The number of elements is normally given in the numbering of most types except military etc.
Alex
The information below, is a few selected items from :
studentweb.eku.edu/justin_holton/tubes.html
How are tubes named?
In the United States vacuum tubes are identified by a naming system that is fairly standard.
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.
The second part of a tube designation is a code letter (or group of letters) for identifying the tube types and functions.
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.
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.
What is the purpose of the getter?
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.
Why is there a vacuum inside a vacuum tube?
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.
But aren't some tubes supposed to have gas inside them?
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.
What is basing?
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.
What is distortion, really?
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.
What causes microphonics?
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.
What is crossover distortion?
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.
Are black plates a gimmick?
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!
Why is the output voltage inverted when it's amplified?
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.
What's the difference between Class AB1 and AB2?
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.
How are tubes named?
In the United States vacuum tubes are identified by a naming system that is fairly standard.
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.
The second part of a tube designation is a code letter (or group of letters) for identifying the tube types and functions.
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.
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.
What is the purpose of the getter?
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.
Why is there a vacuum inside a vacuum tube?
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.
But aren't some tubes supposed to have gas inside them?
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.
What is basing?
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.
What causes microphonics?
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.
