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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 For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit.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.


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

        vacuum tubes for guitar amps                         buy vintage amps, we have amps for sale
      Diode                                                                 Triode

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:// /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

                                       Fender Transformers    (from information in VG magazine)
Model Name Model Power Output Choke Reverb
Bandmaster 6G7A 67233 45217 125C1A *
AA763 125P7D 125A6A 125C1A *
AC568 125P7D 125A6A * *
Bandmaster Reverb AA270 125P5D 125A6A 125C1A 022921
AA768 125P5D 125A6A 125C1A 022921
AA1069 125P5D 125A6A 125C1A 022921
4 x 10 combo's
and Heads
5F6 8087 45249 14684 *
5F6A 8087 45249 14684 *
6G6 125P5A 125A5A 125C1A *
6G6A 125P7A 125A13A 125C1A *
AA270 125P7D 125A13A 126C1A *
AA864 125P7D 125A13A 126C1A *
AB165 125P7D 125A13A 126C1A *
AC568 125P7D 125A13A 126C1A *
Bassman 70 - 013897 013897 * *
Bassman 135 - 013692 013691 * *
Champ AA764 125P1B 125A35A * *
Concert 6G12 67233 45249 125C1A *
AB763 125P7D 125A9A 125C1A *
Deluxe and
Deluxe Reverb
- 6452 1839 * *
6G3 125P2A 125A1A * *
AA763 125P23B 125A1A 125C3A 022921
AB868 125P23B 125A1A 125C3A 022921
Harvard 6G10 125P1A 125A2A * *
Princeton and
5F2A 66079 265 * *
AA1164 125P1B 125A10B * 022921
B1270 125P1B 125A20B * 022921
Pro and
Pro Reverb
5C5 6516 1846 * *
5D5 6516 1846 * *
5E5A 6516 1846 * *
6G5A 125P7D 125A7A 125C1A *
AB763 125P5D 125A7A 125C1A 022921
AA270 125P5D 125A6A 125C1A 022921
AA1069 125P5D 125A6A 125C1A 022921
Reverb Unit 6G15 68319 * 125C3A 125A12A
6G15 125P24A * 125C3A 125A12A
Showman 6G14 67233 45550 125C1A *
AA763 125P34A 125A30A 125C1A *
Dual Showman AB763 125P34A 125A29A 125C1A *
Super and
Super reverb
6G4 8087 45216 125C1A *
6G4A 125P7A 125A6A 125C1A *
AB563 125P5D 125A9A 125C1A 022921
AA763 125P5D 125A9A 125C1A 022921
AB763 125P5D 125A9A 125C1A 022921
AA270 125P5D 125A9A 125C1A 022921
Tremolux 5G9 8160 108 14684 *
6G9 125P6A 45217 125C1A *
6G9A 125P6A 45217 125C1A *
6G9B 68409 125A6A 125C3A *
AB763 125P26A 125A6A 125C3A 022921
Twin and
Twin Reverb
5G8A 7993 45268 14684 *
6G8 67233 45548 125C1A *
AB763 125P34A 125A29A 125C1A 022921
AC568 125P34A 125A29A 125C1A 022921
AA769 125P34A 125A29A 125C1A 022921
AA270 125P34A 125A29A 125C1A 022921
Twin Reverb
7591 Tubes
AB763 125P19A 125A18A 125C1A 022921
Vibrolux and
Vibrolux Reverb
6G11 125P6A 45217 125C3A *
6G11A 68409 125A7A 125C3A *
AB763 125P26A 125A7A 125C3A 022921
AA965 125P26A 125A6A 125C3A 022921
AB568 125P26A 125A6A 125C3A 022921
Vibroverb 6G16 125P6A 45217 125C1A 022921
AA763 125P5D 125A7A 125C1A 022921
Notes: * indicates that no transformer was used

fender tubes, dating fender amps and tube charts

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.

Fender Amp Date Codes
Letter Code Vintage Year Reissue Year Month
A - 1990 January
B - 1991 February
C 1953 1992 March
D 1954 1993 April
E 1955 1994 May
F 1956 1995 June
G 1957 1996 July
H 1958 1997 August
I 1959 1998 September
J 1960 1999 October
K 1961 - November
L 1962 - December
M 1963 - -
N 1964 - -
O 1965 - -
P 1966 - -
Q 1967 - -



dating marshall amps, marshall tubes and amp charts

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
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

Tube Chart (work in process)

Tube Part Number

   A Few Brands  Making This Tube


6L6 Electro-Harmonix, Sovtek, Svetlana, Tesla, JAN-Phillips power output tubes, up to 50 watts/pair, a mainstay of Fender
EL34 Electro-Harmonix, Matsushita, Mullard,Sovtek, Svetlana, Tesla Euro power pentodes, up to 50 watts/pair, many Marshalls
6V6 Electro-Harmonix, JAN-Phillips, JJ Tesla smaller, lower power cousin of the 6L6, 10-14 watts per pair; used in smaller Fenders
6CA7 Electro-Harmonix 6CA7-EH Power Tube
6550 Tung-Sol, Electro-Harmonix, Svetlana Power Tube
KT66 Sovtek, Saratov, Shuguang Power Tube
KT77 EL-34 replacement   JJ/ Tesla Power Tube
KT88 Interchangeable with KT88, KT90, and KT100.  Sovtek, JJ/ Tesla, Electro-Harmoinx

Power Tube

KT90 Interchangeable with KT88, KT90, and KT100.  Electro-Harmonix KT90EH Power Tube
KT100 Interchangeable with KT88, KT90, and KT100.  Sovtek, JJ/ Tesla, Electro-Harmoinx Power Tube
EL84(6BQ5) Sovtek, Electro Harmonix, JJ/Tesla Power Tube, fits a 9 pin socket like an 12AX7 but 2x as tall.  Used in small Vox amps
6K6 Replaces 6K6GT types Pre-Amp Tube
6F6 Replaces 6F6GT types Pre-Amp Tube
6BQ5 (EL84) Same as EL84 Power Tube, (miniature pentode with pinout 9CV)
12AX7 Svetlana, Tung-Sol, Sovtek Preamp and driver tubes
12AT7 JJ/Tesla ECC81 Preamp and driver tubes
12AY7 Electro Harmonix 12AY7EH Driver Tube
6EU7 Sovtek 6EU7 Dual triode used in some older amps for preamp tube

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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.


Guitar Amps use tubes like the 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 , 6K6, 6F6, 6L6 tubes.

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 Amps using tubes like the 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 , 6K6, 6F6, 6L6,            

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