7A5

The 7A5 is a loctal beam audio output tube designed for relatively low anode voltages. Tung-Sol specifies output stages of AC receivers and car radios; Sylvania targets about 110 V anode and relates it to 35A5 except for heater ratings. This relationship concerns electrical context and does not remove the heater-supply difference.

The envelope is T-9 glass and the eight-contact locking loctal base uses diagram 6AA. Heater pins 1 and 8, anode 2, screen 3, control grid 6, cathode 7; any mounting position. Tung-Sol gives 6.3 V at 0.75 A AC or DC. Sylvania and RCA also distinguish nominal 7 V and about 0.8 A from typical 6.3 V conditions; Sylvania’s French manual uses another nominal-current convention. The particular sheet must therefore remain the reference.

The point common to Tung-Sol and the English Sylvania sheet uses 110 V anode and screen and −7.5 V grid. Quiescent anode current is 40 mA and screen 3 mA, rising to 41 and 7 mA at maximum signal. Transconductance is 5800 µmho, or 5.8 mA/V. A 2500 Ω anode load gives stated output 1.5 W at 10% total harmonic distortion. This is the load seen by the anode through the output circuit, not directly loudspeaker impedance.

The English Sylvania sheet and RCA also give 125 V anode and screen, −9 V grid, 44 mA quiescent anode current and 2700 Ω load: 2.2 W at 10% distortion. Sylvania’s French manual instead assigns 1.4 and 1.9 W to its 110 and 125 V points, with 10 and 11% distortion and different anode currents. Edition differences belong to the available data and must not become one averaged power figure.

Complete sheets specify limits of 125 V anode, 125 V screen, 5.5 W anode dissipation, 1.2 W screen dissipation and 90 V heater-to-cathode. Anode dissipation watts are not audio output power. Grid-circuit resistance must not exceed 0.1 MΩ with fixed bias or 0.5 MΩ with cathode bias. RCA also recommends transformer or impedance input coupling, stressing resistance introduced into the grid circuit. These notes supply the conditions needed to interpret the power figure.