12AX4GT

The 12AX4GT is an indirectly heated vacuum diode designed chiefly as a damper tube in television horizontal-deflection circuits. The RCA sheet describes it as a half-wave rectifier for this duty and notes the heater-to-cathode insulation needed to withstand the strong pulses in the circuit. Its stated application is sweep damping under its own rated conditions, not a general-purpose example of a power-supply rectifier.

The 12AX4GT heater requires 12.6 V and 0.6 A on AC or DC; RCA also considers the tube suitable for sets with series-connected heaters. The Sylvania sheet lists beside the GT the 6AX4GT, which uses 6.3 V at 1.2 A, and the 12AX4GTA. Sylvania explicitly assigns the controlled 11-second warm-up only to the latter: this figure must not be attributed to the 12AX4GT.

The two sources give ratings that must remain distinct. Under the design-center system, RCA gives a maximum peak inverse voltage of 4000 V, peak plate current of 600 mA and DC plate current of 125 mA. Sylvania gives 4400 V peak inverse as an absolute maximum, 750 mA steady-state peak current and again 125 mA DC. In both sheets, the inverse-voltage rating assumes pulses no longer than 15% of the horizontal scanning cycle specified for a 525-line, 30-frame television system; it is not a DC voltage permitted without those conditions.

The RCA drawing shows a short intermediate-shell octal base with five pins: cathode on 3, plate on 5, heater on 7 and 8; pin 2 is unconnected. Sylvania also permits a six-pin base with basing diagram 4CG and warns against using contacts 1, 2, 4 and 6 as tie points. Both makers’ drawings set maximum overall length at 84.1375 mm, length above the base seating plane at 69.85 mm, maximum bulb diameter at 30.1625 mm and base diameter at 32.54375 mm. These are separate limiting dimensions, not measurements of every specimen.