7K7

The 7K7 is a loctal tube combining two diodes and a high-mu triode. In radio receivers it allows signal detection and automatic-gain-control rectification alongside audio voltage amplification. Its defining construction separates cathodes: the two diodes share their own cathode, while the triode has another. Sylvania stresses the greater circuit-design flexibility compared with ordinary common-cathode combinations.

T-9 glass envelope, eight-contact lock-in base, any mounting position. RCA 8BF pins: heater 1 and 8, triode cathode 2, anode 3, grid 4; diode anodes 5 and 6, shared diode cathode 7. Thus the triode grid connects at the base, without a top cap. Operating heater 6.3 V, 0.3 A AC or DC; RCA and Sylvania distinguish nominal 7 V, 0.32 A.

At 250 V anode and −2 V grid the triode draws 2.3 mA, with about 44 kΩ internal resistance, 1600 µmho or 1.6 mA/V and amplification factor 70. RCA, Sylvania and Tung-Sol agree at this point. µ is not automatically achieved stage gain; load and circuit matter. Sylvania makes the triode characteristics identical to one 7F7 section and refers there for curves and resistance-coupled amplifier data. Brimar instead refers to 6SL7GT for further characteristics and curves. These compare the amplifier section, not equivalence of the complete tube.

RCA’s 20 May 1949 sheet limits triode anode voltage to 250 V design-center. Sylvania and Tung-Sol’s tentative 1950 sheet instead list 300 V maximum and 1 W anode dissipation, heater-cathode limit 90 V. Differences remain tied to maker and edition. For each diode Sylvania and Tung-Sol specify maximum 10 V drop at 1.5 mA, a conduction test condition rather than power-supply output-current data.

With the external shield at cathode as specified, Sylvania and Tung-Sol list triode grid-anode 1.7 pF, input 2.4 pF, output 2 pF. Each diode anode to triode grid is maximum 0.25 pF; each diode anode to shared diode cathode maximum 2 pF. Residual coupling is documented alongside cathode separation. Brimar’s manual marks the 7K7 obsolete and retains data for reference: this is that publication’s classification, not an introduction date.