1231 — High-Mu Loktal Triple-Grid Amplifier

The 1231 is a high-transconductance, triple-grid (pentode) amplifier tube featuring a compact Loktal construction. Designed primarily for television amplifiers and similar high-frequency or wideband applications, the 1231 incorporates several mechanical and electrical design innovations to minimize parasitic effects and optimize performance at high frequencies.

Mechanical Construction and Shielding

The 1231 is built using a compact T9-F glass envelope with an 8-pin Loktal base (designated 8-V). This construction is significantly smaller than standard glass tubes of its era, offering several physical and electrical advantages:

• Short Electrode Leads: The Loktal design allows for extremely short internal connections from the active electrodes to the base pins, minimizing lead inductance and resistance.
• Low Dielectric Losses: The materials and physical layout reduce high-frequency dielectric losses.
• Direct Grid Connection: Unlike older high-frequency pentodes that require a top-cap connection, the control grid of the 1231 is connected directly to a base pin. This eliminates the need for flexible external grid clips and leads, simplifying wiring and reducing stray capacitance.
• Electrostatic Shielding: The tube features robust internal shielding for the grid connection. Additionally, the metal guide pin (broche-guide) of the Loktal base acts as an effective electrostatic shield between the external grid and plate leads, reducing unwanted feedback.

The tube is designed to be mounted in any position.

Heater Ratings and Operating Conditions

The 1231 features a heater designed for flexible operation, with nominal ratings and specific operating-point parameters:

Calculated Heater Power:

• At nominal ratings: 7.0 V × 0.48 A = 3.36 W
• At typical operating points: 6.3 V × 0.45 A = 2.835 W

Direct Interelectrode Capacitances

When operated in its standard pentode configuration, the 1231 exhibits very low direct interelectrode capacitances, which is critical for maintaining stability and bandwidth in television and high-frequency amplifier circuits:

• Grid to Plate: 0.015 µµF
• Grid to all other elements: 8.5 µµF
• Plate to all other elements: 6.5 µµF

Operating Configurations and Electrical Characteristics

The 1231 is highly versatile and can be configured as a pentode, tetrode, or triode. In all configurations, the tube must be operated with automatic bias (polarisation automatique). The electrical characteristics under these three operating modes are detailed below:

Analysis of Operating Modes

• Pentode Mode: This is the standard high-gain configuration. It provides the highest internal resistance (approximately 700,000 Ω) and a very high amplification factor of 3,850, combined with a mutual conductance of 5,500 µmhos.
• Tetrode Mode: By connecting the suppressor grid to the screen grid, the tube operates as a tetrode. In this mode, the mutual conductance increases to 6,500 µmhos, while the screen grid current drops significantly to 0.5 mA, and the internal resistance decreases slightly to approximately 540,000 Ω.
• Triode Mode: By triode-connecting the tube (suppressor grid and screen grid tied to the plate), the 1231 functions as a medium-mu triode. It delivers a low internal resistance of approximately 5,200 Ω, a mutual conductance of 6,300 µmhos, and an amplification factor of 33.

Technical Summary

The 1231 is an innovative, high-transconductance triple-grid amplifier tube utilizing a compact Loktal 8-pin envelope. Its design is optimized for high-frequency applications, such as television amplifiers, where low interelectrode capacitances, short lead lengths, and effective shielding are paramount. With a nominal 7.0 V heater, the tube is typically operated at 6.3 V in three distinct configurations—pentode, tetrode, or triode—offering designers a flexible range of gain, impedance, and transconductance characteristics.