GUIDED GRID VALVES

The first of the following two extracts comes from the Practical Television magazine for March 1960 edition of Practical Television magazine. The second artical is from the same magazine but in Dcember 1962 following the introduction by mullard of the PC97 guided grid valve.

A NEW TYPE FOR HIGH GAIN

THE trend in modern television design is to reduce the size of components and receivers so that entertainment can be obtained without the apparatus taking up valuable space. The size of picture tubes is gradually being reduced so that the large screen types need only shallow cabinets. Reductions in the sizes of other sections of the receiver, such as I.F. strips and time base units have been made, but it is only recently that the design engineers have begun to make smaller tuner units.

New Turret

Several new British designs have been announced which are now widely used and a new turret tuner is now being marketed in the U.S.A. and is substantially smaller than other current types. The reduction in size is obtained for the most part by using a new type of valve known as a " guided grid " valve. The new miniature turret occupies only a quarter of the volume of earlier tuners but has a superior performance and, with suitable coil strips, can be used for UHF reception. The tuner is the product of Standard Coil Products Co. Inc., and the R.F. amplifier valve was developed as a result of the co-operative effort of R. C. A. Eland, Director of· Research of Standard Coil Products, and two of the major American valve manufacturers. The new valve - a 6ERS or 6ES5 - is used in the R.F. amplifier stage in an improved version of the neutralised triode or Neutrode circuit.

Additional elements are included in the valve and are connected to the screen grid. These elements are termed by the inventor "the guided grid," and act as shield plates which draw no current (Fig. 1). Their inclusion in the valve reduces the anode-to-grid capacitance, thereby increasing the anode impedance. Radiation from the local oscillator into the aerial circuit is also reduced and with the increased anode impedance the coupling coil in the anode circuit is less heavily damped. As a result, the signal-to-noise ratio of the tuner is improved and its gain is considerably higher.

Although guided grid valves are not available here as yet, no doubt British manufacturers will introduce suitable types in the future. The introduction of frame grid valves in this country has enabled substantial improvements to be made in TV front end design. With these new valves excellent reception can be obtained with simpler, neater, and less bulky tuners and the increase in efficiency enables reception to be obtained even at the limits of the fringe areas.

Triode R.F. Amplifier

Until recent months all television tuners featured a double-triode r.f. amplifier valve connected in the well-known cascode circuit. Recently, however, the new Mullard PC97 triode has halted this convention and tuners are now being made with a single triode r.f. stage.

The general idea of the r.f. circuit is shown in Fig. 23. It may surprise some of our readers to see that the valve is connected in the earthed-athode mode instead of the more usual (from the v.h.f. point of view) earthed-grid mode. The earthed-grid configuration was demanded in past days to avoid instability resulting from the inter-electrode capacities of the r.f. amplifier triode. With the grid earthed, the capacitances are broken up by the grid and feedback is usually avoided.

However, the new PC97 has such a low grid-to-anode capacitance (about 0·5pF compared with 2 to 4pF of the double-triode cascode valve) that the circuit can be arranged in the earthed-cathode mode without the need for elaborate neutralising.

Generally speaking, a higher stage gain can be obtained from an earthed-cathode circuit than from an earthed-grid circuit using the same valve, and since the slope of the new triode is somewhat higher than that of its older counterparts - brought about by the frame grid construction - the overall gain of the new r.f. stage is almost the same as that of the former stages using double-triode valves. Moreover, the noise performance is improved as the result of the valve itself.

The low internal capacitance is achieved by the employment of a specially shaped earth screen situated partly between the grid and the anode assembly, as shown in Fig. 24. In effect, the screen acts as an electrostatic shield between the side of the grid assembly and the anode, without affecting the normal function of the valve.

In essence, the low capacitance means that the stage can be designed with a non-critical neutralisng circuit, and in Fig. 23 the neutralising trimmer is C1. This is connected between the low signal potential side of the anode coil L5 and the grid. By normal adiustment, a signal from the anode is fed back to the grid, and since this is of equal amplitude and opposite phase to the signal reflected into the grid circuit from the anode via the anode-to-grid capacitance of the valve, adequate neutralisa- tion is effected.