Guitar Tube / Valve Amp FAQ

Not really, you should allow your amp to "warm up" before playing through it. (See next question for explanation.) Also, use that amp cover when you take it outside! You never know when it will suddenly start raining. Other than that, just be sure to treat it with as much care as you would any other amp. (i.e. don't store it in a damp place, etc.) Use common sense.

The rule-of-thumb is to flip the power switch ON, but leave the left-hand switch on STANDBY. This will allow the tubes to warm up. It takes no longer than 30 seconds to do so, but wait at least 10 to 15 seconds. I've heard some people say, "wait a few minutes" or "wait about five minutes"—this is unnecessary. The worst is, "the longer you leave the amp on standby the better it will sound." Totally false. The standby switch essentially allows the tubes to heat up before applying your guitar's signal to them. This prevents the tube from being "shocked" with high voltage before it's ready to go. This is why amps without standby switches sort of fade in when you turn them on—the tube isn't hot enough to amplify yet.

The popular opinion is that using the standby correctly will increase the longevity of the tubes. This is true for some amps, but not so for others. Whether it truly makes a difference in the Deluxe and Deville is rather questionable. I'll unravel this later in the Intermediate section.

There is no correct way. You can turn it straight off, unplug it from the wall, put it on standby first, etc. It flat out does not matter.

No. The lives of the tubes may be slightly shortened, this happens anytime they're used, but it will not cause any major problems. I've heard of people accidently leaving their amp on standby then going on vacation for a week or longer, and it didn't hurt their amp at all.

Again, this will not hurt the amp. It will, on the other hand, shorten the lives of the tubes much more than just leaving it on standby. I wouldn't worry too much about it. Playing your guitar through it for the same amount of time will cause more damage to tube life than just leaving it on.

Distortion is the sound most associated with rock music, and is produced by amplifiers. Metal often uses extreme amounts of it, while jazz players rarely use any at all. On the other hand, rock and blues players usually like light to modern amounts. Most modern amplifiers have a clean and dirty (distortion) channel. The amount of distortion put out by our amplifier is usually controlled by a knob labeled "drive," "gain," "saturation," "preamp," or simply "distortion." (technical explanation of distortion)

The tube is what amplifies your guitar's sound. The signal coming out of your guitar is very weak and needs to be intensified in order to drive a speaker.

They don't actually look like a hollow tube, but like a weirdly shaped glass light bulb. They come in all kinds of shapes and sizes. In combo amps you can almost always see them just by looking in the back of the amp. A lot of amplifier heads (like Marshall) require that you look inside the enclosure. This can usually be done without taking the head apart.

Yes. The filament, also called the heater, is supposed to glow. The plate on the other hand usually means the tube is about to die. If the plate glows a cherry red color YOU'LL KNOW IT!

An example of florescence in a vacuum tube.

Does the amp sound fine? More than likely you're perfectly okay. Blue glow is not necessarily a bad sign. The only type of blue glow we should ever see in audio equipment is known as florescence—which is "completely harmless."

"Fluorescence: this type of glow is usually violet in color and most noticeable around the inside surface of the glass bulb. It is most pronounced on power tubes and is the product of electron bombardment of the glass taking place within the tube. It generally has no adverse effect upon receiver performance, and in fact, tubes displaying this phenomenon are particularly good with respect to gas content."

REFERENCE: http://barney.webace.com.au/~electron/tubes/blueglo.htm, January 10th, 2005.

Thanks go to Michael John Rench for submitting this helpful photo, which demonstrates florescence.

Yes, it's supposed to be there. The "burned" area is actually a substance stuck on the inside of the glass envelope that absorbs air to ensure the vacuum. (Technical explanation)

You probably don't need to, but having a small fan circulate air into the back of a tube amp isn't a bad idea. A small US$5-10 fan would work great.

The preamp of an amplifier is used to "color" the tone. For example, your tone controls (treble, mid, bass) are located in the preamp, as is the reverb. The preamp's purpose is to shape the sound, then feed it to the power amp. The preamp usually consists of one to nine preamp tubes, depending on the amp.

The power amp takes the signal from the preamp then does most of the amplification that will "drive" the speaker. The power amp's original purpose was to only make the preamp signal "louder," though the power amp also colors the tone. The power amp has at least one power tube, though usually two or four, and also consists of an output transformer and a phase inverter.

The big tubes are the power tubes, the smaller ones are the preamp tubes.

The power tubes put out a lot of wattage, so they also create a lot more heat. Our power tubes are made to handle about 25 to 30 times more wattage than our preamp tubes, so they must be larger. For example, say I have two pieces of metal: One is larger, and one is smaller. If I tried melting both pieces, which would take longer? The larger would, of course. All electronics made to handle extra heat, whether it's a resistor, tube, transistor, diode, etc, will generally be larger in size. This is for reliability.

Yes.

No. In the USA they're known as 'vacuum tubes,' and to the Brits they're known as 'electron valves.' Somewhere along the line people started mixing and matching words. But it all means the same thing.

Yes, touching your tubes too much will make you go blind! Errmm.. when any tube amp is ON the tubes will get very hot—especially the power tubes. This is essential since tubes operate on the principle of thermionic emission. If they didn't get hot, or at least warm, they wouldn't work. Touching them with your hand may leave a burn.

My dad even told me this one! I've read that the oil from your hands will build up on the tube causing it to run hotter than it should. Yeah, right. 1. In combo amps most of the heat goes up into the base of the tube, rather than out the side of the glass. 2. A minute bit of oil from your hand is NOT going to affect the tube's reliability/performance. Manufacturers print their logo on them don't they? Does that cause heat dissipation problems? Come on, I think cramming the tube up inside a metal chassis has more affect on heat dissipation than your fingerprint. Please, do not worry yourself over something as silly as this. You should, on the other hand, refrain from touching the tube's pins a lot. If you do accidently touch the pins there's a 99.99% chance that nothing will happen, but if you're having some weird problems cleaning the pins will sometimes solve them. Occasionally, after moving your amp to a gig or practice, you should make sure the tubes haven't shaken loose. Just give them a little push upwards from the bottom with, you guessed it, your hand.

False. The person who said this probably believes that the tubes are still hot and therefore more fragile. Sort of, but not really. If you go tossing your amp around like a football you're going to damage something, and the first to go may or may not be the tubes. Simply moving the amp from the stage to your car is not going to make a difference—I've done it hundreds of times. Trust me, the low frequencies generated by our amp will cause way more long term damage than moving your tubes before they've cooled to room temperature. Once again, it's nothing to worry about it. A lot of amps just have the tubes suspended and held into place by a socket. They're not touching anything that would cause them to be damaged simply from moving your amp. Tube shields are there for support and to help heat dissipation, so they wouldn't be a problem either. Some amps, like Hot Rods have foam that helps protect the tubes from microphonics and other general roughness.

When they die or become microphonic. Before the other guitarist in my band met me, he used to change tubes once a year. What a waste! The average tube, when gigged by a professional musician, lasts about 3 or more years before burning out. NOS tubes were made to smaller tolerances and are usually higher quality, therefore tending to last longer. I'd learn how to bias your amp and keep a spare set of tubes just in case one prematurely flies south or goes microphonic. This way if you're at a gig you can just replace the problematic tube.

If a tube is "microphonic" it will "howl," or feedback, like a microphone. No tube is free of microphonics, and some say that a little is actually wanted for a livelier sound. Microphonics are caused when the insides of a tube start to loose their solid construction from vibration. There is no fixing this. All we can hope for is that our tubes are well constructed. (technical explanation of microphonics)

If a tube rings like a telephone it suffers from "filament rattle." Like a microphonic tube it should be replaced. Filament rattle isn't technically the same since the tube isn't acting like a microphone. Often you'll hear people mistakenly identify filament rattle as microphonics.

It's well known that you can tap on tubes to hear if they're microphonic or suffer from filament rattle, but BE GENTLE. You can damage the tube when it's hot. Also, don't use your finger! If you hear the tapping being amplified don't freak out, as said before all tubes are microphonic. Tapping should be reserved for when you know you have a badly microphonic tube, but need to identify which one.

Wait for the glass envelope to cool to room temperature. Simply pull the tube the tube out of its socket. Take a look at the new tube's base. If you're installing power tubes or a tube rectifier, often referred to as octal type, there is a 'key' in the center of the pins. Before inserting it you must align the key with the hole in the socket that accepts the key. You'll figure it out. Preamp tubes are a little different, they are one pin short of having a full circle of pins. The preamp tube socket is the same way. Position yourself so you can see how the pins go into the socket. Align the preamp tube and insert. Simple.

There are so many different levels of quality when it comes to vacuum tubes it's hard to say. One thing is certain though—if a tube is ran out of spec its longevity will be decreased.

Headroom is a term used to describe how loud a clean channel will go before 'breaking up' into distortion. This is often associated with the amp's wattage, but this is rarely accurate. Yes, a 100 watt Fender Twin has much more headroom than a 6 watt Fender Champ, but there are many other factors that contribute to headroom.

• The amount of gain stages in the preamp.
• The efficiency of the speaker and output transformer.
• Whether the rectifier is tube or solid state.
• The output (or signal strength) of the pickups
• How hot the preamp and power tubes are biased.

When we 'bias' our amp we're setting the working condition of our tubes. 'Setting the bias' on your amp is a lot like setting the idle on your car. If it's set too high (or "hot") the car's running away with you, but if it's too low (or "cold") it will choke when you give it some gas. To put that into guitar terms, too high (or "hot") and your tubes may run out of spec and die prematurely; too low (or "cold") and your tubes will sound cold and sterile. We don't want to be at one extreme or the other, but preferably at a nice spot in the middle. We want our car to run its best just like we want our tubes to last long and sound their best. In that sense we're trying to find the perfect in-between point when biasing. That's basically it! We bias to make sure our tubes are operating in a safe range and sounding their best. Click here to learn more about biasing, and how to bias a Fender Hot Rod.

We'll only need to bias the power tubes. I often hear people say, "the preamp tubes don't need to be biased"—this is right and wrong. Technically, all tubes must be biased, but power tubes are the only ones you need to think about. If a tube wasn't biased it simply wouldn't work! Preamp tubes use 'cathode bias,' also known as 'self-biasing.'

Only if getting the best sound possible is of any importance to you, but in a fewer words—probably not. As long as you keep using the same type and brand of tubes you previously had, then the bias shouldn't be that far off. If you go switching from 6L6WXTs to 6L6GCs, or from Sovtek to JJ, then rebiasing becomes an important issue in my opinion. If you put new tubes in, and the bias is too high, don't expect the tubes to last long. If the bias is too cold your signal will lose a great deal of its linearity from crossover distortion, and the amplifier may sound lifeless. These are really the only concerns. Biasing your Hot Rod is easy and takes little time or effort. Learn how.

False. It depends on the class of operation (Class A or Class AB). Due to the nature of their design, the bias of Class AB power amps is more sensitive to tube changes than Class A—the latter typical biased at maximum plate dissipation.

Without getting too technical, all you really need to know about Class A is that it's a method of amplification that's been blown out of proportion—mostly due to marketing hype. Whenever an amplifier is referred to by a class of amplification it is always assumed that we're talking about the power amp, because all preamps are Class A. (Now it is certainly possible to set your preamp up as Class AB, but it would be so cumbersome no one has bothered to do so, nor will anyone ever bother.)

I think the reason Class A has become so hyped is because laymen tend to think Class A is synonymous with "Grade A." In other words, they believe Class A is somehow better because of the letter "A." This is an incorrect assumption. The reason Class AB was developed was to make up for the inefficencies of Class A power amplifiers. The reason for the letter "A" was because it's the first method of amplification that was developed, and therefore is also the simplest. This is why many old "student level" amps were Class A, because they are simple and therefore less expensive.

The vast majority of guitar amplifiers that claim to be Class A are actually something called "cathode biased," which really has nothing to do with the class of operation. This has led to the invention of a new non-technical phrase, "true Class A."

The exact same thing as Class A. "True Class A" is a catch phrase that was invented by boutique builders who wanted to let buyers know that their amp actually was Class A like it claimed. However, like the phrase "Class A" it has also been subjected to abuse and hype. Here is an example..

"Good features but the tube thing is really lame and ultimately just a marketing gimmick by Korg. Only true Class A tube circuitry is worth raving about and that would have cost Korg another 1000 dollars to include that- any other forms are just marketing gimmicks. Just like all the tube circuitry in effects pedals and cheap 200 dollar tube preamps- they are not Class A either."

Anonymous review of the Korg Triton Extreme synthesize on harmony central

While I agree there is a certain level of hype when a tube is used in a synth and called "Extreme", the reviewer has absolutely no idea what he's talking about as far as Class A is concerned. The synth uses a 12AU7 preamp tube which, because of the nature of preamplification, is undoubtfully cathode biased Class A—or what the reviewer calls "true" Class A. The same goes for pedals with preamp tubes—all cathode biased Class A.

Remember, the only time Class AB is ever used is for power amps because of its greater power efficency. Preamplification is used to shape your tone—it is not used to create large amounts of power! The synthesizer (and aforementioned pedals) do not have power tubes to drive a speaker—which would also require an output transformer. The reviewer also fails to realize that Class A is far less expensive than Class AB to implement as preamplification—the $1000 extra claim is just ludicrous. I included this quote because it was a perfect example of the many who regurgitate misinformation and unwittingly spread hype. Be careful where you get your information! I've even met professional techs who honestly don't have a clue. Also, take everything your local GC guys say with a very large grain of salt.

Basically, the audio-practicality of Class A combined with the efficency of Class B. Hence, Class AB.

It depends on who you talk to, but it depends more so on the overall design. The most sought after amps in the world are probably the tweed Fender Bassman, Marshall Plexi, tweed Fender Deluxe, blackface Fender Twin Reverb, blackface Fender Super Reverb, Vox AC30, Ampeg SVT, and Marshall JCM800. The vast majority of modern amps are simply clones of these amps, or are slightly modified versions (i.e. more features). I'll let you in on a little secret.. they're all Class AB! Dun Dun Dunnn! Yes, the Vox AC30 isn't Class A like it claims, but cathode biased Class AB that's biased on the hot side. These amps are loved because their overall design is good; it really has little or nothing to do with the class of their power amp.

The most famous Class A amp is probably the Fender Champ.

There's one way to be certain: if the amp only has one (1) power tube—like an old Fender Champ. If there's two or four power tubes the amp is more than likely Class AB. We could make a slightly better guess at ruling out Class A if we have the amp's schematic, but to know for sure we'd have to break out a function generator, oscilloscope, and dummy load—then run tests. Another hint: due to their inefficency and heat generation Class A amps tend to be very low wattage (i.e. if the amp is over 15 watts it's probably not Class A).

Only if packed with dynamite.

In the United States vacuum tubes are identified by a naming system that is fairly standard.

1. The first part of a tube designation is a number which tells us the approximate voltage required for the heater of the tube. For example, the heater of a 6J5 requires 6.3 volts, the heater of a 5U4 requires 5 volts, and the heater of a 50L6 requires 50 volts. 12AX7s, which are dual triodes, require 12.6 volts when wired in series, but when wired in parallel, as in almost all musical instrument amplifiers, they require 6.3 volts. It's no coincidence that the 6L6-GC requires 6.3 volts, this allows us to hook them to the same voltage source.
2. The second part of a tube designation is a code letter (or group of letters) for identifying the tube types and functions.
3. The third part of a tube designation is a number which sometimes indicates the number of useful elements brought out to the base terminals. For example, a 12AX7 has seven elements if we consider the filaments (pins 4, 5, and 9) as one element.
4. The fourth part of a tube designation, if included, will consist of a letter or letters describing some particular characteristic of the tube. For example, a 5Y3-G has a glass (G) envelope. A 5Y3-GT is the same as the 5Y3-G except it's glass envelope is smaller (GT) than that of the G.

It must be emphasized that the following descriptions are generalized, and that there are many exceptions to what's listed here. Also note that Europeans use a different system. The only method of retrieving accurate information about a tube is to refer to its spec sheets.

REFERENCE: Zbar, Paul B., and Sid Schildkraut. "Job 28, Vacuum Tubes; Diodes Characteristics" Basic Electronics: Laboratory Manual for Radio and Television Technicians, McGraw-Hill, 1956.

The getter is usually a halo shaped metal structure typically near the top of the tube. It is coated with a highly volatile powder, usually a barium compound similar to that used on the cathode for electron emission. Once as much gas as possible has been evacuated by mechanical means, the tube is sealed and the getter is heated. The barium powder explodes, which consumes any remaining gas and completes the vacuum. The force of the explosion throws molten barium onto the inside of the glass envelope, which appears as a "dark" or "burned" looking area of the glass. Care is taken so that the getter material does not condense where it could interfere with the normal operation of the elements of the tube. Usually, the getter material is condensed at the top of the tube, which creates a mirror-like surface. If the dark area (getter material) has turned white the tube has lost its vacuum and should be trashed.

REFERENCE: Eastman, Austin V. "Getter" Fundamentals of Vacuum Tubes, Second Edition, pg. 32-33, McGraw-Hill, 1941. New York.

If air is not evacuated from the tube, electrons will likely collide with air molecules while traveling from the cathode to the plate. Often the electrons will stick to an air molecule, creating a negatively charged ion. (Some air molecules will glow an unusual color when ionized.) Overall, "gassy" tubes will behave erratic and are not useful as amplifiers or rectifiers. By removing as much air as possible we greatly lessen the chance of an electron's path to the plate being obstructed. According to Austin Eastman, if a tube is properly evacuated an electron will pass between two atoms as readily as a pea will pass through a hole with a diameter as large as the earth. Therefore, the tube will operate predictably since it is unlikely that an electron will collide with a stray atom.

REFERENCE: Eastman, Austin V. "Effect of Gas" Fundamentals of Vacuum Tubes, Second Edition, pg. 45-46, McGraw-Hill, 1941. New York.

Yes, but these tubes are not used for amplification. Gas filled tubes were used mostly for simple ON/OFF operations in industrial machinery back before transistors. Gas filled tubes were useful in that they allowed more current to flow than vacated tubes. The negatively charged ions would flow towards the plate and the positive ions to the cathode. So we could have current flowing backwards to the cathode with gassy tubes, making gassy tubes useless for rectification.

REFERENCE: Eastman, Austin V. "Effect of Gas" Fundamentals of Vacuum Tubes, Second Edition, pg. 45-46, McGraw-Hill, 1941. New York.

Example of similar basing between the octals EL34 and 6L6GC.

• Pin 1: suppressor grid (6L6GC has it tied to cathode internally)
• Pin 2: heater
• Pin 3: plate
• Pin 4: screen grid
• Pin 5: control grid
• Pin 6: (not used)
• Pin 7: heater
• Pin 8: cathode

Basing refers to the order that the tube's elements (cathode, plate, grids, heaters) are connected to the pins at the base of the tube. The 6L6, 7581A, 6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. are all common guitar amplifier power tubes, and all share similar basing. The whole 12AX7 family (12AT7, 12AY7, 12AU7) which are commonly used in guitar amplifiers also share the same basing.

REFERENCE: Zbar, Paul B., and Sid Schildkraut. "Job 28, Vacuum Tubes; Diodes Characteristics" Basic Electronics: Laboratory Manual for Radio and Television Technicians, McGraw-Hill, 1956.

There are many types of distortion, but the type most referred to in "tone creation" is amplitude distortion. This is caused when an amplifying device, such as a tube or transistor, is conducting as much current as it possibly can or if it has gone into cut off with no current. In both cases the intended current is being distorted. This can sound gritty and nasty, or smooth and beautiful depending on the device being used and the level of conduction. Tubes are usually chosen for musicial instrument amplification because of the way they distort. There are solid state devices, called FETs, that distort very similar to tubes and therefore can be designed to sound similar.

Distortion is actually used to describe the relationship between the input signal and output signal of a gain stage. The tube itself doesn't distort, but actually reaches a point called saturation where it is fully conducting. Since the tube or transistor can not conduct anymore, it can not continue reproducing a linear (or exact) copy of the input signal, and therefore the output signal looks "distorted" when compared to input signal. Distortion is also called "clipping" since the amplitude of the distorted AC alternation will be smaller than the other.

Loose electrodes, the metal pieces inside a tube which conduct electricity, are affected by vibrations generated by the loudspeaker. This causes the space between the electrodes to vary, and as a result the electrical fields to vary within the tube. This changes sound into current vibrations, which is positively fed back through the tube, and an annoying howl results. These vibrations can affect the electrodes through the socket or the base, or by sound waves striking the glass envelope of the tube.

To help control microphonics Fender surrounds some tubes with a spongy, sound-absorbing material. In other amps and old radios mounting the sockets on rubber may also help. If both of these precautions fail the tube is too microphonic to be used, and should be replaced. You don't necessarily have to throw it away, as it may work fine in an amplifier with a separate head and cabinet.

REFERENCE: Marcus, Abraham. "The Electron Tube: Service Notes" Radio Servicing: Theory and Practice, Third Edition, pg. 165, par. 2, Prentice Hall Inc, 1960.

Crossover distortion is caused when a Class AB amplifier is biased too "cold." Instead of the power tubes biased so that each half of the alternation is put back together to form a full sine wave, there are "gaps" between the alternations, which form a "broken" sine wave. The gaps between each alternation can be heard by the guitarist, and are interpreted as unpleasant high odd order harmonics by the brain. Note that the crossover distortion in the picture is exaggerated for educational clarity.

Black plates are making a comeback. Many think that modern tubes with black plates are just trying to look like the old RCA power tubes from the 50s, and have no real purpose. The motive of the manufacturers is unknown by me, but there is a benefit to having black plates—believe it or not.

"The plates of power valves and rectifiers are frequently blackened to increase their heat radiation and thereby reduce their temperature."

- F. Langford-Smith, ed. "Introduction to the Radio Valve; Section 2: The Component Parts of Radio Valves; (iii) Plates" Radiotron Designer's Handbook. Fourth Edition. pg 5, paragraph 5. Wireless Press, 1953.

Now you know! Feel free to impress (or bore) your neighbor with your new found expertise on black plated vacuum tubes!

First, the voltage is only inverted by a common cathode gain stage where the the output voltage is taken off the plate. You also need to know that AC voltage and AC current are 180° out of phase. The reason the voltage is inverted is because the voltage on the grid controls the current through the tube. Since the input voltage and output current are in phase with each other, the output voltage with therefore be 180° out of phase with the input voltage.

All the books I've seen that define this assume the amp is fixed bias, but the amp can be cathode biased as well. Let's say there's -35DCV on the control grid, which isn't all that unusual. If the signal's amplitude is ever greater than 35ACV the control grid will go positive (i.e. 0 volts or above). Grid current will begin to flow. The reason grid current flows is because the control grid is positive, and the negatively charged electrons boiling off the cathode are attracted to it—this results in a sort of negative feedback.

In a Class AB₁ amplifier the voltage on the control grid will always be negative. That means that if there's -30DCV on the control grid, the AC signal will never be greater than ± 30ACV—or 60ACV peak-to-peak.

In a Class AB₂ amplifier the voltage on the control grid will be driven positive by a very large AC input signal whose amplitude is greater than the voltage applied to the control grid.

REFERENCE: Marcus, Abraham, and William Marcus. "Vacuum-Tube Amplifiers: The Class AB Amplifier" Elements of Radio, Second Edition, pgs. 578-580, Prentice Hall Inc, 1948.