Interpretation Of Tube Ratings

A thorough understanding of the significance of published ratings is necessary if optimum results are to be obtained. The following explanation is intended to clarify the meaning of the ratings tabulated under each individual tube type.

The filament or heater voltage given in the tabulations is a normal value unless otherwise stated. Transformers and resistances in the filament circuit should be designed to operate the filament or heater at the rated value for fulMoad operating conditions with an average line voltage. Variations from the rated value due to line-voltage fluctuations or other causes should not exceed plus or minus 5 per cent, unless otherwise stated under the tube type.

In general, the filament of a transmitting tube may be operated with either an a-c or a d-c supply. An a-c source is usually employed because of its convenience and economy, unless a d-c source is necessary to avoid hum. With a-c operation, the grid return and the plate return should be connected to the mid-point of the filament circuit. This point may be the center tap of the filament winding or of a low resistance shunted across the filament circuit. When direct current is used, the return leads should be connected to the negative filament terminal.

Where it is found desirable to use d-c filament excitation on any filament-type tube for which data are given on an a-c basis, the grid-bias values as shown in the tabulated data should be decreased by an amount equal to approximately one-half the rated filament voltage. The grid-bias voltage should be measured from the negative filament terminal.

In the rating of RCA transmitting tubes, certain tabulated values are given as maximum. These are limiting values above which the serviceability of the tube will be impaired from the standpoint of life and satisfactory performance. If these limiting values are not to be exceeded, it is necessary to determine the amount of voltage fluctuation due to line-voltage variation, load variation, and manufacturing variation in the apparatus itself. Average design values can then be chosen so that the maximum ratings will never be exceeded under the usual operating conditions.

Each maximum rating should be considered in relation to all other maximum ratings, so that under no condition of operation will any maximum rating be exceeded. If the product of the maximum rated plate voltage and d-c plate current exceeds the maximum rated d-c plate input, then either or both the plate voltage and plate current should be reduced an appropriate amount. For example, the 808 in class C telegraphy service has the following ratings: 1500 max. plate volts; 150 max. plate milliamperes; and 200 max. d-c plate input watts. It is apparent that when the maximum plate voltage of 1500 volts is used, the d-c plate current must be reduced so that the maximum d-c plate input will not be exceeded. If the maximum plate current of 150 milliamperes is used, then the plate voltage should be reduced accordingly.

The data tabulations also show typical operating values for each respective tube type in the classes of service for which the tube is recommended. These values should not be considered as ratings, because the tube can be used under any suitable conditions within its maximum ratings, according to the application. The output value for any operating condition is an approximate tube output—that is, plate input minus plate loss. Circuit losses must be subtracted from tube output in order to determine the useful output. Output values are approximate and are not to be considered as output ratings. The actual output in any case depends on a number of variable factors, important among which are circuit efficiency and operating frequency.

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