7J7

The 7J7 combines oscillator and mixer for superheterodyne receivers in one loctal envelope. The triode produces the local oscillation; the multigrid section converts frequency. They share a cathode, and the triode grid is internally connected to the mixer injection grid, providing the electronic coupling described by Sylvania. Sources retain two constructions: Tung-Sol’s 1940 sheet and Sylvania’s French manual describe a triode-hexode; RCA in 1947 describes a triode-heptode with fifth grid internally at cathode. Edition context must accompany these names.

T-9 glass construction, eight-contact lock-in base, any mounting position. Operating heater 6.3 V, 0.3 A AC or DC, distinguished from nominal 7 V, 0.32 A. RCA 8BL pins: heater 1 and 8, mixer anode 2, triode anode 3, triode grid/heptode grid 3 node 4, screen grids 2 and 4 at pin 5, signal grid 6, cathode/fifth grid/internal shield 7. Shared connections mean these are not two wholly independent tubes.

RCA’s 250 V mixer-anode, 100 V screens, −3 V signal-grid point gives 1.4 mA mixer anode, 2.8 mA screens, about 1.5 MΩ internal resistance. Triode supply is 250 V through a properly bypassed 20 kΩ dropping resistor; triode anode current 5 mA, grid current 0.4 mA, grid resistor 50 kΩ. Total cathode current 9.6 mA, conversion conductance 290 µmho. This describes converter operation, not static triode transconductance.

At the same general operating regime Tung-Sol gives 300 µmho conversion, 1.3 mA mixer anode, 2.9 mA screens and 5.4 mA triode; Sylvania gives 310 µmho and 5.7 mA triode. Differences remain source-specific. RCA also lists 100 V operation with 280 µmho, and about 2 µmho at signal-grid bias −20 V. Nonoscillating triode at 150 V anode, −3 V grid: RCA gives 6.6 mA, 10.7 kΩ, 1400 µmho, µ 15; Tung-Sol and Sylvania give 7.5 mA, 10.4 kΩ, 1350 µmho, µ 14. Static figures do not replace oscillating-circuit data.

RCA design-center maxima: mixer anode 300 V, screens 100 V, screen supply 300 V; triode anode directly 150 V, supply 300 V. Dissipation maxima are mixer anode 0.5 W, screens 0.3 W, triode anode 1.25 W, total cathode current 14 mA and peak heater-cathode 90 V either polarity. Supply versus electrode voltage is essential to interpreting the 250 V point with dropping resistor. Maximum positive signal-grid bias is 0 V.

Sylvania’s manual claims reduced frequency drift against many contemporary converters, allowing less oscillator-anode supply filtering without the beats it describes. At 100 V it also permits both anodes and screens at the same DC potential, omitting the screen dropping resistor in that configuration. Capacitances differ by edition: RCA RF input 4.6 pF and mixer output 3.2 pF versus 5.5 and 7.5 pF in the triode-hexode sheets, with specified shielding. Circuit reconstruction therefore requires the documented version and conditions as well as the designation.