Amp Dates, Tube Charts, Transformers and Information

What is a Vacuum Tube ? 

In electronics, a vacuum tube (U.S. and Canadian English) or (thermionic) valve (outside North America) is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in several specialized applications such as guitar amplifiers (also called a valve amp outside the U.S.) and high power RF transmitters, as a display device in television sets and in microwave ovens.

Operation

Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement.

The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal "plate" inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. In it's simplest form a vacuum tube can be created to operate as a diode: a device that conducts current only in one direction. A third element called a "control grid" can be added to the design which provides the ability to amplify a signal.  Other configurations are also possible including the Pentode, a tube with 5 active elements  providing an additional amplification factor.   There are a large number of tube varieties and uses.  This is only a very brief overview and we suggest consulting additional resources if you are interested in additional information.  Some Information provided by wikipedia.org.

Why Use Tubes in Guitar amps ?

Most good guitar amplifiers use tubes rather than solid-state components.  Why tubes ?  The amplifier is a critical element in achieving the sound the musician desires.  Tubes provide the tone that musicians want.  Tube amps are warmer, richer and have a more desirable tone than solid-state amps.  The distortion and speaker-damping characteristics of a tube amp with an output transformer matched to the speaker load is hard to replicate with solid-state devices.  Tube amps are particularly popular with serious musicians.  Many musicians prefer to play vintage Fender, Marshall and Gibson amps.  Replacement tubes and transformers are readily available for these amps however there are many boutique amp manufacturers making new tube amps with a vintage sound.

Amplifier classes

Amplifier circuits are classified as A, B, AB and C for analog designs, and class D and E for switching designs. For the analog classes, each class defines what proportion of the input signal cycle (called the angle of flow) is used to actually switch on the amplifying device.

What's a Class A Amp ?

In a class A amp 100% of the input signal is used.  The amplifier is passing current at all times even when you are not playing.  The instant you strike a note it's immediately fed to the speakers resulting in a "fast" sound.  Class A is very inefficient but usually gives very low distortion and is generally a better sounding amp at low volumes.  Class A amps are often more expensive boutique amps.  Some of our Divided by 13 amps are Class A.

What's a Class B Amp ?

A class B amp uses 50% of the input signal. Class B is different from Class A in that there is no current flowing when the output is at idle and turn on from zero current when a signal is present.  In a push-pull Class B amp design each of the output circuits produce one half the audio waveform with each circuit not producing any current flow when the other circuit is operating. Class B designs tend to have more crossover distortion and require a less beefy power supply.   Many popular guitar amps use class B designs including Fender and Gibson amps.

What's a Class AB Amp ?

As the name implies class AB amps exhibit some characteristics of class A amps and some of class B amps.  In a class AB amp design, more than 50% but less than 100% of the input signal is used .  If an amp uses class A mode for a portion of it's output then has to apply additional circuitry for the remainder of it's output then it is considered a class AB Amp.  Class AB amps are also more efficient than a straight class A therefore does not require as large a power supply.

What is an Amp Transformer ?

A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts.  It consists of a minimum of two coils, the primary and the secondary, wound on the same core. An alternating current in one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. Transformers are used to convert between high and low voltages, to change impedance, and to provide electrical isolation between circuits.  This is useful in converting the voltage from a wall outlet, typically 120 or 240 volts, into a higher voltage required by tubes in tube amps . typically 400V or more, and a lower voltage for the tube filament, typically 6.3 or 12.6V. There are several transformers used in tube amps.  Some information provided by http:// en.wikipedia.org /wiki / Transformer

Output transformer - An output transformer is used to match the low impedance of a speaker voice coil to the high impedance of a tube output stage.  Output transformers consist of at least two windings, a primary and a secondary. Some output transformers have multiple impedance taps on the secondary side, to allow matching to different speakers, typically 4, 8, and 16 ohms

Power transformer - A Power transformer converts the incoming line voltage to a higher or lower value for use in the guitar amplifier. Typically, the power transformer will have at least one primary, but sometimes two or more, to allow use at 120V or 240V. In an amp the power transformer will generally increase the voltage to 400 volts or more for the tube plate.  There will also usually be a 6.3V filament winding. There is also sometimes a 5V. winding for use with a tube rectifier. 

Choke - Another term used for an inductor, most commonly an inductor used as a power supply filter.

Power transformer and output transformer

Fender Transformer Chart

Dating Fender Amps - Using the Fender Tube Chart

Look inside the amp (but don't stick you hand in there, even after being unplugged the amp may retain a dangerous electrical charge), there should be a tube chart on most amps.  On this chart there is a hand stamped date code consisting of 2 letters.   For example AD would be April 1990 and DG would be July 1954.

Dating Marshall Amps

In 1969 Marshall introduced a date coding system. Some of the older Marshall amps have an inspection sticker on the top of the chassis which usually has the day, month and year the amp was actually made or inspected.  Here's a chart with date codes for Marshall amps.  Note, "A" Date Code ran for 18 months (July 1969 to December 1970) so the "B" date Code was never used and has been omitted.  Use the serial number to determine the date code.  The serial number is generally located on the back of the chassis but from 79 to 80 it was on the front panel.  From 1969 to 1983 the date code was after the serial number.  From 1984 to 1992 the model number was first, then the date code, then the serial number.

A=1969      A=1970
C=1971      D=1972
E=1973      F=1974
G=1975      H=1976
J=1977      K=1978
L=1979      M=1980
N=1981      P=1982
R=1983      S=1984
T=1985      U=1986
V=1987      W=1988
X=1989      Y=1990
Z=1991      Z=1992

Early Marshall Model Codes (approx Mid 69 to Late 83)

/A = 200 Watt
SL/ = 100 Watt Super Lead
SB/ = 100 Watt Super Bass
SP/ = Super PA
ST/ = 100 Watt Tremolo
S/ = 50 Watt
T/ = 50 Watt Tremolo

Later Marshall Model Codes (approx Early 84 to Late 92 )

A/ = 200 Watt
SL/A = 100 Watt Super Lead
SB/A = 100 Watt Super Bass
SP/ = Super PA
ST/A = 100 Watt Tremolo
S/A = 50 Watt
T/A = 50 Watt Tremolo
RI = Reissue

Early Marshall Date Code Example
EXAMPLE: SL/A 25353 E
SL/A = Model Code
24523 = Serial Number
E = Date Code
This amp would be a 100 Watt Super Lead 1973

Later Marshall Date Code Example
EXAMPLE: S/A S 24523
S/A = Model Code
S = Date Code
24523 = Serial Number
This amp would be a 50 Watt 1984

Common Tubes used in Vintage Amps and modern replacements

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Disclaimer: These charts are for informational purposes only and are not a substitute for consulting your amp tube chart, amp repair shop and amp manufacturers instructions.  We are not responsible for typographical errors, inaccuracies and omissions on this web site.  Amps should only be serviced by a qualified technician.