280

The 280 is a full-wave vacuum rectifier intended to derive DC from an AC supply in radio power units. RCA's 1932 R-10 manual titles its sheet “UX-280” and calls it “80” in the text: the foreword explains the transition from older three-digit designations to two-digit ones. The two names should therefore be read in the context of that manual, without treating the book's date as the tube's introduction year. The two plates conduct alternately from the high-voltage transformer secondary, while the filament is the common electrode from which rectified current is taken.

The directly heated filament requires 5.0 V AC at 2.0 A. RCA specifies supply through a step-down transformer and requires 5.0 V at the filament terminals under operating conditions and at average line voltage. This is why the 280 must not be described as an indirectly heated cathode tube. The manual's example circuit shows a complete power unit with high-voltage transformer and filter; the voltages and currents obtained depend on those parts, not on the designation printed on the glass alone.

RCA's table gives three separate combinations of RMS AC voltage per plate and maximum DC output current: 350 V with 125 mA, 400 V maximum with 110 mA maximum, and 550 V maximum with 135 mA maximum. The last combination is permitted only with a choke-input filter whose first choke is at least 20 H. The three rows describe distinct conditions, and the 550 V case retains the choke-input requirement; the AC voltages are not guaranteed DC output voltages after the filter.

The manual permits either capacitor-input or choke-input filtering. With choke input the available DC voltage is lower, but regulation improves and peak current falls: the following page's curves show how output changes with load and AC voltage in the two circuits. For capacitor input, RCA also calls attention to the instantaneous peak, about 1.4 times the measured RMS AC voltage, when choosing a capacitor able to withstand it. This is not a fixed multiplier for calculating DC output in every circuit.

The base is a medium four-pin type: RCA's connection figure is a top view showing two plate contacts and two filament contacts, without numbering them. RCA prefers vertical mounting; if the tube is horizontal, the filament-pin openings must be at the top or bottom, and air must circulate around the hot envelope. The manual also describes using two 80s as half-wave rectifiers in a full-wave circuit, with each tube's plates tied together: that is a different arrangement from a single 280. RCA's maximum dimensions are 142.875 mm overall length and 55.5625 mm S-17 envelope diameter; they are not a guaranteed measurement of every specimen. Source: RCA Radiotron Manual, R-10 series, 1932, pp. 39, 42 and 78–79.