RCA transmitting tubes are designed to give long, reliable, and trouble-free service when they are operated within their maximum ratings in properly designed equipment. Even in a well-designed transmitter, however, a tube can be subjected to an overload which may be destructive if allowed to persist. Such an overload may be caused by the failure of a driver stage. In this event, the following amplifier tube will, if biased by means of a grid leak, lose its grid bias; unless the tube has a fairly high mu, it will then draw excessive plate current. The tube must dissipate the entire d-c plate input because, with no excitation present, the plate efficiency of the tube is zero. Unless the overload is promptly removed, the tube will be damaged.
Although fixed bias from a rectifier may be employed for an r-f amplifier tube, the bias can still be lost because of rectifier trouble. Even if the grid bias and the grid excitation do not fail, an overload may result from inadvertent detuning of the plate tank from resonance. Such detuning causes a large increase in plate current and a rapid decrease in efficiency.
In view of these considerations, it is evident that radio transmitters should be equipped with suitable protective devices. D-c meters in the various circuits, while invaluable for tuning and testing purposes, as well as for power calculations, offer little assistance in preventing damage due to sudden overloads. A meter will show when the overload exists, but valuable apparatus may be destroyed before the operator can open the power-supply switch.
Protective devices, in order to be effective, must operate very rapidly when an overload occurs, so that the power input to the tube is either greatly reduced or entirely removed. Four commonly used protective devices are: (1) plate-supply series resistor; (2) cathode resistor; (3) high-voltage fuse; and (4) d-c overload relav.
A series resistor placed in the positive plate-supply lead is useful as a protective device when an amplifier stage is being adjusted initially, or when circuit changes and tests are being made. A sudden rise in plate current will increase the voltage drop across the resistor and automatically decrease the effective plate voltage. Data for calculating resistor values are given in TRANSMITTING-TUBE INSTALLATION. A series resistor in the plate circuit wastes power, and, therefore, is ordinarily not used in normal transmitter operation.
A cathode resistor, used to furnish part or all of the required d-c grid bias, acts to protect a tube against heavy overloads. The method of calculating the correct value for a cathode-bias resistor is explained under GRID-BIAS CONSIDERATIONS. The proper value for normal operating conditions may not be adequate to prevent exceeding the maximum rated plate dissipation of a tube when the grid excitation fails; however, the severity of the overload will be greatly reduced.
High-voltage fuses of the proper rating, placed in the positive plate-supply lead, protect vacuum-tube circuits very effectively. In the case of a screen-grid tube where the screen voltage is obtained from the plate supply by means of series resistor, the fuse is placed in the common positive lead so that its opening will remove both the screen voltage and the plate voltage; otherwise, with voltage on the screen only, the screen may draw excessive current.
High-voltage fuses are generally designed to blow at a current about 50 per cent higher than their rated value. Fuses designed for small currents are usually intended to carry continuously somewhat less than their rated current. For example, a typical fuse rated at 0.25 ampere has a maximum d-c load rating of 200 milliamperes for continuous operation.
The continuous-duty current rating of the high-voltage fuse employed in an amplifier stage should be about equal to the normal d-c plate current of the tube being protected. Thus, when the d-c plate current reaches a value about 50 per cent greater than the rated value for the tube, the fuse should blow promptly.
Where a fuse is used as a protective device in a low-power stage which is followed by other stages employing grid-leak bias, it is not usually desirable to use fuses in these other stages. It is apparent that opening of the first fuse may cause fuses in the following stages to blow, due to the removal of grid excitation from all tubes following the low-power stage. If the tubes in the higher-power stages have a fairly high mu, or employ a fixed bias sufficient to reduce the plate current to a low value when grid excitation fails, fuses can be used satisfactorily. Otherwise, a d-c overload relay is preferable.
A d-c overload relay, although initially more costly than a fuse, is one of the most satisfactory protective devices. Operating on the magnetic principle, such a relay can usually be adjusted to function on a predetermined value* of d-c current. In addition, a relay can be used almost indefinitely, because it can be reset after each opening. The contactors are about the only parts subject to appreciable wear, and they can usually be replaced.
A relay is seldom used directly to open a high-voltage d-c circuit. Instead, the holding coil of the relay is placed in the negative plate-supply lead and the contactors are used to open the primary circuit of the high-voltage transformer. In some cases, it may be desirable to place the holding coil in the filament-to-ground return lead, although the coil then carries both the d-c grid current and the d-c plate current. When the holding coil is placed in either of the two positions mentioned, the coil should be shunted by a resistor having about 20 times the resistance of the relay winding. This arrangement serves to maintain the ground connection in the event that the relay winding should develop an open circuit. The relay contactors must be heavy enough to carry the relatively large a-c current flowing in the primary of the plate-supply transformer. Examples of circuits employing d-c overload relays are included in the CIRCUIT SECTION.
Fig. 16 shows a very effective method of using a small d-c relay in conjunction with a cathode resistor to protect a grid-leak-biased tube against grid-excitation failure*. The holding coil of the relay, inserted in the grid-return circuit, causes the relay contactors to short circuit the cathode resistor Rk as long as normal d-c grid current flows; thus, the development of bias voltage across Rk is prevented. When the grid excitation fails, however, the relay contactors open and Rk adds enough cathode bias to the circuit so that the plate current drops to a small value. The resistance value of Rk is not critical. It should be about five or more times that of the resistor which would normally be used for cathode bias.. A resistance of 10,000 to 25,000 ohms is suitable for most tubes. The wattage rating of Rk depends on the d-c plate current which will flow against the bias voltage developed across Rk. This type of protective device does not guard against d-c plate-current overloads caused by plate-circuit detuning and is, therefore, not as universally effective as a d-c overload relay.
Some radio amateurs may feel that the use of protective devices for vacuum-tube circuits is not necessary for home-built transmitters. It should be remembered, however, that a fuse or a d-c overload relay will not only protect the amplifier tubes but may prevent the destruction of meters, power transformers, rectifier tubes, and other circuit elements. One heavy overload removed in time may represent a saving many times the cost of a good protective device.
* D. A. Griffin, "Automatic Protection with Grid-Leak Bia«", QST, October, 1935.
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