August 1944 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Yeah, you're right, almost nobody needs a chart of
vacuum tube schematic symbols, but for the few nostalgic types
who do, here is one from a 1944 edition of Radio-Craft
magazine. As with schematic symbols for transistors, these renditions
are representative of the general appearance you will find, but
the exact manner in which tube symbols appear vary greatly. Perusing
all the various electronics publications of the day will reveal
Electronic Tube Symbols
Adherence to Fundamental Principles Prevents Costly Mistakes
By Carl E. Winter
Fig. 1 - Standard methods of showing internal elements
in tubes, also a tube base diagram.
In reading or drawing radio schematic wiring diagrams, the
tendency to ignore the drawn symbol and to rely upon the written
designation to. explain the tube's function is very strongly manifested.
The reason for this is obvious. Many technicians and engineers
lack the practice to fully interpret tube symbols and thus gain
the greatest efficiency and speed in working with blueprints.
It is not always true that a tube symbol indicating a diode's
function should be drawn as a diode. If the tube used is a 6Q7 and
only the diode portion is used in the circuit application, the tube
symbol must, none the less, be drawn representative of the 6Q7 tube.
If a tube possesses two plates for full wave rectification and
only one is used as shown by the schematic, the other cannot merely
be left out of the symbol, it must be shown so that the engineer
or technician can tell at a glance what type of tube is actually
used and can quickly estimate its operating characteristics and
circuit requirements. The unconnected elements of the tube should
be terminated at the circle indicating the tube's envelope.
By adhering to this method of direct representation, the possibility
of errors getting by all concerned is greatly reduced.
Careful use of tube symbols will also simplify the drawing of
the necessary correlated tube socket, tube base, and wiring diagrams.
If, for example, a 6X5 rectifier is drawn as a diode and marked
6X5, the reader of a schematic can usually identify the tube as
possessing two plates. But if, by some error the legend "6X5" is
not shown or is, accidentally written as "6Q7", the tube socket
drawings, tube base diagrams and wiring diagrams may all be prepared
for the 6Q7 tube's connections. Many headaches follow when eventually
the error is discovered. If a tube symbol is drawn to represent
a ,particular tube such confusion is reduced to a minimum.
Many tubes which have their elements internally connected are
used in present day circuits. Similarly, only certain elements of
other tubes are actually wired to circuit components. These are
factors which must not be ignored in drawing schematics.
While it may not be essential to show unused elements and connections
in a schematic they are definitely needed in wiring diagrams and
tube socket drawings.
As in most things this principle of adhering too rigidly to graphical
symbolism can be overdone, There are only a few basic types of tubes
although the tubes of each type may run well into the hundreds.
The basic types have generally recognized symbols to represent them
and if these few are studied and known they will serve to reduce
the major problems arising from tube symbolism in radio circuit
The use of graphical tube symbols in actual wiring diagrams is
not general practice. It is customary in this type of drawing to
layout pictorially the tube socket with its pin -indicating numerals
and draw the wires leading to them.
In this pictorial work the possibility of confusion is very small.
Wiring diagrams are usually used in conjunction with schematics
and tube socket drawings.
Tube socket drawings picture the tube base itself with call outs
to indicate which prongs are "hot" and which are not used.
Fig. 2 - Symbols for the commonest types of tubes. Slight
modifications are needed for others. The cathode-ray tube
is also shown.
The type of illustration shown in most tube manuals is not a
true graphical symbol of the sort used in radio schematics. Rather,
it serves to portray the elements of the tube in a simplified pictorial
fashion and to indicate which tube base pins are connected to each
of the tube's elements.
Because of the diversity of types of drawings required on various
blueprints to indicate a given tube, it follows that a graphical
tube symbol is a cross between a pictorial portrayal of the tube
and a symbolic semi-wiring diagram of the tube itself.
At present we are concerned only with representative drawings
of tube elements as they appear on a graphical symbol within the
glass or metal vacuum envelope of the tube itself.
This envelope is usually indicated by a circular or oval outline
and the leads from each element are carried through this circle
into the wiring of the schematic itself. It is not customary procedure
to indicate on the schematic that the tube is either metal or glass.
Many engineers, in drawing rough circuits, or to illustrate a
point during discussions, will leave the enveloping circle of the
tube's case off entirely, but in drawing a schematic to be used
for production or design it is best to encircle the tube elements
to avoid confusion.
As any tube will contain at least two elements and even the most
complex will be confined to grids, plates, cathodes and heaters
in various quantities and positions, a primary requisite is the
ability to identify each tube element as shown in the symbol.
The "heaters" are usually drawn with no direct connections to
their cathode. Standard methods of showing heaters are given on
this page. Cathodes used in conjunction with heaters, and directly
heated cathodes are also shown (Fig. 1-a).
Plates (anodes) may be drawn in any one of several ways but the
illustrations given here are customary methods of representation.
The commonest of these are shown in Figs. 1-b and 1-c.
A gas tube is always drawn with a distinguishing dot or circle
within the tube outline and pilot lights or neon bulbs are drawn
in such manner that confusion is impossible.
When tube elements are connected within the tube the symbol is
drawn as indicated in Fig. 1-d, but if elements are interconnected
outside of the tube's envelope, even if the interconnections are
in the pins, they must be drawn external to the tube symbol, as
in Fig. 1-e.
Television tubes are in a class by themselves. An example of
a symbol for a television tube is shown in Fig. 2. The elements
of these tubes, while similar to those of receiving and transmitting
tubes, serve different purposes. Consequently the drawing of graphical
symbols for television tubes should be treated and studied as a
In graphical tube symbolism, as in all schematic drawings of
radio circuits, the important thing is to standardize. Standardization
of all graphical symbols and of tube symbols in particular will
repay the radio engineer, draftsman or technician a hundredfold
in eliminated errors and in quicker, more ready interpretation of
Posted August 15, 2014