6D8G

The 6D8G is a pentagrid converter serving as oscillator and mixer in superheterodyne circuits. Tung-Sol recommends it where heater-current economy matters: at 6.3 V it requires 150 mA, AC or DC. The cathode is indirectly heated. Sylvania highlights heater consumption as the main difference from the 6A7 and 6A8G, whose general characteristics it resembles; this does not imply direct replacement without checking connections and circuit.

The Hygrade Sylvania bulletin is dated September 29, 1936 and cites RMA registration 76 of June 12, 1936. This is precise documentary evidence, not automatically a first commercial-use date. Its ratings are still tentative: at 250 V plate, 100 V screen and −3 V signal grid it gives 3 mA plate current and 500 µA/V conversion conductance. The later Tung-Sol sheet and French Sylvania manual give 3.5 mA and 550 µA/V in the corresponding conditions. Different editions remain identifiable.

Tung-Sol at 135 V plate, 67.5 V screen and −3 V signal grid gives 1.5 mA plate current, 1.7 mA screen current, 3 mA oscillator-anode current and 6.4 mA total cathode current. At 250 V plate and 100 V screen the respective currents are 3.5, 2.6 and 4.3 mA, with 10.8 mA total cathode current. The oscillator-grid resistor is 50 kΩ. In the 250 V condition, the oscillator-anode figure of 250 V is supply voltage applied through 20 kΩ, not actual electrode voltage.

Gain control is documented by conversion conductance, the parameter describing signal transfer in frequency conversion. At −3 V Tung-Sol gives 325 µA/V in the 135 V condition and 550 µA/V at 250 V; at −10 V these fall to 35 and 100 µA/V. Sylvania’s manual also gives about −20 and −40 V to reduce conductance to 2 µA/V in its two conditions. These are not zero-current voltages. With external shield connected to cathode, Tung-Sol lists 8 pF RF input, 5.5 pF oscillator input, 4.6 pF oscillator output and 11 pF mixer output capacitance.

Tung-Sol ratings distinguish 300 V at the main plate and screen supply from 100 V actual screen voltage. Likewise the oscillator anode allows a 300 V supply but 200 V actual voltage. Maximum dissipations are 1 W main plate, 0.3 W screen and 0.75 W oscillator anode; maximum total cathode current is 13 mA. Distinguishing supply from electrode voltage is essential when reading ratings and operating points.

Connection G-8A uses a small eight-contact octal base: 1 unconnected, 2 and 7 heater, 3 main plate, 4 internally joined screen grids, 5 oscillator grid, 6 oscillator anode and 8 cathode. The signal grid is at the top cap. Any mounting position is allowed. Tung-Sol’s ST-12 drawing gives maximum diameter 1 9/16 inches, about 39.69 mm, height including cap but excluding pins 3 29/32 inches, about 99.22 mm, and overall length 4 15/32 inches, about 113.51 mm. Sylvania’s 1936 drawing confirms diameter and overall length but separately measures 3 19/32 inches, about 91.28 mm, from base to glass top without cap. These heights have different reference points.

Documentation consulted: Hygrade Sylvania Corporation, Technical Data, Sylvania Type 6D8G, Pentagrid Converter, tentative September 29, 1936 data citing RMA 76; Tung-Sol Lamp Works, 6D8G, Pentagrid Converter, two pages dated December 30, 1940, plates 920-2 and 921-2; French Sylvania manual, Type 6D8G, Convertisseur pentagrille, page 65. All four available sheets were read, and drawings, photograph and input data checked. An additional linked file could not be retrieved and was not used as a source.