6SF7

The 6SF7 combines one diode and a gain-controlled pentode in a metal valve with octal base. Tung-Sol specifies combined IF amplifier, detector and automatic-volume-control rectifier service. Heater 6.3 V, 300 mA, AC or DC. The diode surrounds the cathode sleeve shared with the pentode: the functions share a cathode reference rather than being completely independent valves.

Two class-A1 pentode conditions are specified. At anode 100 V, screen 100 V, grid −1 V: anode current 12 mA, screen 3.4 mA, internal resistance about 0.2 MΩ, transconductance 1.975 mA/V. At anode 250 V with screen 100 V and grid −1 V: 12.4 mA, 3.3 mA, about 0.7 MΩ and 2.05 mA/V. One transconductance figure cannot represent all operating conditions.

Control is documented by about −35 V bias to reduce transconductance to 10 µA/V in both tabulated conditions. This is residual slope, not exactly zero current. Curves show anode current, screen current and transconductance against grid voltage; the diode graph relates rectified current, developed DC voltage, RMS signal and load resistance. Detection performance therefore also depends on external load.

Stated limits include anode 300 V, actual screen 100 V, screen source 300 V; maximum anode dissipation 3.5 W and screen 0.5 W, without positive DC control-grid bias. The sheets recommend minimizing heater-cathode potential difference. With shell at cathode, maximum grid-anode capacitance is 0.004 pF, input 5.5 pF, output 6 pF; pentode anode to diode 0.8 pF, pentode grid to diode maximum 0.002 pF.

Bottom-view octal 7AZ assigns 1 shell, 2 pentode control grid, 3 common cathode, 4 screen, 5 diode anode, 6 pentode anode, 7 and 8 heater. The sheets describe eight contacts and allow any mounting position. This numbering is specific: an octal base does not mean contacts match another metal pentode.

Tung-Sol gives maximum overall 2 5/8 inches, about 66.68 mm; body 2 1/16 inches, about 52.39 mm; metal-cylinder diameter 1 1/16 inches, about 26.99 mm; base outline 1 5/16 inches, about 33.34 mm. RCA gives the same overall, maximum seated height and maximum outline diameter. Base diameter must not become cylinder diameter. Photos show RCA and Raytheon; recorded dimensions remain those stated by the respective datasheet manufacturers.

Tung-Sol notes the same characteristics and application as 12SF7 except heater ratings, an essential distinction in the family reference. RCA refers its single-diode discussion to 6B8-G and diode curves to 6B7, documentary cross-references rather than complete substitution claims. RCA copies preserve tentative December 1941 data and February 1941 curves; Tung-Sol is 1945. These date documents, not a claimed introduction year.

Documentation consulted: RCA Radiotron Division, 6SF7, Diode–Super-Control Amplifier Pentode, tentative data 1 December 1941, both copies, and curves 20 February 1941; Tung-Sol, 6SF7, plates 1608–1610, 15 October 1945, three pages; Precision 660 separate diode and pentode rows as test settings. All eight technical collection pages were read without converting tester settings into operating limits.