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
many "standards." There are still many people around the world who service and
restore vintage vacuum tube equipment.
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 diagrams.
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
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
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
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 separate subject.
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 23, 2021(original