12SF5GT — High-Mu Triode Vacuum Tube

The 12SF5GT (including the 12SF5 designation) is an octal-based, indirectly heated high-mu triode designed for voltage amplification in Class A amplifier circuits.

Construction and Pinout

The 12SF5 and 12SF5GT are constructed with an indirectly heated cathode (heater-cathode type) and utilize an octal base. The physical dimensions and envelope configurations correspond to outline drawings 8-1 and 9-11.

The tube uses the 6AB base connection diagram. Viewed from the bottom of the base, the pin assignments are as follows:

• Pin 1: Shell (S) / Metal envelope shield
• Pin 2: Heater (H)
• Pin 3: Grid (G)
• Pin 6: Plate (P)
• Pin 7: Heater (H)
• Pin 8: Cathode (K)

Pins 4 and 5 have no internal connections. The heater is designed to operate with a nominal supply of 12.6 V at a current of 0.15 A.

Electrical Ratings and Capacitances

The maximum electrical limits and interelectrode capacitances for the 12SF5 and 12SF5GT are specified as follows:

• Maximum Plate Voltage: 300 V
• Input Capacitance: 4.0 pF
• Output Capacitance: 3.6 pF
• Grid-to-Plate Capacitance: 2.4 pF

These capacitance values are specified without an external shield.

Class A Operating Points

Under typical Class A amplifier operating conditions, the tube is operated with the following parameters:

• Plate Voltage: 250 V
• Grid Voltage: −2.0 V (specified as 2.0 Neg Grid Volts)
• Plate Current: 0.9 mA
• Plate Resistance ($r_p$): 66,000 Ω
• Transconductance ($g_m$): 1,500 µmhos
• Amplification Factor ($\mu$): 100

The high amplification factor of 100 indicates that the tube is optimized for high-gain voltage preamplification, where a small input signal at the grid produces a significantly larger voltage swing at the plate.

Technical Summary

The 12SF5GT and 12SF5 are high-mu triode vacuum tubes featuring a 12.6 V, 0.15 A heater and an octal base with a 6AB pinout configuration. With a maximum plate voltage rating of 300 V, the tube is typically operated in Class A amplifier service at a plate voltage of 250 V and a grid bias of −2.0 V. Under these conditions, it provides an amplification factor of 100, a transconductance of 1,500 µmhos, and a plate current of 0.9 mA, making it highly effective for low-current, high-gain voltage amplification.