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A feature of post-war valve production in the
United Kingdom was the wide measure of standardisation that had been
achieved by the different manufacturers. Lists of "preferred types" for use in domestic broadcast
receivers were prepared for the guidance of designers in an effort to
ensure that in the future the radio service engineer would not be
confronted with such enormous numbers of different valve types as had
been used during the past twenty years.
Manufacturers' lists specified whether a particular valve was "current
equipment" type, " Replacement"
(i.e., not recommended for new
equipment, but still being manufactured for replacement purposes), or
"obsolete" (i.e., manufacture discontinued).
By the beginning of the 1950s,
current-equipment valve types and bases were generally restricted to a
limited range within the following main groups:
(1)
6.3-volt heater, international octal base
(2)
0.3-amp. heater, international octal base (there is considerable
overlapping in groups (1)
and (2)
(3)
0.15-amp. heater, international octal base (12.6-, 25-, 35-volt heaters,
etc.)
(4)
0.2-amp. heater, international octal base
(5)
0.1-amp. heater, glass-button
miniature construction, B8A, B9A bases
(6)
1.4-volt filament, international
octal base
(7)
1.4-volt filament, glass-button miniature construction, B7G base
(8)
6.3-volt heater, B8B, B8G, " loctal " and " lock-in " bases.
A wide variety of other types may be
encountered, especially in earlier sets (pre-war and immediately post-war).
These include:
(1)
4-volt heater, British 5-, 7- and 9-pin bases;
(2)
2-volt filament, British 4-, 5- and 7- pin bases;
(3)
2.5-volt heater, American UX bases;
(4)
bases such as the Mazda Octal, side-contact, "
acorn " and footless
American coding system for octal valves
The suffixes G, GT and MG appended to
the basic type number of valves so coded refer to the physical and not to the
electrical characteristics.
G, glass; GT,
glass-tubular (bantam); MG, metal-glass; no suffix, metal.
Conditions for correct use
The valves in well-designed sets should still
be working under correct conditions and will remain so provided that service
information is correctly followed when repairs are carried out. Unfortunately,
there were – and still are - many ‘bodgers’ and short-cut merchants around.
Typical of their ‘tricks of the trade’ is the dropped mains tapping, overrunning
the valves and other components, the fitting of incorrect valve types and the
shorting out of fuses. As the useful life of a valve is governed to a
considerable extent by the conditions under which it is operated, these
procedures can result in the quite rapid demise of initially good valves, as
well as make an originally safe set dangerous and a possible fire hazard. It is
also worth pointing out that under-running a heater can poison the cathode and
is therefore as damaging as overrunning.
The following notes are based on the "Code of
Practice on the Use of Radio Valves in Equipment" (B.S.1106) and on the
recommendations of members of the British Radio Valve Manufacturers'
Association.
Apart from the
application of incorrect potentials, excessive dissipation may be caused by
incorrect tuning of associated circuits, unnecessarily high no-signal currents
or parasitic oscillation in A.F. and R.F. stages.
Filament and heater voltages should generally
be maintained within ± 7 per cent of the rated values.
The heater current of valves connected in series should be maintained within ± 5
per cent of the rated values.
Thoriated-tungsten* and oxide-coated filaments should be maintained within
closer tolerances than the above figures: 5 Per cent voltage fluctuations are
permissible, but permanent deviation from rated value will reduce valve life. Directly heated and indirectly heated valves having similar
filament current ratings should not be connected in series.
With 2-volt filaments a tolerance of ±
10
per cent is permissible, though this may result in some variation of the
valve characteristics. I.4-volt
filaments are designed for use with dry-cell batteries having a rated terminal
voltage of 1.5 volts, but when
mains operation or accumulators are used, the voltage drop across each 1.4-volt
section of filament should have a nominal value of 1.3 volts.
With certain types of valves under-running the heater is as harmful as
over-running. If the voltage at the
filament or heater terminals rises by 7 per cent, the current will increase by
considerably less than this figure, since the resistance of the filament or
heater increases as its temperature rises.
The following table shows voltage fluctuations in terms of voltage and
per cent tolerance for 220-volt mains:
Nominal Mains Voltage
Tolerance
Voltage
Range
220 V.
± 2%
2I5.6-224.4
220 V.
± 5%
209 -231
220 V.
± 7%
204.6-235.4
220 V.
± 10%
198 -242
220 V.
± 15%
187
-253
Cathodes:
The maximum D.C. potential difference
(i.e., A.C. peak value) between heater and cathode should not exceed I50 volts,
except with valves specially designed for A.C./D.C. operation.
A D.C. path should exist between the cathode and each electrode and
between the cathode and all internal and external screens.
It is not good practice for the heater-dropping resistor to be common to
both H.T. and heater supplies; this, however, is not important where the
resistor is of low value.
Control Grids:
The resistance between the grid and cathode should be the minimum practicable.
Typical figures are: R.F. pentodes and frequency changers - not greater
than 1Mohm, or 3.5Mohm where automatic bias circuits are used; mains-output
valves 0.1Mohm, or 0.5Mohm where automatic bias circuits are used; 1.4-volt
battery valves are an exception, and values up to 10Mohm may be used in certain
circumstances.
Valves should not normally be operated under conditions that cause grid
current.
Screen grids:
The voltage supply for the screen grids of frequency changers and beam tetrodes,
should be obtained from a potentiometer network, the resistor values being as
low as possible to prevent undue variation of voltages.
Mounting:
Valve-holders which incorporate floating contacts have them for a reason and
should not be stiffened by having heavy wire connections made to them.
When replacing holders, replace like-for-like, especially where cushioned
or sprung holders have been used where the valves would otherwise be subjected
to continuous vibration. Chassis
mounting methods should be preserved. To reduce microphony, the chassis should
not be rigidly fixed to the cabinet containing the loudspeaker.
The characteristics of valves placed in a strong magnetic field may be
affected: miniature I.4-volt valves, in particular, should not be mounted in
close proximity to loudspeaker magnets.
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