7A4

The 7A4 is a medium-mu triode for oscillation, detection and amplification, with loctal construction. Sylvania describes characteristics similar to 6J5G or 6J5GT, but credits shorter leads, reduced capacitance and lower base losses with better high-frequency performance. One sheet states oscillator service up to about 225 MHz: a maker-stated performance dependent on the circuit, not a universal limit for every application. One preserved sheet references RMA release No.165 of March 8, 1939.

The eight-contact loctal base has a central spigot mechanically locking into the socket to retain the tube. The envelope is T-9 glass, with all connections at the base and no top cap: heater 1 and 8, anode 2, grid 6 and cathode 7. RCA marks pin 5 as an internal connection not to use, while other sheets show no connection. Electrical similarity to 6J5 therefore does not imply direct socket compatibility.

Heating deserves historical clarification. Sylvania and RCA give nominal 7 V and 0.32 A, but characteristic conditions use 6.3 V and about 0.3 A; Tung-Sol directly lists the latter. Sylvania explains that nominal 7 V corresponded to a 130 V mains condition in the stated AC service and was also nominal for automotive use. These two sets of numbers are not an invitation to alternate supplies without considering circuit and document convention.

In class A, 90 V anode and 0 V grid gives 10 mA anode current, about 6700 Ω internal resistance and 3000 µmho transconductance, or 3 mA/V. At 250 V anode and −8 V grid: 9 mA, about 7700 Ω and 2600 µmho, or 2.6 mA/V. Amplification factor is 20 in both cases. Curves show variation of current, resistance and transconductance; that factor is not guaranteed gain of an actual stage.

Sylvania and Tung-Sol specify 300 V maximum anode and 2.5 W anode dissipation. Sylvania and JETEC give 90 V heater-to-cathode, while Tung-Sol’s August 1944 sheet allows 100 V DC: the difference stays with its source. Tung-Sol adds 20 mA maximum cathode current and at most 1 MΩ DC grid-circuit resistance. With external shield at cathode, capacitances are about 4 pF grid-to-anode, 3.4 pF input and 3 pF output. Sylvania also calls output capacitance about one-third that of 6C5G, explaining the original high-frequency interest.