standards

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.

Valve Defects  

Some common valve defects which may affect the performance of a receiver include: 

(1)       heater or filament discontinuity  

(2)       loss of cathode emission

(3)       leakage or contact between any two electrodes, especially between cathode and heater

(4)       presence of gas

(5)       grid emission

(6)       microphony

(7)       mechanical damage

Negative grid current in a valve can be caused by the presence of gas, internal leakage paths within the valve or valve base or grid emission, and may, in turn, give rise to distortion, lack of sensitivity and output, broad tuning, excessive anode currents, etc.

The effects of grid emission may not become apparent until a few minutes after the receiver is switched on: a gradual loss of sensitivity suggesting that grid emission is taking place in an R.F. or I.F. valve; while a gradual increase of distortion may indicate its presence in an A.F. stage.  Negative grid current may be checked by inserting a 50 microAmp f.s.d. meter in series with the earthy end of the grid ‘leak’ resistor.

A form of mechanical damage, more common in “all-glass” types is the accidental bending of the pins.  As these may be damaged during inserting or removing from their holders, great care is necessary, especially if an attempt is made to straighten bent pins with normal tools. Best not to bend them in the first place! Special jigs were available for this purpose on valves using B7G, B8A and B9A bases.

* Thoriated tungsten is one of the metallic elements used to make cathodes and filaments. Plain tungsten was first used but it did not create an adequate supply of free electrons. Thoriated tungsten was a great improvement.

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