6SG7

The 6SG7 is a remote-cutoff, also described as semi-remote, pentode for high-gain radio-frequency and intermediate-frequency amplifiers. The sheets emphasize high transconductance, low grid-anode capacitance and dual cathode connections. Bias-controlled gain is central to receiver controlled stages. The construction described here has a metal shell and octal base; heater 6.3 V, 300 mA, AC or DC.

Dual cathode connections reduce unwanted coupling through common impedance in input and output cathode circuits, as GE explains and Tung-Sol states for the 6SG7/6SG7GT family. Maximum grid-anode capacitance of the metal version is 0.003 pF, input 8.5 pF, output 7 pF with shell connected to cathode. Tung-Sol separately gives the glass 6SG7GT maximum grid-anode capacitance 0.035 pF with external shield. Related designations do not erase that difference.

A shared typical point is anode 250 V, screen 150 V, grid −2.5 V: anode current 9.2 mA, screen 3.4 mA, transconductance 4 mA/V and internal resistance above 1 MΩ. At anode and screen 100 V, grid −1 V: 8.2 mA, 3.2 mA, 4.1 mA/V and about 0.25 MΩ. The third point, anode 250 V, screen 125 V, grid −1 V, gives 11.8 mA, 4.4 mA, 4.7 mA/V and about 0.9 MΩ. Keep conditions separate instead of selecting favorable figures from different columns.

Gain-reduction criteria also matter. At anode 250 V and screen 150 V, Tung-Sol and RCA give about −17.5 V grid for transconductance 40 µA/V; GE gives about −23 V for 10 µA/V. They are different criteria. Curves also show several screen supplies, including a 250 V source with 30 kΩ series resistance. Circuit behavior depends on the supply network as well as valve type.

Stated limits include anode 300 V, actual screen 200 V, screen source 300 V; maximum dissipations anode 3 W and screen 0.6 W. Positive DC control-grid bias is not specified for operation. GE and Tung-Sol give 90 V heater-cathode; RCA recommends keeping this difference as low as possible when they are not directly connected. Dissipation limits are not audio-output ratings.

Bottom-view octal 8BK assigns 1 shell and internal shield, 2 and 7 heater, 3 cathode and suppressor, 4 control grid, 5 second cathode connection, 6 screen, 8 anode. Any mounting position is allowed. Maximum overall is 2 5/8 inches, about 66.68 mm; maximum base outline 1 5/16 inches, about 33.34 mm. Tung-Sol and RCA give maximum seated height 2 1/16 inches, about 52.39 mm; GE reaches the same upper limit through nominal dimension and tolerance. Metal-cylinder diameter differs from base outline: maximum GE 1 1/32 inches, Tung-Sol 1 1/16 inches.

Photographs preserve concrete service and distribution evidence: RCA examples marked JAN, CRC-6SG7 and VT-211; a carton overprinted U.S. Army and U.S. Navy carries February 1944 acceptance. One specimen bears Brimar distribution marking. Another says Sub-Standard, a marking on that specimen, not a definition of the family. The National Union photograph instead shows a glass 6SG7GT, which should not inherit metal 6SG7 construction and dimensions.

Documentation consulted: RCA Radiotron Division, 6SG7, H-F Amplifier Pentode, tentative data 1 May 1942 and three curve sheets 16 April 1942; Tung-Sol, 6SG7/6SG7GT, plates 2053–2056, 2 August 1948; General Electric, 6SG7, Description and Rating, ET-T334A, October 1952, three pages; Precision 660 6SG7 row as test data. All eleven technical pages, photographs and cartons were checked.