Details of the Sargrove -Tungsram Type UA -55.

Wireless World Dec, 1947, Page 483.

Designed primarily to reduce the cost of valves used in receivers manufactured by the ECME system (see Wireless World April 1947) the UA-55 is capable of performing all the stage functions in a straight or superhet receiver and is yet of comparatively small dimensions and robust construction. Effective limitation of size has been achieved by accepting a reasonably small output ( of the order of 1 watt) when the valve is used as a power amplifier. The valve is a combination of two beam tetrodes disposed symmetrically on either side of a common cathode. The heater is rated at 55 V, 100 mA and is therefore convenient for series connection in 110 and 220-volt mains supply circuits. A central screening plate with extensions between the leads to each section of the valve is connected to the cathode and is shaped to act also as the beamforming plates. In cross-section, it is " E" shaped and shields the first and second grid supports from the anode. The accelerator grid wires are aligned behind the grid wires, and a method has been found of doing this without optical aids. It is claimed that a ratio of anode to screen current of the order of 10:1 is maintained throughout the life of the valve.

With a screen potential of +15 V a high-impedance voltage amplifier with a high-impedance voltage amplifier with a slope of 4.5 mA is obtained. The anode grid inter-electrode capacity is of the order of 0.07 pF per section, and it is therefore necessary to use somewhat unconventional circuits for R.F. and I.F amplification. A neutralized circuit using a capacity centre tap on the secondary of the input transformer may be employed with one half of the valve, the other half being used as second detector. Alternatively, the two halves may be arranged as an amplifier of the Colebrook R-C coupled type, when stable gains of the order of 300 can be obtained at 465 kc / s.

In circuits where the input can be applied to both control grids simultaneously, variable-mu characteristics can be obtained by applying dissimilar voltages to the screens. Otherwise, remote cut-off characteristics can be obtained by supplying the accelerator grid potential through a comparatively high series resistance.

With anode and screen strapped, a high-slope low-impedance triode is obtained. One half of the valve can be used as an oscillator under these conditions, the other half being employed as a tetrode mixer. With this arrangement and a line voltage of only 90, a conversion conductance of 0.7 mA/V is claimed, with a cathode current of 9 mA.

When the two sections are connected in parallel and both screen and anode are supplied from a 90-volt H.T. line, the valve functions as a power amplifier with a slope of 7 mA/V at - 5 volts bias and gives over one watt into a load of 2,500 ohms with an anode dissipation of 3½ watts.

It may seem rather wasteful to use a valve of this type as a power rectifier, but the makers consider that this is economically justified if specimens that do not meet the required standards for general circuit use, and which would normally be rejected, are earmarked for use as rectifiers. Resistors must be used in series with the first and second grids to limit current, and the D.C. output voltage is substantially the same as the R.M.S. input voltage for currents up to 20-25 mA, the value drawn by the simple two valve receiver illustrated on page 438 of the November issue.

A special nine-pin valve holder is necessary. The possibility of using existing nine-pin holders was considered, but it was decided that it would be less expensive to go for optimum design in the valve and to call for a special holder. The pitch circle is 11/16th in diameter and the pins 1mm in diameter and ¼” long, are spaced 36 degrees apart except the anode pins, which are spaced at 72 degrees.

Full characteristics of the UA55 are given in a paper which will be read at the Brit. I.R.E. discussion meeting on Dec. 11th. 

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