14R7 — Duplex-Diode Remote-Cutoff Pentode

Overview

The 14R7 is a multi-unit vacuum tube containing a semi-remote-cutoff pentode and two independent diodes within a single envelope, sharing a common cathode. It is designed primarily for service in AC-DC radio receivers as a combined detector, automatic volume control (AVC) rectifier, and audio-frequency amplifier. Alternatively, its pentode section can be utilized as a radio-frequency (R.F.) or intermediate-frequency (I.F.) amplifier.

The tube is constructed with a locking-in eight-pin (Loctal) base and is designed for operation in series-string or parallel heater arrangements, particularly in receivers employing 150 mA heater tubes. The design features exceptionally low coupling capacitance between the diode units and the pentode control grid, minimizing unwanted signal interaction.

Construction, Heater, and Base Pinout

The 14R7 is housed in a T-9 glass envelope with a locking-in 8-pin base (RMA 8AE / D8-1). It features an indirectly heated, coated unipotential cathode that is shared by the pentode and both diode sections. The tube can be mounted in any position.

The physical dimensions of the tube are as follows:

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

The heater is rated for both AC and DC operation. Under standard conditions, the heater requires:

• Heater Voltage: 12.6 V
• Heater Current: 0.15 A

Under certain design ratings, nominal heater values are specified as 14.0 V and 0.16 A.

The locking-in 8-pin base provides the following pin connections (viewed from the bottom):

Electrical Characteristics and Class A1 Operation

The pentode section of the 14R7 operates as a Class A1 amplifier. Typical operating conditions for this section are documented at two different plate voltage levels:

The diode sections are designed for detection and AVC rectification. When 10 V DC is applied to either diode plate, the average diode current is typically 2.0 mA, with a minimum limit of 0.8 mA per plate.

Maximum Ratings

The maximum ratings for the 14R7 are established under the design-center system:

Resistance-Coupled Amplifier Operation

The pentode section of the 14R7 can be configured as a resistance-coupled audio-frequency voltage amplifier. Typical operating parameters are specified for supply voltages of 90 V, 180 V, and 300 V, using either cathode bias or grid-leak bias:

Cathode-Bias Configuration

In this mode, a cathode resistor ($R_k$) provides the necessary negative grid bias, and the grid resistor ($R_{g1}$) is non-critical.

• With Plate Load ($R_p$) = 0.10 MΩ and Grid Resistor ($R_s$) = 0.10 MΩ:

  • At 90 V Supply: $R_k$ = 900 Ω, Screen Resistor ($R_{sg}$) = 0.22 MΩ, Voltage Gain = 39, Peak Output ($E_o$) = 9.2 V RMS (at 5% THD).
  • At 180 V Supply: $R_k$ = 540 Ω, $R_{sg}$ = 0.28 MΩ, Voltage Gain = 59, Peak Output ($E_o$) = 18 V RMS.
  • At 300 V Supply: $R_k$ = 360 Ω, $R_{sg}$ = 0.38 MΩ, Voltage Gain = 80, Peak Output ($E_o$) = 31 V RMS.

• With Plate Load ($R_p$) = 0.24 MΩ and Grid Resistor ($R_s$) = 0.24 MΩ:

  • At 90 V Supply: $R_k$ = 2,000 Ω, $R_{sg}$ = 0.69 MΩ, Voltage Gain = 57, Peak Output ($E_o$) = 8.7 V RMS.
  • At 180 V Supply: $R_k$ = 1,100 Ω, $R_{sg}$ = 0.79 MΩ, Voltage Gain = 85, Peak Output ($E_o$) = 18 V RMS.
  • At 300 V Supply: $R_k$ = 760 Ω, $R_{sg}$ = 1.00 MΩ, Voltage Gain = 125, Peak Output ($E_o$) = 28 V RMS.

Grid-Leak Bias Configuration

In this mode, the cathode is grounded ($R_k$ = 0), and bias is developed across a high-value grid resistor ($R_{g1}$ = 10 MΩ).

• With Plate Load ($R_p$) = 0.24 MΩ and Grid Resistor ($R_s$) = 0.24 MΩ:
  • At 90 V Supply: $R_{sg}$ = 0.64 MΩ, Voltage Gain = 57, Peak Output ($E_o$) = 8.5 V RMS.
  • At 180 V Supply: $R_{sg}$ = 0.74 MΩ, Voltage Gain = 91, Peak Output ($E_o$) = 18 V RMS.
  • At 300 V Supply: $R_{sg}$ = 0.88 MΩ, Voltage Gain = 131, Peak Output ($E_o$) = 30 V RMS.

Interelectrode Capacitances

The direct interelectrode capacitances of the 14R7, measured with an external shield connected to the cathode (Pin 7), are:

• Input (Control Grid to all other electrodes except Plate): 5.6 µµf
• Output (Plate to all other electrodes except Control Grid): 5.3 µµf
• Control Grid to Plate (Maximum): 0.004 µµf
• Diode 1 Plate to Control Grid (Maximum): 0.005 µµf
• Diode 2 Plate to Control Grid (Maximum): 0.002 µµf

These low coupling values, especially between the diode plates and the pentode control grid, prevent RF/IF signals in the detector stage from feeding back into the amplifier input.

Related Designations

Brimar documentation indicates that the electrical characteristics and application data for the 14R7 are closely related to the 7R7, which utilizes a different heater voltage (typically 6.3 V). For further detailed operating information, reference is made to the 7R7.

Technical Summary

The 14R7 is a versatile, loctal-base duplex-diode remote-cutoff pentode designed for multi-function service in compact radio receivers. Combining a high-gain, remote-cutoff pentode with two independent detector diodes, it allows a single tube envelope to perform detection, AVC rectification, and voltage amplification. With a 12.6 V, 0.15 A heater, a maximum plate rating of 250 V, and low internal coupling capacitances, the 14R7 represents a highly integrated solution for mid-century superheterodyne receiver designs.