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Recent years have been noteworthy for the number of new valves introduced, and in particular for the multi-electrode frequency-changers and the multiple-diode types. although plenty of fresh valves will make their appearance at this year's Exhibition, there are signs that the flood has been stemmed, for new valves are not so much new and revolutionary types as modifications and improvements to existing ones. It is, of course, necessary for the valve maker to retain his older valves, and will be for years to come, since they are required for replacement purposes in existing sets. No reduction, but rather an increase, in valve types is still to be expected, therefore, but it seems probable that there will be a decrease in the number of valves which can strictly be listed as current specimens.
The Mullard deaf-aid midget valve can be seen on the left, while the new Acorns are on the right compared with a cigarette to emphasise their small dimensions.
One of the few valve types which is truly new in this country is the Acorn. Osram will be showing a triode of this type, the HA1, and Mullard a triode and a pentode The former of these is the AT4, and has a mutual conductance of 2.0 mA/V with an AC resistance of 12,500 Ω, while the pentode, AP4, has a mutual conductance of 1.4 mA/V. and a resistance of 3.5 MΩ. They are indirectly heated and consume 0.2 Ampere at 4 Volts. In view of their small size, the inter-electrode capacities are very low, and they are suitable for operation at frequencies as high as 430 MHz (0.7 metre). The triode is priced at 50s and the pentode at 60s.
Marconi and Osram have introduced a range of AC mains valves with heaters consuming only 0.6 A at 4 Volts in most types. These comprise the W42, an HF pentode with a 'top-grid' and a mutual conductance of 1.5 mA/V, the X42, a heptode frequency-changer, the H42, a triode with a 7 pin base and a 'top-grid'. This triode has a mutual conductance of 1.5 mA/V and an AC resistance of 66,000 Ω and is intended chiefly for use in resistance-coupled amplifiers. The pentode in this series, the N42, consumes 1.0 A and is capable of an output of 3 Watts. The well-known S23 battery screen-grid valve is now marketed in the modern dome-type bulb, as is also the MHL4. The MH4 also has this bulb, and has been improved in that the noise level has been reduced to an exceptionally low figure. There will also be a new rectifier, the U18, rated for 250 mA output at 500 Volts.
The Harries Thermionics All-Purpose valve, and right, the Osram N43 output pentode for television amplifiers.
An interesting development is the All-Purpose valve which will be shown by Harries Thermionics. This is a multi-electrode valve of the indirectly-heated type, having five grids and an anode. By connecting the valve appropriately it can be made to function as a heptode frequency changer, a screened amplifier, duo-diode-triode, or as an output valve of the critical-distance anode type. When used as a frequency-changer or HF amplifier, AVC bias is not applied the control grid in the usual fashion, but to a special grid.
Output valves of the Harries type will be shown by Hivac, and the ACYY is a particularly interesting specimen, since an output of 10 Watts is claimed from two in push-pull for an input of only 22 Volts peak.This firm will also have at range of AC/DC type valves with heaters consuming 0.3 A at 13 Volts, and there will also be battery valves fitted with ceramic bases for short-wave working, as well as a midget triode enclosed in a metal sheath.
This form of construction has been adopted by Mullard for their new Deaf-Aid valves, which are of midget dimensions. The DA1 has a mutual conductance of 0.5 mA/V and a resistance of 60,000 Ω, while the DA2 has a mutual conductance of 0.78 mA/V and at resistance of 9,000 Ω. Both valves are rated for 100 Volts maximum anode potentials and have filaments consuming 0.05 A at 2 Volts. This firm will also be showing a cathode-ray tuning indicator, the TV4, which comprises a triode amplifier and a cathode-ray tube built into a bulb of rather smaller dimensions than the average triode. A triode-hexode, the TH4, for AC operation, will also be on view, and for this a conversion conductance of 11.0 mA/V is claimed.
The Mazda range will include a new triode output valve. This is the PA20; it has characteristics identical with these of the PP3/250, but has a filament rated at 2 Volts 2 Amperes, This low voltage is adopted in order to reduce the possibility of mains hum being introduced by the LT supply when this is AC. There will also be an interesting range of transmitting Valves.
British Tungsram will have a wide range of valves, among which the VP4B is worthy of mention. This valve is designed to operate with the same screen and anode voltages and has a mutual conductance of 2.8 mA/V. Its particular feature is a variable-μ characteristic which is almost exactly exponential. A triode-hexode is included in the range.
The 402-Pen is one of the more interesting valves on the Cossor stand an output pentode of the AC/DC type with a heater consuming 0.2 A at 40 Volts; it has a mutual conductance of 7.0 mA/V. When operated at 200 Volts the anode current is 40 mA.
Large output triodes will form a portion of the exhibit of 362, and will include the PX100, a valve rated for an output of 35 Watts and requiring 1,000 Volts for the HT supply.
A number of firms will be showing valves developed especially for television purposes. The Mullard TSP4 is one of these, and is an HF pentode having the high mutual conductance of 4.73 mA/V. It is capable of an output of 30 V RMS into a 10,000 Ω load tor 3.4% second harmonic distortion. The Osram MSP41 is intended for use in television amplifiers, and has characteristics similar to those of the MSP4, but is rated for a screen-grid potential of 240 Volts, and this enables a mutual conductance of 3.2 mA/V to be obtained with a grid bias as high as 4 Volts. This firm will also have the N43 output pentode for television sets; it is similar to the N41, but has a top-grid connection and a very low grid-anode capacity.
A number of high-voltage rectifiers for use in cathode-ray equipment will also be on view.
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