6AX4GT

The 6AX4GT is an indirectly heated vacuum diode designed as a damper in television horizontal-deflection circuits. Tung-Sol specifies direct-drive circuits, where anode, cathode and heater withstand high pulses at line frequency. Its role is the scanning-circuit damper; the half-wave rectifier designation should not obscure that specific purpose. Heater requirements are 6.3 V, 1.2 A; mounting may be in any position.

Ratings depend on the maker’s stated system. Sylvania, September 1955, gives design-center ratings: peak inverse voltage 4400 V absolute maximum, anode dissipation 4.8 W, steady-state peak current 750 mA and DC anode current 125 mA. RCA, March 1961, and Tung-Sol, January 1963, instead give design-maximum values of 4400 V, 5 W, 825 mA peak and 137 mA DC. These sets must not be merged by selecting each larger number. Conditions refer to 525-line, 30-frame-per-second television systems; pulse duty cycle must not exceed 15% of a scanning cycle. Tung-Sol does not recommend power-rectifier service.

Heater-cathode insulation is essential. With the heater negative to the cathode, the maximum DC component is 900 V and total DC plus peak 4400 V; with the heater positive, the corresponding limits are 100 and 300 V. Polarity matters, and a pulse rating does not replace a DC limit. The stated drop at instantaneous current 250 mA is 32 V: a diode test condition, not a nominal heater or supply voltage. Published curves show anode current increasing with forward voltage.

The base is octal-family, but sheets document versions with five or six actual contacts and different shells. In bottom-view diagram 4CG, 3 is cathode, 5 anode and 7–8 heater. Contacts 1, 2, 4 and 6 must not be used as wiring tie points: RCA identifies internal connections on 2 and, where present, 1; some bases omit 1, 4 and 6. An unusable contact is not a spare terminal. RCA specifies maximum overall length 3 5/16 inches, about 84.14 mm; seated length 2 3/4 inches, 69.85 mm; maximum diameter 1 9/32 inches, about 32.54 mm. Other makers’ glass and base dimensions remain separate.

A useful Sylvania family note states that 6AX4GT, 12AX4GT and 12AX4GTA are identical except for heater characteristics. The 12.6 V versions draw 600 mA; controlled 11-second warm-up belongs to 12AX4GTA, not 6AX4GT. This is a documented technical relationship, not a direct replacement instruction. Documentation: Sylvania Engineering Data Service 6AX4GT/12AX4GT/12AX4GTA, September 1955; RCA 6AX4-GT, March 1961, two examined copies; Tung-Sol 6AX4GT, 1 January 1963. Dates identify sheets, not introduction years.