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Footless Valves

Wireless World, May 25, 1939.
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Valve development proceeds very rapidly nowadays and the changes are more often than not in the mechanical form in order to obtain increased efficiency on short waves. One of the latest trends - the abolition of the pinch and top cap - is described in this article.

New Tungsram range for short waves.

One of the greatest disadvantages of the conventional form of valve construction is the length of the internal connecting leads between the base pins and the electrodes. The promixity of these leads adds very considerably to the inter-electrode capacities, and their length makes their inductance appreciable at high frequencies.

The inductance of the cathode lead is probably the most important, for it makes proper de-coupling of the circuits impossible. One serious result is that on short waves the input resistance of the valve is considerably lower than would be the case if the electron transit time were the only factor to be taken into account. The inductance of the screen-grid lead is also disadvantageous, since it makes the effective capacities rise with frequency.

Recent valve development has lain very largely in the direction of reducing the length of the internal leads, and considerable improvement in valve performance has resulted. This process was carried to its logical conclusion some years ago in the well-known acorn, in which the very small electrode clearances also greatly reduced electron transit time effects.

While the acorn is the only suitable type of valve for some applications, it is too difficult to manufacture and too fragile for general use. Recent development, therefore, has had for its aim the attainment of electrical characteristics approaching those of the acorn combined with a mechanical construction which is not inferior to that of ordinary valves.

This was first approached by adopting conventional forms of construction, but reducing the length of the valve and using a special form of base to reduce the length of. the internal leads. One example of this development is the well-known 'E' series with the side-contact base.

The next stage was to abandon the use of the glass pinch for supporting the electrodes and to substitute for it a glass ring of the same diameter as that of the circle of base pins. This ring forms part of the glass wall of the bulb, and not only gives a considerable reduction in the length of the leads, but the leads themselves are much more widely spaced in the glass support. This results in a reduction of the inter-electrode capacities and at the same time gives greater mechanical rigidity. In addition, it makes it possible to bring both anode and grid connections out to the base, instead of having to take one of them to a top-cap, because there is now room to introduce internal screening between the leads.

This in itself is by no means an unimportant development, because it permits a considerable reduction in the length of the connecting leads external to the valve. The top-cap has long been a nuisance in receiver design.

Valves of this type with an all-glass construction have been described in The Wireless World, and the practice has been to retain the usual vertical electrode assembly. In Germany, however, Telefunken have produced a range of valves with the electrodes arranged horizontally and use an all-metal construction with glass beads for insulating the leading out wires. The horizontal construction is claimed to give increased rigidity, because the electrodes can readily be anchored at each end.

A new range has now been introduced by Tungsram. The valves are in some ways similar to the Telefunken, for they have a horizontal electrode assembly and have the same arrangement of the base pins, but they are of all-glass construction with an external metal cover for screening and protection.

The base is a modified octal, but with a larger pin circle to increase the spacing; the pins are provided with a waist to give a locking action in the socket.

In this way mechanical rigidity and easy assembly are combined with the desirable electrical properties of the footless valve, and at the same time the advantage of the well-tried evacuation technique of a glass valve is retained.

The makers claim that in a comparison of these new valves with the usual types with a pinch construction the following advantages are gained:-

  • Lower inter-electrode capacities and reduced internal lead length, giving improved performance on short and ultra-short-Waves.
  • Grid and anode connections at the same end, thus reducing external lead lengths.
  • Greater mechanical rigidity (every electrode has a three or four-point suspension instead of the usual two-point).
  • Smaller physical dimensions.
  • Greater reliability because of the simpler construction.
  • Reduced leakage, due to the absence of the pinch and the use of fewer mica spacers, thus giving quieter operation.

The valves so far produced in this range are a variable-mu RF pentode EF11, a straight abrupt cut-off RF pentode EF12, a double diode RF pentode EBF11, a triode-hexode frequency- changer ECH11, an output pentolde EL11N, and a rectifier AZ11. Further valves to be produced area cathode-ray tuning indicator EM11, and an AC/DC output pentode and rectifier CL11N and CY11.

The EF11 is rated for 250 Volts and 100 Volts anode and screen potentials, and at the normal bias of -2.2 Volts the anode and screen currents are 6 mA. and 2 mA. respectively. When the screen potential is fixed, a bias of 17 volts is needed to reduce the mutual conductance from its normal value of 2.2 mA/V. to 0.022 mA/V. If the screen is fed from the 250 Volt line through. a dropping resistance of 75,000 Ohms, however, 45 Volts bias is needed to obtain this value of mutual conductance. The valve can thus be arranged to have either a short or a long grid base, as desired. The maximum cathode current is 10 mA and the input and output capacities are 6.1 pF and 6.5 pF respectively with a grid-anode capacity of 0.002 pF. The straight pentode EF12 has similar general characteristics, but the mutual conductance is 2.8 mA/V.

The Triode Hexode

The triode-hexode ECH11 is rated for 250 Volts anode and 100 Volts screen potentials on the hexode section. with currents of 2.3 mA and 3 mA. at 2 Volts grid bias, with fixed screen potential, 13 volts bias is needed to reduce the conversion conductance from 0.65 mA /V. to 0.00325 mA/V, whereas if the screen is fed through 50,000 Ohms a bias of 21 Volts is necessary to bring the conversion conductance down to 0.0016 mA/V.

The triode section, which is used as the oscillator, should have a 50,000 Ohm grid leak, and with the correct amplitude of oscillation the grid current is 200 μA The anode should be fed through 30,000 Ohms from the 250 Volt line and it takes a current of 3.3 mA. This section has a mutual conductance of 2.8 mA/V with an amplification factor of 20

The hexode section has input and output capacities of 5.3 pF and 9.1 pF respectively; the triode grid-cathode capacity is 4.3 pF, the anode-cathode capacity 2.5 pF and the grid-anode capacity 1.5 pF. The capacity between the hexode grid and anode is 0.001 pF, between the hexode grid and the triode grid 0.2 pF, and between the hexode grid and heater 0.001 pF.

The characteristics of the pentode section of the EBF11 double diode RF pentode are very similar to those of the EF11. The anode and screen currents are 5 mA. and 1.8 mA. respectively, however, and if a dropping resistance is used for the screen supply it should have a value of 85,000 Ohms.

The particular feature of this valve is the extremely low inter-electrode capacities. The input and output capacities are 4.9 pF and 6.2 pF respectively with a grid-anode capacity of less than 0.002 pF. The-diode capacities are D1 cathode 2.3 pF, D2 cathode 2.7 pF and DI - D2 0.5 pF. The capacity between the pentode anode and either diode anode is less than 0.015 pF and between the pentode grid and either diode anode it is less than 0.001 pF.

These four valves all have heaters rated at 6.3 Volts 0.2 Ampere, but the output pentode EL11N takes 0.9 Ampere at this voltage, The similar CL11N will have a heater rated at approximately 40 Volts 0.2 Ampere.

The EL11N is rated for 250 Volts anode and 275 Volts screen potentials with 6 Volts grid bias, the currents being 36 mA and 4.0 mA respectively. The bias resistance should be 150 Ohms and the load impedance 7,000 Ohms. The output is given as 4.5 Watts for 10% total harmonic distortion and the input as 4.2 Volts RMS. The mutual conductance is 9 mA/V. and the AC resistance 50,000 Ohms.

The AZ11 rectifier has a 4 Volt 1.1 Amp filament and is of the full-wave directly-heated type. At 300 Volts RMS input the maximum output current is 100 mA, but at 500 Volts input it is rated at 60 mA only.

The the metal shell is internally connected to the cathode pin. Partly because of this, and partly to reduce the length of the external common cathode connection to the valve, the use of cathode bias is not recommended in the early stages.

Fig. 1.

Bias by means of a resistance in the negative HT lead can be used, but is usually inconvenient. The makers recommend the omission of any initial bias and that the circuit be arranged in the form shown in Fig. 1, where R1C1, R2C2, R3C3, R4C4 are the usual AVC filters. In the EBF11 one diode is used as the detector, with R5 for the load resistance and C5 for the by-pass capacitor. The other diode is connected after R4 to give a small measure of delay on AVC.

Owing to the fact that the diode passes current up to about -1.0 Volt anode potential, the normal no-signal anode potential is, about -0.9 Volt, and the controlled valves consequently have a slightly negative grid potential which prevents their cathode currents from becoming excessive. On any normal signal, of course, bias is provided by the AVC system.

This method of operation considerably simplifies the receiver, and its only disadvantage is some reduction in the initial sensitivity. This may not always be considered a disadvantage however, for the effect is that of a QAVC action.

The EF11, EF12, EL11N and CL11N, are provisionally priced at 10s. 6d.; the ECH11 and EBF11 at 11s. 6d.; the AZ11 and CY11 at 9s., and the EM11 at 8s. 6d.

A sketch of the electrode system of an RF pentode is shown here, together with the arrangement of the base pins. NB. This is yet another pin layout for the footless base. See Base Footless for the Philips arrangement adopted by the museum - ed.
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