10L14 — Double Triode Vacuum Tube

An indirectly heated double triode designed for high-frequency amplification and self-oscillating mixer service in F.M. receivers.

Overview

The 10L14 (also designated as the UCC85) is a high-frequency double triode vacuum tube manufactured by Mazda. It is primarily intended for use as a radio frequency (R.F.) amplifier and self-oscillating mixer in frequency modulation (F.M.) receivers. The tube features two independent triode sections with separate cathodes and an internal shield, housed in a compact glass envelope with a standard B9A nine-pin miniature base.

Construction, Heater, and Pinout

The 10L14 is designed with an internal shield connected to a dedicated pin to minimize capacitive coupling between the two triode sections, ensuring stable operation at high frequencies. The tube has a maximum bulb height (excluding pins) of 49 mm (1.937 inches) and a maximum diameter of 22.2 mm (0.875 inches). It can be operated in any mounting position.

The heater is designed for series-string operation with a nominal current of 0.1 A and a nominal voltage of 26 V.

Calculation of heater power: $P_h = V_h \times I_h = 26\text{ V} \times 0.1\text{ A} = 2.6\text{ W}$

The B9A base pin connections, viewed from the underside of the base, are as follows:

To optimize performance and minimize unwanted coupling, the manufacturer specifies that the triode section on pins 6, 7, and 8 should be utilized as the R.F. amplifier, while the section on pins 1, 2, and 3 should be used as the self-oscillating mixer.

Absolute Maximum Ratings

To ensure reliable operation and prevent premature failure, the 10L14 must be operated within the following absolute maximum limits:

Electrical Characteristics

The electrical characteristics of each individual triode section are evaluated at two distinct operating points:

Verification of the triode relationship ($\mu \approx g_m \times r_a$):

• At Point A (170 V): $6.2\text{ mA/V} \times 8.1\text{ k}\Omega = 50.22$(compared to the nominal$\mu = 50$).
• At Point B (200 V): $5.8\text{ mA/V} \times 8.3\text{ k}\Omega = 48.14$(compared to the nominal$\mu = 48$).

Inter-electrode Capacitances

The high-frequency performance of the 10L14 is heavily influenced by its inter-electrode capacitances. The capacitances are documented under three distinct measurement conditions to assist designers in accounting for socket and shielding effects:

• Condition *: Measured in a fully-shielded socket, without an external screening can (except where noted).
• Condition \dagger: Measured with the socket holder capacitance balanced out.
• Condition \ddagger: Total capacitance including Plessey B9A ceramic holders (type CP180900/1 without can, or CP180024/3 with can).

The addition of an external screening can significantly reduces the direct capacitive coupling between the two anodes ($c_{a'-a''}$), dropping it from 0.028 pF to 0.003 pF under Condition *, which is highly beneficial for preventing parasitic oscillation between the R.F. stage and the mixer stage.

Typical Operating Conditions

The 10L14 is optimized for two primary circuit configurations in F.M. receivers:

R.F. Amplifier Service

When configured as an R.F. amplifier, the triode section on pins 6, 7, and 8 is operated with the following typical parameters:

Self-Oscillating Mixer Service

When configured as a self-oscillating mixer, the triode section on pins 1, 2, and 3 is operated with the following typical parameters:

In the self-oscillating mixer configuration, the application of an intermediate frequency (I.F.) feedback voltage tends to stabilize the oscillator performance. This stabilization permits the use of a relatively high grid leak resistance of 1 M̤.

A typical practical circuit layout for the self-oscillating mixer utilizes a 200 V supply voltage ($V_b$) with an $8.2\text{ k}\Omega$ anode load resistor. The circuit incorporates a series of bypass capacitors (typically 1000 pF) on the heater supply lines and the grid return paths, along with an I.F. feedback component of approximately 100 pF, whose precise value depends on the physical layout and component tolerances.

Characteristic Curves and Design Interpretation

The performance of the 10L14 is further illustrated by its characteristic curves:

• Transfer Characteristics ($I_a / V_g$): The curves show the relationship between anode current and control grid voltage for anode voltages of 100 V, 170 V, and 200 V. At $V_a = 170\text{ V}$, the anode current approaches cutoff as the grid voltage is biased more negatively than $-6\text{ V}$.
• Plate Characteristics ($I_a / V_a$): The family of plate curves plots anode current against anode voltage for grid voltages ranging from $0\text{ V}$ to $-6\text{ V}$ in 1 V increments. The maximum anode dissipation boundary of 2.5 W ($p_{a(max)}$) is superimposed on these curves, defining the safe operating envelope. For example, at $V_a = 250\text{ V}$, the anode current must not exceed 10 mA to remain within the 2.5 W dissipation limit.
• Dynamic Parameters vs. Anode Current: The curves plotting mutual conductance ($g_m$), amplification factor ($\mu$), valve anode resistance ($r_a$), and control grid voltage ($-V_g$) against anode current ($I_a$) at $V_a = 170\text{ V}$ and $V_a = 200\text{ V}$ show that the amplification factor remains relatively stable between 40 and 50 over a wide range of anode currents (from 2 mA to 20 mA), while the mutual conductance rises sharply as anode current increases, peaking near 10 mA to 12 mA.

Technical Summary

The 10L14 (UCC85) is a high-performance, indirectly heated double triode designed specifically for high-frequency applications in F.M. receivers. Featuring a 26 V, 0.1 A heater, a B9A miniature base, and an internal shield, it is optimized to function as a combined R.F. amplifier and self-oscillating mixer. With an amplification factor of approximately 48 to 50 and a mutual conductance of 5.8 to 6.2 mA/V under typical Class A conditions, each section provides excellent gain and low noise. By utilizing the recommended pin assignments and incorporating appropriate shielding and I.F. feedback, designers can achieve stable, low-noise frequency conversion and amplification in the VHF band.