14F7 — High-Mu Twin Triode Vacuum Tube

An indirectly heated, lock-in double triode designed for voltage amplification, resistance-coupled cascade circuits, and phase inverter service.

Overview

The 14F7 is a twin high-mu triode vacuum tube featuring lock-in construction. It contains two electrically independent triode units within a single T-9 glass envelope. Each triode section has its own cathode, control grid, and plate, allowing them to be operated independently or in cascade. The tube is designed with a common heater rated for 12.6 V and 150 mA (0.15 A), suitable for AC or DC operation.

The 14F7 is primarily applied as a voltage amplifier in resistance-coupled cascade amplifiers or as a phase inverter. Its electrical characteristics are identical to the octal-based 6SL7GT and the 6.3 V lock-in 7F7 twin triodes.

Construction, Heater, and Base Configuration

The 14F7 utilizes a T-9 glass bulb and an 8-pin lock-in base with a BAC (or 8AC) basing arrangement. The physical dimensions of the tube are compact, with a maximum overall length of 2-25/32 inches, a maximum seated height of 2-1/4 inches, and a maximum bulb diameter of 1-3/16 inches. The tube is designed to operate reliably in any mounting position.

The cathodes are coated unipotential structures. The heater is designed for series or parallel operation on a nominal 12.6 V supply. The pin connections, viewed from the bottom of the lock-in base, are as follows:

Heater Specifications

• Heater Voltage: 12.6 V
• Heater Current: 150 mA (0.15 A)
• Calculated Heater Power: 1.89 W (calculated as $12.6\text{ V} \times 0.15\text{ A}$)
• Cathode Type: Coated unipotential

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the 14F7 are measured with an RMA #308 shield connected to the cathode. The capacitance values are identical for both triode units:

Electrical Ratings and Operating Conditions

The maximum ratings for the 14F7 are interpreted according to the RMA Standard M8-210. These limits define the safe operating envelope for the tube:

• Maximum Plate Voltage: 300 V (noted as 250 V in some general Class A amplifier tables)
• Maximum Heater-Cathode Voltage: 90 V
• Minimum External Grid Bias Voltage: 0 V
• Maximum Plate Dissipation Per Unit: 1 Watt

Typical Class A1 Operating Points

Typical operating conditions and electrical characteristics for each triode unit are specified at two primary plate voltage levels:

The relationship between amplification factor, transconductance, and plate resistance is consistent with the standard triode formula $\mu = g_m \times r_p$. For Operating Point B, this is verified by calculation: $1,600 \times 10^{-6}\text{ S} \times 44,000\ \Omega = 70.4 \approx 70$

Characteristic Curves and Interpretation

The electrical behavior of each triode unit is illustrated by its characteristic curves. The plate family curves plot plate current against plate voltage for grid bias values ranging from 0 V to -6.5 V. At a typical plate supply of 250 V, the grid cutoff voltage is approximately -4.5 V, where plate current approaches zero.

Additional transfer curves at a plate voltage of 250 V show how plate current ($I_b$), plate resistance ($r_p$), and amplification factor ($\mu$) vary with grid bias:

• As the grid bias becomes more negative (from 0 V to -4.0 V), the plate current falls from approximately 5.0 mA to near zero.
• Concurrently, the plate resistance ($r_p$) rises sharply, starting below 40,000 $\Omega$ at a grid bias of -1 V and exceeding 150,000 $\Omega$ as the bias approaches -3.5 V.
• The amplification factor ($\mu$) remains remarkably flat, holding steady near 70 across most of the usable grid bias range, only dropping slightly near the cutoff region.

Related Designations and Comparisons

The 14F7 is closely related to several other vacuum tube types:

• 7F7: The 7F7 is electrically identical to the 14F7 in its triode characteristics, but it is designed for a 6.3 V, 300 mA heater supply instead of the 14F7's 12.6 V, 150 mA heater.
• 6SL7GT: The electrical characteristics of each triode unit in the 14F7 are identical to those of the 6SL7GT.
• 14AF7: Although sharing the same 12.6 V heater rating and 8AC lock-in base, the 14AF7 is a medium-mu twin triode ($\mu = 16$, plate resistance of 7,600 $\Omega$, and transconductance of 2,100 $\mu\text{mhos}$ at 250 V plate voltage). It is a distinct type and cannot be directly substituted where the high-mu characteristics of the 14F7 are required.

Technical Summary

The 14F7 is a lock-in, high-mu twin triode designed for voltage amplification and phase inverter service. With a 12.6 V, 150 mA heater, it provides two independent high-gain triode units, each featuring an amplification factor of 70, a transconductance of 1,600 $\mu\text{mhos}$, and a plate resistance of 44,000 $\Omega$ under typical 250 V operating conditions. Its robust lock-in construction and stable high-mu characteristics make it a reliable choice for cascade resistance-coupled amplifiers.