Practical Notes on the Micromesh 'Tunograph'.
The Tunograph indicator, image courtesy of the Museum of Technology.
The tuning of a receiver fitted with AVC is apt to be more difficult than that of set not so equipped, for the automatic volume control appears to flatten the tuning, and a precise setting for optimum signal strength cannot readily be distinguished. It is the usual practice, therefore, to tune a receiver of this type for minimum background hiss, or for the best quality of reproduction, instead of for maximum signal strength. There is no real difficulty in this once the operation has become familiar, and after a little practice it is quite possible to tune as accurately as with the aid of an indicator.
There is no doubt, however, that a visual tuning indicator is a help, particularly when the receiver is to be handled by unskilled members of the family.Various methods of visual tuning are possible, but methods depending on the movement of a light or a shadow are probably the most popular. The Micromesh 'Tunograph' comes under the former of these headings, for the position of a spot of light on a screen is made to vary with the strength of the signal.
Fig. 1, The electrode arrangement and base connections of the Tunograph.
The Tunograph is really a miniature cathode-ray oscillograph, and some idea of its structure can be gained from the photograph and from the drawing of Fig. 1. The heated cathode emits electrons which are drawn away by the positive anode and concentrated into a beam by the focusing shield. The beam of electrons passes through an opening in the anode and then between the deflecting plates until it finally strikes a fluorescent screen mounted at the upper end of the bulb. One focusing plate is connected internally to the anode, and the other is available for external connection. If both are maintained at the same potential, the beam of electrons strikes the screen at the extreme right-hand side, with the tube mounted vertically, and gives the characteristic green spot of a cathode-ray tube.
If the free focusing plate be now biased negatively with respect to the other, the spot of light moves towards the left by an amount which depends upon the bias, and some 40 Volts is needed to bring it to the extreme left-hand side of the screen. In practice, therefore, the change of anode current with grid bias on a controlled valve is made to deflect the spot of light, and, since the change of current increases with signal strength, the deflection of the spot is greatest, for a strong signal.
The indicator is not confined to an arrangement of this nature, however, for being practically free from inertia, the electron beam can follow the alternating signal potentials. If the deflecting plates be maintained at.the same steady potential, therefore, in the absence of a signal, the spot of light will appear at the right-hand side of the screen. The application of signal potentials between the plates will then cause the spot of light to vibrate backwards and forwards in a straight line, with the visible result of a line of light the length of which is dependent upon the strength of the applied signal.
Fig.2. The connections for a, line indication are shown here, and it will be observed that the Tunograph is fed from the last HF or IF tuned circuit.
The circuit connections for this arrangement are shown in Fig. 2. and it will be seen that the Tunograph is connected across the last tuned circuit of the receiver. Since the tube has a certain capacity, this circuit will require re-trimming after fitting the indicator but this should present little difficulty. It will be found that the line of light spreads to the left with increasing signal strength, and tuning is carried out for maximum length of line.
This method of indication, while very attractive from some points of view, is open to two objections in practice. In the first place, a very strong signal is necessary to give any decided line, about 13 Volts for a length of 1 cm, and very few receivers are designed to operate with a detector input greater than one-tenth of this. Secondly, there is a reduction in visibility when the spot spreads out into a line, and on a very strong signal the line itself may be barely visible, and it is possible to see only a faint line with a blob at each end.
Fig. 3. With these connections, the position of the light spot varies with the bias on the controlled valve, and hence with signal strength.
For general use, therefore, the alternative circuit, Fig. 3, is preferable, although this necessitates slightly more alteration to the set. A resistance of some 10,000 Ohms must be included in the anode circuit of one of the controlled valves. The anode of the Tunograph is connected to the positive HT side of this, and the free deflecting plate to the other. Assuming an anode current with no signal of 4 mA the free deflecting plate is then biased negatively with respect to the other plate by 40 Volts, so that the spot of light comes to rest at the extreme left-hand side of the screen. When a signal is tuned in, and AVC comes into action the anode current of the controlled valve drops by an amount which is dependent on the signal strength, with the result that the voltage drop across the 10,000 Ohm resistance falls proportionately, and the free deflecting plate becomes less negatively biased, so that the spot of light moves to the right. Tuning, therefore, is carried out for a maximum movement the spot of light.
The disadvantage of this method, when the tube is to be fitted to an existing set is that the necessity for including a resistance in the controlled valve anode circuit means that its anode voltage is reduced. The value of 10,000/20,000 Ohms marked on the circuit diagram is suggested by the makers; it will, however, obviously depend upon the type of valve used. Since, with no signal, a voltage drop across it of 40 Volts is required, 10,000 Ohms is correct for a valve taking 4 mA., and 20,000 Ohms for one taking 2mA. Some variable-mu valves, however, require 8mA and then the requirements are met by a resistance of 5,000 Ohms. In practice the resistance value is best determined by selecting a value such that with no signal the spot of light, settles at the extreme left of the screen. When adding the indicator to a set, it will probably be best included in the anode circuit of the earliest controlled valve, for the reduction of 40 Volts in its supply will probably, be least felt here.
The anode potential of the Tunograph must beat least 180 Volts, and care should be taken to see that the mains transformer is capable of supplying an additional ampere of current for the filament without an excessive drop in heater voltage on the valves. Since the filament of the indicator is rated for only 0.5/0.6 Volt, it must be run from the 4 Volts AC lines through a 4 Ohm resistance rated to carry one ampere. It should be noted that the spot of light is not readily visible in a bright light unless the tube be enclosed in a light-proof box with a viewing window. It is suggested that the tube be mounted inside the cabinet in any convenient position and a small hole cut in a suitable position on the panel for viewing. In general, it will be best to mount the tube horizontally, with the left-hand side of the plate at the bottom when using a spot indication and the right-hand side at the bottom when using a line indication. for the spot or line will then move vertically on tuning in a signal, and the operation of tuning will be carried out for maximum height of spot or line.
It should be pointed out that the spot can be deflected by external magnetic or electrostatic fields, and a small deflection will be observed on touching the glass bulb with the finger, due to the dissipation of the charge which accumulates on the glass. It is not improbable, therefore, that the spot might move right off the screen if the tube be mounted in close proximity to a loud speaker, mains transformer, or smoothing choke.
The Tunograph indicator, showing the light spot on the screen. As the tuning is varied to bring the set into resonance with the signal, the light spot moves to the right.
Thanks to Clive Evenett for sending a copy of the article.