14E7 — Duplex-Diode Remote-Cutoff Pentode

The 14E7 is a multi-unit vacuum tube of lock-in construction, combining a remote-cutoff pentode and a duplex-diode detector within a single envelope. It is designed for versatile service in radio receivers, functioning as a combined detector, automatic volume control (AVC) rectifier, and radio-frequency (RF), intermediate-frequency (IF), or audio-frequency (AF) amplifier.

Construction, Heater, and Pinout

The 14E7 features a lock-in construction utilizing an 8-pin metal-lock-in base (designated as the 8AE basing arrangement) and a T-9 glass bulb. This physical design ensures secure mechanical mounting and reliable electrical contact. The tube can be operated in any mounting position.

The physical dimensions of the envelope are as follows:

• Maximum overall length: 2-25/32 inches
• Maximum seated height: 2-1/4 inches
• Maximum diameter: 1-3/16 inches

The tube incorporates a coated unipotential cathode. The heater is designed for either AC or DC operation.

The 8-pin lock-in base connections, viewed from the bottom of the tube, are assigned as follows:

• Pin 1: Heater
• Pin 2: Pentode Plate
• Pin 3: Grid #2 (Screen Grid)
• Pin 4: Diode Plate #1
• Pin 5: Diode Plate #2
• Pin 6: Grid #1 (Control Grid)
• Pin 7: Cathode, Grid #3 (Suppressor Grid), and Internal Shield
• Pin 8: Heater

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the pentode and diode sections are measured with an external shield (such as an RMA shield #308) connected to the cathode. The typical values are:

• Grid #1 to Plate ($G_1$ to $P$): 0.005 $\mu\mu\text{F}$ maximum
• Input ($G_1$ to $H+K+G_2+IS$): 4.6 $\mu\mu\text{F}$
• Output ($P$ to $H+K+G_2+IS$): 5.3 to 5.5 $\mu\mu\text{F}$
• Diode #1 to Grid #1 ($1P$ to $G_1$): 0.013 $\mu\mu\text{F}$ maximum
• Diode #2 to Grid #1 ($2P$ to $G_1$): 0.003 $\mu\mu\text{F}$ maximum

Maximum Ratings

The maximum electrical ratings for the 14E7 must be observed to ensure reliable operation and prevent premature failure. These ratings are interpreted according to standard design limits:

Typical Operating Conditions and Characteristics

The pentode section of the 14E7 is characterized by its remote-cutoff grid characteristic, allowing for effective gain control. Typical operating conditions for the pentode section as an RF or IF amplifier are detailed below:

Application and Curve Analysis

The remote-cutoff characteristic of the pentode section is clearly demonstrated by the relationship between the control grid voltage and transconductance. At a plate voltage of 250 V and screen voltage of 100 V, a control grid bias of -3 V yields a transconductance of 1300 $\mu\text{mhos}$. When the grid bias is increased negatively to -42.5 V, the transconductance drops to a nominal 2 $\mu\text{mhos}$. This wide range of gain control makes the 14E7 highly effective in automatic volume control (AVC) circuits, where strong incoming signals can be attenuated smoothly without introducing significant distortion.

The plate characteristics of the pentode unit show typical pentode behavior, where the plate current rises rapidly at low plate voltages and then flattens into a relatively horizontal plateau. The screen current curves show that screen current is highest at low plate voltages and decreases to a stable level as plate voltage increases.

The two independent diode units share the common cathode and are designed for signal detection and AVC rectification. The rectification characteristics of each diode unit show the relationship between the developed DC voltage and the rectified current. For instance, with an RMS signal input of 30 V, the developed DC voltage varies depending on the diode load resistance (ranging from 0.1 Megohm to 2.0 Megohms), allowing designers to optimize the detection and AVC time constants for specific receiver designs.