VACUUM TUBE FAQ
FREQUENTLY ASKED QUESTIONS

Question: What is a vacuum tube?
Answer: A vacuum tube is an electronic device consisting of a minimum of four active elements: a heater (filament), a cathode, a grid and a plate, all sealed in a vacuum glass enclosure to prevent parts from burning. Once heated, the cathode begins to emit electrons, which flow from the cathode (which is negatively charged) toward the plate (which is positively charged). The grid’s purpose is to control this flow, in effect, acting as a valve, which is why tubes are called "valves" in the U.K.
 

Question: What is a JAN tube?
Answer: The term JAN stands for JOINT ARMY-NAVY. These are tubes that have been manufactured to a military specification or have been specifically selected for a military application. Most of the current stocks of JAN tubes are NOS (New Old Stock) tubes that were manufactured in the 1970s and 1980s. These tubes were either ruggedized during manufacture or selected from a very tight specification for highest performance and reliability. Since these tubes are no longer in production, when the current supplies are sold there will be no more.
 

Question: What is grid leakage?
Answer: Grid leak is the small current through the grid of a vacuum tube into the circuit feeding the grid. It is an inherent operating characteristic of triodes and other multi-grid vacuum tubes. This current is caused by the small negative voltage present in all vacuum tubes as a result of a space charge within the envelope of the tube. This space charge is part of the thermionic effect, which is the fundamental phenomenon behind all vacuum tubes.

Grid leakage is also a term that is frequently incorrectly used to describe the condition of a tube when tested on a tube tester. The correct term is inter-electrode leakage, which refers to leakage paths between the elements of a tube. Many emission type tube testers have leakage tests that far exceed the application of the tube being tested and will reject many perfectly functional tubes. Sencore sold tube testers in the 1960s that were claimed to have the "most sensitive leakage test in the industry".
 

Question: What are soft, hard, and medium rated tubes?
Answer: When power tubes are matched, they are frequently classified by the distortion and break-up characteristic. The most common method is to refer to the tubes as Soft, Medium, or Hard.

Soft tubes reach saturation and break-up quicker. These tubes have lower plate current (Ip) and transconductance (Gm) matching numbers. They are preferred by blues guitarists for the break-up and sustain they provide.

Hard tubes have the highest amount of clean headroom before break-up and distortion. These tubes have high plate current (Ip) and transconductance (Gm) matching numbers. They take longer to reach saturation and are preferred by jazz, country, and bass players. Hard tubes are also used by guitarists who rely primarily on effects pedals to generate distortion.

Medium tubes fall in between soft tubes and hard tubes. They exhibit good headroom, but will break up and distort when pushed. These tubes have plate current (Ip) and transconductance (Gm) matching numbers in the middle of the range. Classic rock guitarists and players who play a wide variety of styles generally use medium rated tubes.
 

Question: What is the best 12AX7 for my amplifier?
Answer: The actual sound of a 12AX7 depends on the design of the amplifier. Other factors include the type of sound the user is looking for, the gain structure of the amplifier, and the type of 12AX7 that was used during the design process of the amplifier. The best way to decide which 12AX7 to use is actually purchase several different types and actually try them in the amplifier to determine which one sounds the best in your amplifier. Experienced technicians do this. They know that a certain type of 12AX7 tube may sound better in one amplifier, where a different 12AX7 may sound better in another amplifier. Since 12AX7 tubes are self-biasing, they can readily be swapped without any adjustment to the amplifier. The following description of the 12AX7 tubes manufactured by New Sensor Corporation will provide a helpful guideline to assist with the selection process when trying different types of 12AX7 tubes.

The Sovtek 12AX7WA/WB/WC series of tubes are general-purpose workhorse types that provide reliable performance in a variety of applications. These tubes are frequently found as OEM tubes in many different brands of amplifiers. The 12AX7WB has about 6% more gain than the 12AX7WA and a slightly darker sound signature, which is great for smoothing out harsh sounding amplifiers. The 12AX7WC has closely matched sections making it ideal for phase inverter circuits. The 12AX7WA/WB/WC types hold up better than other 12AX7 tubes in cathode follower circuits where the maximum cathode to heater rating of 100 volts is often exceeded.

The Sovtek 12AX7LP/LPS series has a large plate format that gives a large soundstage with a lot of detail. The 12AX7LPS also has a spiral filament for reduction of heater to cathode induced hum in amplifiers with AC powered heaters.

The Svetlana 12AX7 has performance that falls between the Sovtek 12AX7WA/WB/WC series and the Electro-Harmonix 12AX7EH. It has a slightly different internal geometry that makes an additional "flavor" of 12AX7 available.

The Electro-Harmonix 12AX7EH is a high-gain, low noise type with a short plate format for reduction of microphonics in high gain amplifiers. It also features the spiral filament for hum reduction.

The reissue Mullard 12AX7/ECC83 has the same features as the Sovtek 12AX7LPS, with a geometry and transfer characteristic that replicates the sound signature of the large plate Mullard ECC83 tubes that were popular in the 1960s and 1970s.

The reissue Tung-Sol 12AX7 has the highest gain of all of the 12AX7 types. It has a very full and musical sound signature. This is an excellent tube to improve the sound of bland and thin sounding amplifiers.

The Sovtek 5751 is a special version with 70% of the gain of a normal 12AX7. This tube has low microphonics and is useful for lowering the gain of an amplifier to provide a clean, bell-like tone that is reminiscent of Stevie Ray Vaughan’s "Riviera Paradise".

The Electro-Harmonix 12AY7EH/6072A is another tube that is useful in lowering the gain of an amplifier. It has 44% of the gain of a 12AX7 with a higher transconductance making it more touch sensitive. 12AY7 tubes were frequently used as the first preamp tube in most Tweed-Era Fender amplifiers.
 

Question: How does New Sensor/Electro-Harmonix match power tubes?
Answer: The New Sensor/Electro-Harmonix Tube Matching System uses a test fixture that is based on the information provided in MIL-STD-1311 Test Methods for Electron Tubes. This document is the military standard for vacuum tube testing. This test fixture is connected to a special computer controlled switching matrix that selects each tube individually on a tray that holds forty tubes. This allows us to test and match large numbers of tubes to meet the demands of the musical instrument amplifier and hi-fi audio industries, as well as service shops and technicians that maintain this equipment. The system is powered by heavy-duty regulated power supplies that are continuously set and calibrated for each tube type through proprietary computer software that is referenced to a Hewlett-Packard/Agilent 34401A digital multi-meter. The calibration is tracable to NIST (National Institute of Standards and Technology). Each tube, when selected by the switching matrix, is put through a battery of tests, including inter-electrode leakage, positive grid current, screen grid current window, and high/low plate current limits, prior to the actual matching process. When the tube passes these tests, it is then matched to within 1 ma. plate current (Ip) and 100 microohms of transconductance (Gm). When the system has tested and matched the entire tray of forty tubes a sheet of labels indicating test results and matching information is printed by the computer. The tube tray is then removed from the matching system, replaced with another, and the process is repeated. The tubes are then put into pairs, quartets, or sextets with the labels indicating Ip and Gm attached to the boxes and the tubes banded together. The matching is very accurate and repeatable. This precision matching ensures that the sets of tubes will wear evenly, sound better, and last longer.
 

Question: What is the truth about tube testers?
Answer: The best test for a tube is in the actual piece of equipment the tube will be used. It is common for design engineers to build a mockup of the circuit being designed with meters and oscilloscope connections in order to evaluate the performance of the tube under the various operating parameters encountered. Early in the development of radio special tube/set testers were used where the tube was removed from the radio, the tester was plugged into the tube socket, and the tube was plugged into the tester socket. The radio and tester were turned on and the condition of the tube was read on the tester meters. This worked well on old 4 pin simple rectifiers and triodes. As more tube types were developed and circuits became more complex, these simple testers did not work or provide enough information to properly evaluate the condition of tubes operating at wide ranges of voltage, current, and signal waveform. The high cost for many adaptors and wide range of equipment required became impractical and not cost effective.

The service tube tester was developed for the telephone, radio-TV, communications and industrial electronics industries to provide basic tube testing capabilities to help technicians and engineers locate defective tubes. The early testers only tested cathode emission. They worked fine in the early days of the industry before the circuits and tubes became more sophisticated.

Over the years there were many approaches to tube tester design. The features, accuracy, and the tests available differ widely by make and model. Some manufacturers wanted to focus on simple low cost units to find weak or just bad tubes. In all cases tube testers were at best a set of balances and trade-offs in the evaluation of the tubes vs. the cost of the tester. Even the best service testers made trade-offs in design to allow for simplicity of use, the ability to test many different types of tubes, features, and accuracy, balanced by the price of the tester. The service tester was a piece of test equipment to be used by professional engineers and electronic technicians to aid in the process of repairing tube electronic equipment. Testers were designed with the understanding that those using them were knowledgeable in tube operation, the equipment the tube was used in, and how the tester worked in evaluating tubes. This is more often not the case today when someone acquires a tube tester and attempts to use it!

There are many types of service tube testers and most of them date from the early 1950s to late 1960s. The usefulness of each make and model will vary with the type of equipment you are servicing. Considering the fact that these testers are vintage, the age, actual usage, and condition of the tester will have a big impact of how useful it is in testing tubes today. Over the years, moisture, dust, and dirt can be absorbed by tube sockets, switch wafers, and wiring causing many leakage paths that would contribute to false readings. Calibration of the tester is another factor that will affect accuracy and usefulness of the tester. Many simple emission testers do not have any provisions to make internal calibration adjustments. Another interesting note concerns the military TV-series of mutual conductance testers designed by Hickok. The TV-series testers were ruggedized military versions of commercial Hickok testers. When purchased from military surplus the buyer would usually find a sticker affixed to the tester that reads "CALIBRATION NOT REQUIRED-NOT USED FOR QUANTITATIVE MEASUREMENT". The only time these testers were calibrated was when they were manufactured or when they were repaired or overhauled.

Service testers fall into two main categories- emission testers and mutual conductance testers. Other types of testers would include laboratory and special purpose testers.

Emission testers are the most common testers. Popular brand names include Eico, Heathkit, Mercury, B&K, and Sencore. There are a few problems using any emission tester. The emission test basically connects all of the elements of the tube together, except the cathode, and tests it as a diode for cathode emission. The meter scale is most commonly labeled "Bad-?-Good". Most tubes other than diodes depend on the control of the electron flow rather than the amount of electron flow. This important shortcoming means that emission testers will miss the output tube with a cathode ‘hot spot’ which is hidden when the grid is tied to the plate. When a cathode has a ‘hot spot’ most of the emission current stems from this restricted area on the cathode. A control grid does not have the current control action under this condition and when the tube is placed in an amplifier, it draws excessive current and goes into thermal runaway. Also, cheaper emission testers used a low current power supply. A 5U4 rectifier tested on a tester with a 100ma power supply may have enough emission to yield a "Good" reading on the meter, but when the tube is placed in an amplifier that is drawing 150ma to 200ma, the tube may not be able to provide sufficient current for operating at peak efficiency. Emission testers frequently operate at low voltages, with some testers as low as 30 volts. These testers only provide a static test of the tube that does not represent the conditions the tube will be subject to in an actual circuit in which it would be used. Some emission testers apply too much current to small signal tubes and when the emission test button is held down on the tester for an extended period of time, the cathode is stripped rendering the tube useless. Another shortcoming of many emission testers is the leakage test. In testers where all of the elements are connected together all of the leakage paths are in measured in parallel, which could cause perfectly functioning tube to fail the leakage test. Some testers, such as the Sencore Mighty-Mite series testers were advertised as having the most sensitive leakage test in the industry. This many times was more stringent than circuit application requirements and again causing many functional tubes to fail testing.

Most mutual conductance testers work by applying an AC voltage to the control grid of the tube, while maintaining DC voltages on the plate and screen grid. Most of these testers use the 60Hz line frequency transformer coupled as the input test signal. The cathode can be biased with a small positive DC voltage, or the control grid can have a small negative DC voltage. This setup actually dynamically measures the AC gain of the tube, rather than the actual transconductance. A variation of this type of tester is the "grid shift" approach. This uses a DC voltage on the control grid that is shifted (usually 1 volt) and the change in plate current is measured. Tube theory tells us that transconductance is the ratio of change in grid potential to the change in plate current. The "grid shift" method is a static test. Engineers believe the AC (dynamic) method is superior because it reflects true RMS values, regardless of waveform distortion. If the line voltage is not a true sine wave, which is common in heavy industrialized areas, the dynamic tester will still indicate correct values. Hickok held the patents for and made most of the mutual conductance testers. Hickok also designed testers for Western Electric, the military TV-series of tube testers, and licensed patents to Stark in Canada. Many low priced "conductance" testers used AC voltage on all of the elements can actually damage high transconductance tubes. The heavy rectification current caused by driving the control grid positive can overheat the grid wires, resulting in critical spacing to be upset. The tube then actually loses transconductance.

The remaining types of tube testers include laboratory grade testers and special purpose testers. Included in this group of testers is the Hickok Cardmatic. This was a sophisticated tester used primarily by manufacturers and the military that employed punched cards instead of switches to provide tube test setup. A punched card was provided for each type of tube to be tested. An example of a top end laboratory tester is the New London Instruments Model 901A Transconductance Analyzer. This tester is setup by using a tube manual. Tube pin connections are selected by a set of push-button switches. Voltages to all of the elements can be adjusted and current of each element can be monitored on separate meters. It also directly measures transconductance. A very complete analysis of the condition of a tube can be made on this device. This instrument has jumpers that can be removed for each tube element so it can actually be used as a design platform for tube circuits. The flexibility of this tester allows it to be used to plot tube curves. Special purpose tube testers include those types that are used for a specific purpose, such as the small signal tube testers made by George Kaye Audio Labs and Vacuum Tube Valley. These are used to test preamp tubes for noise, microphonics, gain, and triode balance. Other special purpose tube testers are usually special test fixtures that are used to test a specific tube for a specific purpose. These special test fixtures are usually not available commercially and are usually built by the manufacturer or individual who intends to use them. These are usually found on assembly lines for grading tubes to be used in a certain application.

As was stated earlier, the best test for a tube is in the equipment in which it will be used. If you intend to acquire a tube tester, be aware of its limitations and do not take every tube tester reading as the gospel truth. Some good tubes will test bad and some bad tubes will test good under certain conditions, as noted above in the descriptions of the various types of testers. If you are in doubt concerning a tube tester reading, substitute a known good tube in the piece of equipment the tube is used in. Tubes and vintage tube testers are analog devices. If you avoid using a "digital" frame of mind when using these analog devices, you will find tube testers to be very useful.
 

Question: What is Amplifier Bias?
Answer: Setting the bias of an amplifier involves adjusting the voltage to the control grids of the power tubes so the tubes draw the correct amount of operating current. This is much like setting the idle on a car when doing a tune up. On amplifiers that have adjustable bias, it is a potentiometer that is provided for the technician to set the bias voltage to the control grids while monitoring the plate or cathode current of the output tubes. If the operating current is set too low, the amplifier will sound weak and have a gritty non-musical sound caused by crossover distortion. If the operating current is set too high, the tube will draw excessive current, which will shorten the life of the tubes. Setting the bias correctly will give the best sound and longest tube life. Due to the dangerous high voltages encountered in tube amplifiers in addition to the knowledge and equipment required to set the proper amount of operating current, this adjustment is best left to be done by an experienced technician. Many technicians will also perform minor preventive maintenance, such as cleaning the controls and soldering loose connections, while they have the amplifier open to adjust the bias. If you use New Sensor matched power tubes, make note of the Ip (plate current) numbers on the tube boxes. Once the bias is set with this set of tubes, you can order replacement sets with the same or close Ip (plate current) numbers and install them without needing to reset the bias.

Some amplifiers provide a bias voltage to the grids of the power tubes, but do not have a way to adjust the voltage. Most Mesa Boogie amplifiers do not have a bias adjustment control. Many of the ‘70s master volume Fender amplifiers provided a balance control instead of a bias voltage adjustment. For amplifiers that do not have a way to adjust the bias voltage, it is best to use New Sensor matched tubes that have middle Ip (plate current) numbers. This will ensure that the amplifier is operating at or close to the optimum desired plate current.

Cathode-biased amplifiers use a resistor and bypass capacitor in the cathode circuit of the output tubes to provide a positive voltage to the cathode. This sets the grid voltage negative in relation to the cathode and establishes the proper operating current. Fender Champ, Fender Tweed Deluxe, and VOX AC-30 are common examples of cathode-biased amplifiers. There is no bias adjustment on these amplifiers.

Preamp tubes are cathode biased, so there is no need to check or set bias on any preamp tube.
 

Question: When should I change the Tubes in my amplifier?
Answer: First pay attention to your amplifier’s performance. When you hear the sound begin to deteriorate, it may be time to change the tubes. You may notice that chords sound muddy, the amplifier is losing high or low frequency response, there is poor balance in the level of various notes, or the amplifier lacks punch and sounds weak. If the amplifier is making funny noises such as ringing, popping, or ghost notes or power is fading up and down, these are all indications that it is most likely time to replace the tubes.

When one power tube starts to go bad, it will draw down the other power tubes in the set. In this case it is best to replace them with an entire set of New Sensor matched output tubes. This will ensure that all of the tubes in the replacement set will wear evenly and provide the best tone. Many amplifier technicians also replace the phase inverter (driver) tube when replacing the power tubes, because it is the hardest working preamp tube in the signal chain.

Amplifier manufacturers generally use the least expensive tubes in their products. New Sensor/Electro-Harmonix has created and reissued many great "tone tubes" to help musicians get the best possible tone from their amplifiers. These tubes can used to get an improvement in the sound of most amplifiers. The original Tweed Fender Bassman came with Tung-Sol tubes during the 1950s. If the factory installed tubes in the reissue ’59 Bassman are replaced with reissue Tung-Sol 5881s in the power section, Electro-Harmonix 12AY7EH/6072A in the first preamp stage, and a Sovtek 5AR4 for the rectifier, the amplifier will have a sound that is closer to the original ’59 Bassman sound. Marshall amplifier originally came from the factory in the late ‘60s and ‘70s with Mullard tubes. Outfitting Marshall amplifiers with reissue Mullard tubes will give these amplifiers that classic British vibe that made them famous. The Electro-Harmonix 6CA7EH is a copy of the Sylvania/Philips 6CA7 that was used in Music Man amplifiers and is the most rugged replacement for these high voltage amplifiers. The Tung-Sol 12AX7 has a very full and musical sound signature and is the perfect tube to improve the tone of a bland and thin sounding amplifier.
 

Question: What is the difference between Class A and Class AB amplifiers?
Answer: Class A and Class AB are terms that describe how the power tubes work within the power section of a guitar amplifier. To properly explain the technical differences between these classes of operation would require a lengthy discussion.

In Class A circuits the output tubes are conducting and drawing current all of the time even when no signal is present. Class A amplifiers sound more vintage, are very touch responsive and have a somewhat spongy feel with a singing response when overdriven. They also have an interesting harmonic content with asymmetrical clipping that is highly desired by many musicians. Since the output tubes are drawing current all of the time, they run much hotter than Class AB amplifiers. They run at lower voltage and have less output. All single ended (one output tube) guitar amplifiers, such as the Fender Champ, are Class A. Some push-pull amplifiers are Class A, the most notable being the Vox AC30.

All guitar amplifiers that are Class AB are push-pull. In this circuit one output tube conducts plate current and amplifies during the positive cycle of the input signal. The other output tube conducts plate current and amplifies during the negative cycle. When one tube is conducting and amplifying the signal the other tube is basically "resting" because it is drawing little or no plate current. In this case, the amplifier runs much cooler. Another advantage of Class AB operation is the plate voltage can be higher and the bias can be much deeper allowing for more efficient operation and higher power output.

A typical Class A amplifier using a pair of 6L6 tubes will have a maximum power of about 20 watts, while operating the same pair of tubes in Class AB will easily yield 50 watts. All Fender and Marshall amplifiers with an output of more than 40 watts are Class AB. Class AB amplifiers tend to have greater dynamics, sound punchier, are cleaner, and have cooler running tubes.
 

Question: What is an Octal Tube?
Answer: An octal tube is a vacuum tube that has an 8-pin base with an 11/16" diameter keyed locating prong called a spigot in the center. This tube base was introduced by RCA in 1935 for their new line of metal tubes, which they called an "Octal base". The octal base soon caught on for conventional glass tubes as well. The eight available pins allowed more complex tubes to be constructed, including dual triodes. The glass envelope of an octal tube was cemented into a Bakelite or plastic base with a hollow post in the center, surrounded by eight metal pins. The wire leads from the tube were soldered into the pins and the pinched-off nub through which the air was evacuated from the envelope fit into the post. The post had a protrusion along one side that matched the indexing slot in the socket so the tube could only be inserted in the correct orientation.

Until the introduction of miniature (Noval base) tubes, such as 12AX7 and EL84, the octal base was the most common and popular tube base. Some common octal tubes are 6V6GT, 6L6GC, EL34, 6550, KT88, 6SL7, and 5AR4. When looking at the bottom of the tube with the indexing protrusion facing you, pin 1 is the first pin to the left of the indexing protrusion.
 

Question: What are NOS and NIB tubes?
Answer: "NOS" stands for "New Old Stock", which means that the tube has never been used and is from old production. In most vacuum tube circles NOS generally implies that the tube is in the original box.

"NIB" stands for "New In Box". These are old production tubes that have never been used and are usually placed in plain white boxes. This is done because the original box has deteriorated over the years or the tube was from a manufacturer who purchased bulk-packed cartons of tubes that were not individually boxed and used for assembly line installation.

If you are planning to outfit your tube equipment with NOS/NIB tubes, it is wise to have knowledge about the actual types of tubes you are intending to purchase. When purchasing NOS/NIB tubes, be aware that there are dealers who will rebrand current production tubes with famous desirable brand names from the golden days of tube production and sell them at high prices as NOS or NIB. As the supply of popular NOS types dries up, sometimes tubes that were originally rejected by the manufacturer and left in a warehouse will find their way to Ebay and other Internet dealers. Used tubes will also be found from these sources that are sold as "tests as NOS." There is no such thing. The simple bottom line in this case is that the tube is either used or it is new. NOS/NIB tubes that are sold by New Sensor Corporation are either military surplus purchased at government auctions or have been sourced from reputable tube dealers, so you can be assured of quality when purchasing tubes that are no longer made.
 

Question: Why is there a difference in Tube and Transistor sound?
Answer: Engineers and musicians have long debated the question of tube sound versus transistor sound. Conventional methods of frequency response, distortion, and noise measurement have always assumed linear (clean) operation of the test amplifier and have shown that no significant difference exists. In actual operation most amplifiers are often severely overloaded with signal transients. Under this condition there is a major difference in the harmonic distortion of tube and transistor circuits.

There are also significant differences in the construction of tube and transistor amplifiers that contribute to the sound. Tube amplifiers require output transformers to match the high impedance of the power tubes to the low impedance of the loudspeaker. The transformer has a natural high-frequency roll-off that makes the tube amplifier sound warmer. When the amplifier is overdriven the transformer also has a point of core saturation that provides a form of compression. This gives the tube amplifier sustain and a singing quality that sounds very musical. Transistor power amplifiers are either direct or capacitor coupled to the speaker load and do not have this natural compression. Negative feedback circuits used to reduce distortion in tube amplifiers are relatively simple. Negative feedback circuits used to reduce distortion in transistor amplifiers are more complex. The different types of negative feedback circuits in tube and transistor amplifiers react very differently to the harmonic content of the signal when the amplifier is overdriven. Tube amplifiers generally have a smoother, rounded waveform, where the overdrive of a transistor amplifier is more abrupt and resembles a square wave.

The harmonic content of an overdriven tube amplifier consists primarily of 2nd order and 3rd order harmonics with some 4th order harmonics. The harmonic content of an overdriven transistor amplifier is primarily 3rd order with suppressed 2nd order harmonics. 2nd and 3rd order harmonics are the most important from a viewpoint of electronic distortion. Musically the 2nd harmonic is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller. The 3rd harmonic is a musical 12th. Instead of making the tone fuller, a strong 3rd harmonic makes the tone softer. The odd harmonics (3rd, 5th, etc.) produce a "stopped" or "covered" sound. The even harmonics (2nd, 4th, etc.) produce a "choral" or "singing" sound. Adding a 5th to a strong 3rd harmonic give the sound a metallic quality that gets annoying in character as the amplitude increases. A strong 2nd with a strong 3rd harmonic tends to open the "covered" effect. Adding the 4th and 5th harmonics to this gives an "open horn" character. The higher harmonics, above the 7th, give the tone "edge" or "bite."

The basic cause of the difference in tube and transistor sound is the weighting of harmonic distortion in the amplifier’s overload region. Transistor amplifiers exhibit a strong component of 3rd harmonic distortion when driven into overload. This harmonic gives a "covered" sound with a restricted quality. A tube amplifier when overdriven generates a whole spectrum of harmonics. Particularly strong are 2nd, 3rd, 4th, and 5th overtones that give the sound a full-bodied "brassy" quality. Combining this reinforcing harmonic content with the compression and high-frequency roll-off of the output transformer in a tube amplifier is why your tube amplifier will "give it up" while your friend’s transistor amplifier will sound restricted and harsh.
 

Question: Why do some tubes have several number designations?
Answer: There are several different numbering schemes for vacuum tubes. The two most common are the European system developed by Mullard and Philips. The other is the American RETMA (Radio Electronics Television Manufacturers Association) standard. These systems were employed as a method of standardization in the late 1930s when large numbers of vacuum tubes were being developed to meet the demands of a growing technology.

In the European system the first letter indicates filament as follows:
A 4 Volts
B 0.18 Amps (series)
C 0.2 Amps (series)
D <=1.4 Volts (series/parallel)
E 6.3 Volts (series/parallel)
F 12.6 Volts
G 5 Volts (parallel)
H 0.15 Amps (series)
K 2 Volts
L 0.45 Amps (series)
P 0.3 Amps (series)
U 0.1 Amps (series)
V 0.05 Amps (series)
X 0.6 Amps (series)
Y 0.45 Amps (series)

The second and subsequent letters indicate the construction of the tube as follows:
A Diode (excluding rectifiers)
B Double diode
C Triode (signal, not power)
D Power output triode
E Tetrode (signal, not power)
F Pentode (signal, not power)
L Power output tetrode or pentode
H Hextode or heptode (of the hextode type)
K Octode or heptode (of the heptode type)
M Tuning indicator
Y Half wave rectifier
Z Full wave rectifier

The first digit indicates the basing arrangement as follows:
1 Miscellaneous
2 Miniature 10 pin
3 International octal
4 Miniature 8 pin
5 Magnoval
8 Noval (9 pin miniature)
9 Miniature 7 pin

The remaining digits are the sequence number. Note that signal pentodes and tetrodes that end in even numbers are sharp cutoff. Those ending in odd numbers are remote cutoff.

The European system has the advantage of providing a lot of information about the tube by deciphering the tube number. Here are some examples:
EF86: 6.3 Volt filament, signal pentode, Noval base, sharp cutoff.
GZ34: 5 Volt filament, full wave rectifier, International octal base.
ECC83: 6.3 Volt filament, two signal triodes, Noval base.
E83CC: Special quality version of ECC83. Swapping the second and third field was commonplace to indicate that the tube was a premium version.
PCL82: 0.3 Amp series string filament, signal triode + power pentode, Noval base.

In the American RETMA standard the first digit indicates the filament voltage as follows:
0 Cold cathode (voltage regulator tube)
1 0.1 - 2.0 Volts
2 2.1 - 2.9 Volts
3 3.0 - 3.9 V0lts
n n.0 - n.9 Volts

The letter or letters that follow indicate the sequence code. When single letter codes were used up, double letter codes were introduced. Note that U to Z generally (but not always) as the only letter or as the last letter are used for rectifiers.

The final number indicates the element count with the filament counting as element 1. The 6SN7 has two cathodes, two grids, two plates, and a filament.

Additional letters indicate improvements to the tube or special properties as follows:
A Controlled heater warm-up time or improved version of a non-A type.
B Improved ratings.
C Further improvement.
G Glass bulb.
GT Glass tubular.
W Ruggedized version.
X Low loss ceramic base.
Y Low loss phenolic base.

With the RETMA standard, you will need a spec sheet or tube manual to determine the function of the tube and what type of basing is used. Here are some examples of the RETMA standard:
6SN7GT: Filament rating is between 6.0 - 6.9 Volts, SN=sequence code, 7 elements, glass tubular envelope.
6V6GTY: Filament rating is between 6.0 - 6.9 Volts, V=sequence code, 6 elements, glass tubular envelope, low loss phenolic base.
12AY7: Filament rating is between 12.0 - 12.9 Volts, AY=Sequence code, 7 elements.

Some companies, such as Mazda and Marconi-Osram (GEC), had their own numbering systems. The most famous are the KT-series of Kinkless Tetrodes, which included the KT66 and KT88 beam tetrodes. The KT66 is an up-rated version of a 6L6 and the KT88 is interchangeable with a 6550.

Many tubes that are intended for special applications, industrial, or military use do not use the RETMA standard. These tubes have a four digit number as the designation. A prime example is the Tung-Sol 5881 that was developed as a heavy-duty replacement for the 6L6. This tube was unique in the fact that it was a prime power tube used in amplifiers built during the 1950s hi-fi movement. It also found a home in the Fender Tweed Bassman, as well as being the power tube used in the servo amplifiers that drove the control surfaces in B-52 bombers. Another tube found in Fender amplifiers is the 7025. This was actually a 12AX7 that was selected for low noise and hum for use in preamplifier circuits. There were versions of the 12AU7, such as the 5814 that had balanced sections and was primarily used in analog computers along with the 6189 that was optimized for mobile, shipboard, and aircraft communications. These premium special purpose industrial/military tubes work very well in guitar and hi-fi amplifiers and are well worth looking into.

There is also the CV series of tubes. The format is CV followed by up to 5 digits. This system was used by the British military to codify vacuum tubes, gas tubes, and some semiconductors. The CV4004 is the British military version of a 12AX7WA. The CV series tubes are of high quality and sought out by audiophiles for use in high-end audio equipment.

The following is a list of equivalent tubes showing the different numbering systems. Some of the tubes listed are premium special versions.
EL34= 6CA7, KT77, E34L
EL84= 6BQ5, 7189, E84L
6L6GC= 5881, 7581, KT66, EL37, 6L6WGC
12AT7= ECC81, 6201, CV4024, E81CC
12AU7= ECC82, 5814, 6189, CV4003, E82CC
12AX7= ECC83, 7025, CV4004, E83CC
6V6GT= 7408, CV511
5AR4= GZ34
 

Question: What is tube rolling?
Answer: Tube rolling is the process of trying out a number of tubes in the same spot in an amplifier and selecting the one that sounds best to you. This can be very helpful in optimizing the tone of the amplifier.

CAUTION: Tube amplifiers run on dangerous high-voltages and the tubes get very hot during operation. The amplifier should always be placed in standby when replacing tubes. Also, an oven mitt or Electro-Harmonix Tube Glove should be used when removing hot tubes to avoid painful burns. It is actually a good idea to let the tubes cool down for a few minutes before removing them. If you feel uncomfortable trying this yourself, you can always take your amplifier to your favorite technician and have him assist you in changing the tubes.

The same type of tube made by different manufacturers and in different variations from a manufacturer can have a definite impact on the actual sound of an amplifier. The most common and easiest tubes to "roll" are the preamp tubes. Preamp tubes are self-biasing and no adjustments are required when they are installed. In guitar amplifiers these tubes are almost universally 12AX7/ECC83. Actually, many technicians will try a variety of 12AX7 tubes when an amplifier is repaired to find the tubes that the amplifier "likes". If the amplifier is thin and bland sounding, a high gain version, such as the reissue Tung-Sol 12AX7, can be substituted to thicken and give definition to the sound. The reissue Mullard 12AX7/ECC83 is a large plate format, which is characterized by having a large soundstage with a lot of detail. If the amplifier has too much gain, a Sovtek 5751, which has 70% gain of a 12AX7 can be substituted. Another tube that works well for this purpose is the Electro-Harmonix 12AY7EH/6072A. This tube has a lower gain and a higher transconductance. In many cases this tube will give the amplifier the sound of a custom "boutique" amplifier and make it very touch responsive. The Sovtek 12AX7WB has a dark (or warm) character and is useful for taming an amplifier that has a harsh sound. You have probably already realized that by hand selecting the preamp tubes, you can actually change the gain structure of the preamp and create a custom sound.

Power tubes will in most cases require resetting the bias. Also, you want to change the whole set of power tubes. Some amplifiers, such as Mesa-Boogie, have no bias adjustment. If you want to "roll" the tubes in these amplifiers, you can order matched sets with middle matching numbers. The Sovtek 5881WXT will give an amplifier an aggressive sound, where a Tung-Sol 5881 will have more of a Fender Tweed character. The Tung-Sol 6L6GC-STR (depending on the amplifier) will lean toward a vintage Fender Blackface GE or RCA sound and the Svetlana SV6L6GC will sound more like the Sylvania 6L6GC tubes found in Fender Silverface amplifiers. Some amplifiers can accommodate either 6L6 or EL34 tubes. These amplifiers most often have a switch to change the bias range between these tubes. If you have an amplifier that can use either 6L6 or EL34 tubes, you will quickly notice that 6L6 tubes are more "Fender", where EL34 tubes are more "Marshall".

As you can see, tube rolling can give new life to an ordinary sounding amplifier. It is also the most logical way to try to find that custom sound you have searching for.
 

Question: How long, under normal use, should I expect my power tubes to last?
Answer: The life expectancy of power tubes depends on a number of factors:
How often and how loud do you play?
What style of music do you play?
What kind of amplifier do you have? Some amplifiers run the tubes harder than others.

In general, the average steadily gigging club musician can expect power tubes in a typical amplifier to last from six months to about a year. Touring professionals will change tubes more often. It is very common for the top acts to have the amplifiers completely checked out and have the tubes replaced just prior to going on tour. They usually also have back-up amplifiers, extra tubes, and may even take a technician on tour to service the amplifiers as needed. The musicians who play for their own entertainment at home or only play occasional gigs may actually get several years of service from a set of power tubes.

As you develop your playing schedule, you will generally begin to notice when the amplifier seems to not be sounding like it should and will take it in to the technician to have the tubes checked and/or replaced at regular intervals. For the average gigging musician, an amplifier checkup every six months is a good rule of thumb.
 

Question: I have heard the word "Microphonic" used to describe a tube. What does it mean?
Answer: Microphonics describes the phenomenon where a tube transforms mechanical vibrations into an undesired signal (noise or feedback). Due to the mechanical construction all tubes exhibit some inherent microphonic tendencies. The most critical application regarding microphonic tubes are high gain preamp stages that are close to the input. This is especially true in combo amplifiers where the chassis is mounted in the same cabinet with the speaker. The preamp stage close to the input of the amplifier has more stages after it to amplify the signal plus any noise and microphonics. Most technicians will handpick the first preamp tube for the least noise and microphonics. A tube that is not suitable for the first preamp stage can many times be used in a later preamp stage in the signal chain. As the signal is amplified, the increasing level of the signal will override the microphonic tendencies. Since there are less stages of amplification following these stages the microphonics will be amplified less.

You can check your tubes for microphonics by performing this simple test:
1. Remove any back cover that may be on the amplifier to gain access to the tubes and any tube shields on the preamp tubes.
2. Plug a guitar (with the volume control at minimum) or a dummy plug into the input of the amplifier.
3. Turn up all controls on the amplifier.
4. Turn on the amplifier.
5. Tap each tube with the eraser end of a pencil. If you get feedback or a bell-like ringing sound, the tube is microphonic. If you hear a dull thump, the tube should be fine and perfectly usable. If you find a microphonic preamp tube, you can many times switch it with another preamp tube of the same type that is farther down the signal path in the amplifier to eliminate the problem. If this does not solve the problem, replace the microphonic tube with a new tube.
6. When the microphonics problem is cured, remove power from the amplifier then replace any tube shields and covers that were removed to gain access to the tubes.
 

Question: Will the tubes in my amp all wear out at the same rate?
Answer: The hardest working tubes in an amplifier are the power tubes. These should be purchased and replaced in matched sets. A set of matched power tubes will work together and wear at the same rate. They will also sound better and last longer than unmatched tubes.

The next hardest working tube is the driver tube. This is the tube that drives the power tubes. In a push-pull amplifier this tube is usually the phase inverter and it is frequently installed next to the power tubes. Many technicians replace this tube when they replace the power tubes.

Rectifier tubes also dissipate a lot of power, but normally when they fail, they either completely lose emission or short out. There have been some cases where the rectifier will lose some emission, but the amplifier will still function, but will sound weak and lifeless. If the other tubes have already been replaced, it may be time to install a new rectifier.

Preamp tubes are generally not driven that hard and will usually last longer than the other tubes in the amplifier. Some high gain amplifiers drive these tubes harder than vintage amplifiers, so they may need to be replaced more often.

If the amplifier starts to sound dull, lacking response and punch, it is a good indication that the tubes need to be replaced. First start with the power tubes and work your way back through the driver and preamp toward the inputs until the proper sound of your amplifier is restored.
 

Question: I am a performing musician, should I bring an extra set of tubes to my gig, just in case?
Answer: If you know how to change tubes and your amplifier is constructed in a way that gives easy access to the tubes (open back vintage Fender), it may not be a bad idea to carry a spare set of power tubes and a couple of preamp tubes to the gig in your roadie bag as cheap insurance. In other cases it is better to have a backup amplifier.

Many modern tube amplifiers require disassembly and removal of covers to gain access to the tubes. This can be time consuming, parts can get lost and further damage to the amplifier can occur. Also, any disassembly will require you to have a tool kit handy and can expose you or others to dangerous voltages. A club environment is definitely not the place to disassemble a tube amplifier.

Another factor in changing tubes at a gig is the fact that when a tube shorts out or fails, it could cause other parts to fail. Sometimes failure of other parts can cause a tube to fail. In this case when the replacement tube is installed it will meet the same demise as the one it replaced.

The only time it really pays to have spare tubes on hand is when you are on tour. If your amplifier breaks down, you can find a technician between gigs and if he doesn’t happen to have the tubes your amplifier needs in stock, you already have replacements to speed the turn around of the repair.