future problems.

Your Amp Sounds Bad

If you’re hearing a big increase in noise, cracking or distortion which comes on all of a sudden, one (or more) of your tubes may be near the end of its life. If your tubes have been in for a while it may be time to change them. Generally, power tubes wear out faster than preamp tubes – so look at them first. It’s a good idea to change all tubes of the same type at the same time if they're of a similar age, but there isn’t always a need to change ALL tubes in the amp.

If you want to be a nerd about it, try to keep a note of the approximate age of a tube. This will help you narrow down the problem tube.

Drop in Volume

In a way this is similar to above. Your amp may be quieter than it once was. When a tube fails the amp can still work, but it’ll only be running off 3 tubes, for example, not 4. So it’s lacking the extra amplification which the tube would have been providing. Look below for visual signs that a tube has failed.

Power But No Sound

This one may be straightforward. Your power switch lights up but there’s no sound. This will mean there’s either a problem with your amp, or your tubes. Hopefully it’s your tubes as it’s more easily fixed. If you heard a popping noise, saw a flash, or your tubes are a few years old, then there’s a good chance that it’s your tubes. The problem is usually a connection breaking somewhere in the tube. It won’t necessarily be a visible problem because tubes can be very sensitive.

Read below for identifying the bad tube or try one of your old stock tubes in each position until you find the culprit.

Intermittent Volume and Power

This is one of the worst and most frustrating problems because it’s so hard to diagnose and solve. Your amp could be working fine but then have a sudden drop in volume, or even turn off completely. You may think it’s dead only for it to start working again. This may be the first sign of a tube slowly giving up. However, it could also be the sign of something else. Try testing with an old stock tube and see if that solves the problem.

Your Amp Won’t Power On

The panic moment – your amp won’t turn on and you’ve made sure it’s plugged in. The first thing you should do is check the fuse. These exist for safety reasons and to protect your amp. If too much current flows through the device the wire in the fuse melts and breaks the circuit. Sometimes this fuse blowing is the fault of a tube – usually a shorted or failed rectifier / power tube. The fuse blowing is a good thing – it’s protecting your amp from extreme voltages!

As a rule, preamp tubes don’t cause your fuse to blow, they’re too low voltage. So if there’s a problem it’ll be your power or rectifier tubes. Also, it’s unlikely that both tubes will have blown, it’ll usually just be one. The odds of 1+  tubes blowing at the same time are incredibly low verging on impossible. If they have both blown then it could indicate a problem with your amp. It may be an idea to get an amp tech to look at your amp.

If you find that is is just one tube, then replacing the fuse and blown tube could sort your problem easily. 

Seeing a Problem

So you think a tube may be bad, but how do you know for sure? And which tube is it? You don’t always want to replace a full set if it isn’t needed.

Is the Tube Glowing?

This is a good way to tell your tube is broken – it isn’t glowing. After reading how a tube works above, it becomes obvious why. If it’s not glowing, then it’s not producing heat to complete its role in the functioning of a vacuum tube. The signal won’t be being passed. However, you can’t guarantee that a tube has broken if it’s not glowing, it may not be receiving a current (the amps fault). But if you replace with another tube, and it starts to glow, you can be pretty certain it was the tube.

If it still doesn’t glow with a tube you know is ok, then it may be amp tech time.

Is the Getter White?

This one is an easy diagnosis. The getter is the silver coating at the top of the tube which absorbs gasses so it doesn’t interfere with the functioning of the tube. If the getter is white, then the tube isn’t operating in a vacuum because air has leaked in and caused the colour change.

This tube is now useless and needs replacing.

Red Plating

Red plating is when the plate (in addition to the filament) glows red and looks kind of sinister. This means too much power is flowing through the tube and is a bad sign. Very soon the tube will die. Sometimes the amp can function ok while red plating but it’s not recommended to use it.

Red plating is the result of a bad bias adjustment or a bad tube. If you notice this after a bias adjustment, or on more than one tube, then take it (back) to the amp tech. It’s probably the amp. Certain tube brands just deal better than others with a bad bias adjustment, so may not red plate.

But this isn’t always a good thing – the bias may cause problems later which you won’t be able to diagnose due to the lack of red plating. Tubes which can red plate easily aren’t always a mark of bad quality either, they’re only red plating because they’re not being used as intended.

If it’s just one tube red plating, out of the blue, then it’s likely to be a faulty tube – replacing the tube may fix the problem.

Tube Microphonics

Microphonic tubes are annoying because they add noise into your tone from small vibrations which are created in your amplifier. Sometimes the sound from the amp vibrates them enough to hear the microphonics.

It’s pretty easy to test for a microphonic tube. All you need to do is take a pen and gently tap the tube while in use. You’ll hear a clear noise coming through your amp if it’s microphonic. Sometimes tubes can arrive microphonic (usually damaged through transportation). Or they can develop the fault over time. You’ll usually want to replace the faulty tube.

What if the Amp Still Doesn’t Work?

Tubes aren’t always the problem if your amp dies. It could be something else. In rare cases it’s possible that a bad tube has damaged something in your amp. So, a part or component will need to be replaced. Although, most of the time it will be a biasing issue and the fuse should stop damage in most cases.

Tube amps are pretty complex and it could be any number of things for an experienced amp tech to diagnose.

Other Terminology

There are two more terms you may come across not mentioned in this guide:

Burning in - Burning in is the process of running a tube before it’s actually used properly. Any potential contaminants on the surface of the internal tube are removed during the burn in phase. JJ tubes do come pre burnt in, as do many other brands, but they should improve within the first several hours of using the tube.

Tube rolling - Tube rolling is where you try out different tubes in the same slot. For example, you'd try and compare a JJ ECC83s and a Sovtek ECC83 and decide which you like best. This means you could end up with a range of brands in your amp. With experience, you'll get to know the brands. So you may think "I wish my tube had more gain", so you could remove 1 of the 4 ECC83 you may have in there and replace it with a Shuguang ECC83. NOTE - when rolling power tubes, it's best to replace a whole set at a time, and not just replace an individual one.

Extra resources

Guide for rebiasing an amp:

https://rockettpedals.com/tube-amp-biasing-at-home/

More in depth info about how a tube works:

https://robrobinette.com/How_Tubes_Work.htm

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