6SJ7

The 6SJ7 is a sharp-cutoff pentode in a metal envelope, intended for radio-frequency and intermediate-frequency amplifier stages. A General Electric sheet also describes it as a biased detector or high-gain amplifier; Tung-Sol lists further use as an RF oscillator or mixer in circuits designed for those roles. Sharp cutoff distinguishes it from the remote-cutoff characteristic of the 6SK7, shown alongside the 6SJ7 in RCA documents: the two designations do not have the same gain-control behaviour.

The 6SJ7 uses a single-ended construction that brings every connection to the eight-pin octal base, with no top cap for the grid. An RCA technical note associates this arrangement, together with shielding of the internal leads, with fewer vulnerable external connections and tidier assembly of RF stages. In the metal 6SJ7 pinout, pin 1 connects to the shell and internal shield, pins 2 and 7 supply the heater, pin 3 is the suppressor grid, pin 4 the control grid, pin 5 the cathode, pin 6 the screen grid and pin 8 the anode. The shell and shield must be treated as parts of the circuit, not merely mechanical details.

The nominal heater requires 6.3 V at 0.3 A. For a typical class A stage, RCA tables specify 100 V on the screen grid and −3 V on the control grid: at 100 V on the anode they give about 2.9 mA anode current and 1,575 µS transconductance; at 250 V on the anode, about 3.0 mA and 1,650 µS. The suppressor grid is connected to the cathode at the socket. These figures describe test conditions and do not justify using arbitrary voltages or loads without checking; the actual circuit must meet the limits in the manufacturer’s data sheet.

In pentode connection, RCA and Tung-Sol specify a maximum control-grid-to-anode capacitance of 0.005 pF with the shell connected to the cathode; for the metal 6SJ7, input and output capacitances are about 6 and 7 pF. The small input-to-output capacitance helps limit unwanted coupling, but the performance of a high-frequency stage also depends on wiring and shielding. The sources also describe a triode connection, made by joining the screen and suppressor grids to the anode: in the RCA table at 250 V, the amplification factor is about 19 and the plate resistance about 7.6 kΩ. These figures must not be applied to the pentode connection.

Published limits also need careful reading. The 1948 RCA sheet gives design-center maximum ratings of 300 V at the anode and 2.5 W anode dissipation, with a maximum of 125 V at the screen grid and 300 V for its supply under the stated conditions; it lists 0.7 W screen dissipation. A 1944 Tung-Sol sheet and a 1953 German sheet instead give 0.3 W for the screen, while a later technical registration revises the limit according to screen voltage. These documents use different criteria and must not be merged into a single universal rating. The technical register confirms registration of the designation in 1938, but that date does not establish the year in which every individual tube was made.