Vacuum tubes, which can also be referred to as electron
thermionic valves are a type of active device that uses thermionic
emission to generate electrons. All tubes will have at least two
elements, an anode and a cathode. The entire assembly is
sealed inside a glass or metal envelope
a hard vacuum. Metal tubes may have the envelope electrically connected
to one of the external pins, so that it can be used as a shield.
The cathode may either be a bare tungsten wire (a
cathode), or more commonly a metal oxide coated metal tube containing NiCr heating
elements electrically isolated from the cathode with mica fibers.
Directly heated cathodes are usually used when the filament will be
running from DC, or in some types of rectifiers. Since indirectly
heated cathodes aren't electrically coupled to the heater, a separate
voltage is used, which may be AC or DC. The combination of heat and a
high-vacuum environment allows the cathode to generate a cloud of free
electrons, which are attracted to the positively charged anode.
Depending on the type of tube, there may be one or more elements
between the cathode and anode to control the beam of electrons.
Some tubes have gain,
be used as an amplifier. The level of gain of triodes and other tubes
with gain is referred to
in technical literature as mutual conductance, expressed as mu or μ.
High mu tubes are tubes with a high level of voltage gain, which are
used to increase the voltage of low level signals like those output
from microphones and phono cartridges. An example of a high mu tube is
the 12AX7, which has enough voltage gain to increase the 4.7mV output
from a phono cartridge to almost line level.
Medium mu tubes have slightly less gain and are used
the signal chain, as a gain buffer in a preamp, for example. A common
medium mu tube that is frequently used as a preamp output tube is the
Low mu tubes have low voltage gain and high current
output impedance. They're used as power output tubes to drive things
like loudspeakers and headphones.
A tube is generally plugged into a device via a standard
socket. The three most common types of tube sockets are 1" 8-pin
sockets (octal), four pin sockets used on a few directly heated triodes
like the 300B, and miniature 9-pin (noval) sockets used for smaller
all-glass tubes. All tubes generate some heat, how much is dependent on
the type of tube. Small preamp tubes only become warm to the touch,
while larger output tubes and rectifiers can get quite hot under load,
although not as hot as a lightbulb. Since heat rises, heat sensitive
components like electrolytic capacitors shouldn't be mounted directly
above a tube.
Types of Tubes
In audio engineering there are four common types of
triodes, tetrodes and pentodes, named after the number of electrodes in the tube. In each type,
there are hundreds of different models, each with their own operating
characteristics and pinouts.
Diodes are the simplest form of tube with only two
cathode. These work the same way as their solid
allowing current to flow in only one direction, like an electrical
version of a check valve. The usual use for this type of tube is in an
AC-to-DC rectifier. Rectifiers with one or more diodes in the same
envelope are available with a wide range of operating voltages and
currents. The common 5AR4 rectifier is composed of two diodes with a
common cathode in the same envelope, and provides full wave
rectification when used with a center-tapped transformer, with the
center tap as "ground".
Smaller diodes are used for signals, such as separating
positive and negative component of an audio signal, or as a detector in
radio sets. For the most part, tube based diodes and solid state diodes
are interchangable, but a lot of new audio equipment designs still use
mostly because it allows for a soft start with a slow rise in voltage
that prolongs the life of the other tubes in the circuit, and it's a
little easier than going through the trouble of adding timers and relays.
A triode is the basic tube with gain,
it performs the same general function as a transistor, it is an
electrical gate valve. As in a diode, the triode has a cathode
generating electrons and a positively charged anode that attracts the
electrons. In between the anode and cathode is a third element, usually
made from a spiral of fine wire, called the grid. Applying a slightly negative
voltage to the grid completely stops the flow of electrons. As the
positive voltage on the grid is increased, more electrons are passed to
the anode, increasing the anode current. Because the flow of electrons
can be controlled with a small voltage to produce large voltage swings
at the anode and cathode, a triode can be used as an amplifier.
Tetrodes are similar to triodes, but with an extra
anode and grid. This screen grid
lowers the capacitance between the grid and anode. Since the grid-anode
capacitance repels some of the electrons, reducing it allows for higher
Pentodes are similar to tetrodes, but with an extra
screen grid and anode, called a suppressor. Under certain operating
conditions, electrons striking the anode dislodge other electrons in a
process called secondary emission.
Since this is undesirable, the suppressor was added to direct the
secondary electrons back towards the anode. In practice, it is normally
connected to the cathode. Pentodes are commonly used as output tubes,
since they can handle larger voltage swings with less distortion.
Beam Power Tubes
This type of tube, which may be a tetrode or pentode has
confining electrodes to shape the stream of electrons leaving the
cathode. Once again, this type is often used for power amplifiers
because of their ability to handle large current swings with relatively
low distortion. The 6V6 is an example of a beam power tetrode, the 6L6
is an example of a beam power pentode.