227

The 227 is a triode for detection and amplification in radio receivers. In RCA manual R-10 the entry is headed UY-227, while the text calls the tube ‘27: both designations therefore appear in the same manufacturer document. Its 2.5 V, 1.75 A heater warms a separate cathode and can operate from alternating or direct current. Feeding the heater from a transformer winding allowed use in mains-powered receivers without making the AC-carrying heater itself the emitting element.

For amplification, RCA lists the ‘27 in radio-frequency and audio-frequency stages. The table gives several operating points, not one performance figure for every circuit: at 135 V on the plate and −9 V on the grid it specifies 4.5 mA plate current, about 9 kΩ plate resistance, an amplification factor of 9, and 1 mS transconductance. At 250 V the tabulated grid bias is −21 V and the current 5.2 mA. A note sets maximum plate voltage at 275 V; the table's 250 V is an operating condition, not that limit.

The same tube can work as a detector. For grid-leak and condenser detection the manual recommends 45 V on the plate, a 1–5 MΩ grid leak, and a 0.00025 µF, or 250 pF, condenser. It also describes grid-bias detection: with plate voltage no greater than 250 V, bias is adjusted for about 0.2 mA plate current with no signal. These are two distinct circuits, and their values should not be confused with the currents in the amplification table.

The RCA base is Medium 5-Pin, and the tube may be mounted in any position. Its connection diagram is a top view of the socket: cathode at upper left, grid at top centre, plate at upper right, and the two heater contacts below; it gives no pin numbers. RCA recommends keeping heater and cathode at the same potential. If this is impractical, the heater is preferably negative relative to the cathode, though it may also be positive, within a 45 V difference. For the S-14 bulb the manual states maximum overall length of 119.0625 mm and maximum diameter of 46.0375 mm; these are maximum dimensions for the documented RCA version. Source: RCA Radiotron Manual, R-10 series, 1932, pp. 33, 39 and 42.