(per leg) 0.408 x Einv. or 0.428 x Eav
2.45 x Erms
Max. Averaqe Plate-Current Rating per Rectifier Tube
a regulation of 10 per cent or less. Good plate-supply regulation is essential in self-excited oscillators to maintain frequency stability; it is essential for class B a-f amplifiers and modulators where the load current varies with the average signal voltage; and it is equally essential in the keyed r-f amplifier stage where key thumps must be minimized and condenser breakdown avoided. The voltage output of a power supply is reduced by the voltage drop through the rectifier tubes (only 15 volts in mercury-vapor types), the transformer-windings, and the filter-choke windings. It is also influenced by the type of filter system. The power transformer should be of substantial size, of generous overload rating, and should have low-resistance windings. A filter choke should have the proper value of inductance for the operating conditions, and a low-resistance winding. The use of "swinging" chokes and choke-input filters helps to provide good regulation. Their use is discussed under FILTERS.
A heavy-duty bleeder resistor connected across the output terminals of the power supply assists in maintaining good voltage regulation. The resistor prevents the filter condensers from charging up to the peak value of the a-c voltage and offers protection against accidental shock from contact with charged filter condensers after the power supply has been switched off. The value of current through the bleeder is frequently made about 10 per cent of the full-load current.
Two or more mercury-vapor rectifier tubes can be connected in parallel to give correspondingly increased output current over that obtainable with a single tube. A stabilizing resistor of 50 to 100 ohms should be connected in series with each plate lead in order that each tube will carry an equal share of the load. The value of the resistor to be used will depend on the amount of plate current that passes through the rectifier. Low plate current requires a high value; high plate current, a low value. When the plates of mercury-vapor rectifier tubes are connected in parallel, the corresponding filament leads should be similarly connected. Otherwise the tube drops will be considerably unbalanced and larger stabilizing resistors will be required. When it is desirable to minimize the small power loss caused by the voltage drop through the stabilizing resistor, an inductance of approximately one-third henry may be substituted. The use of the inductance has the added advantage of helping to limit the peak current to each tube. This is especially desirable if a condenser-input type of filter is used.
Two or more high-vacuum rectifier tubes can also be connected in parallel to give corresponding higher output current and, as a result of paralleling their internal resistances, give somewhat increased voltage output. The use of stabilizing resistors is generally unnecessary with parallel-connected high-vacuum rectifiers.
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