14Q7 Pentagrid Converter

The 14Q7 is a multi-electrode pentagrid converter designed primarily for frequency conversion and mixer service in radio receivers. It features an indirectly heated unipotential cathode and is constructed with a locking-in eight-pin base. The tube's design integrates the functions of an oscillator and a mixer within a single T-9 glass envelope, allowing for efficient superheterodyne receiver designs.

Physical Construction and Pin Configuration

The 14Q7 utilizes a T-9 glass bulb with a locking-in 8-pin base. The physical dimensions of the tube are characterized by a maximum outer diameter of 1 3/16 inches, a maximum overall length of 2 25/32 inches, and a maximum seated height of 2 1/4 inches. The device does not feature a top cap, meaning all electrode connections are routed through the base pins.

The 8ALT basing arrangement provides the following pin connections, viewed from the bottom of the tube:

In this configuration, Grid #3 serves as the signal-input grid, while Grids #2 and #4 are tied together internally to function as the screen grid. Grid #5 acts as the suppressor grid, and Grid #1 serves as the oscillator grid.

Electrical Ratings and Heater Characteristics

The heater of the 14Q7 is designed for AC or DC operation. The nominal heater specifications vary depending on the operating context:

• Under nominal rating conditions, the heater voltage is rated at 14.0 V with a nominal current of 0.16 A.
• Under typical converter operating conditions, the heater is operated at 12.6 V with a current of 0.15 A.

The maximum electrical ratings established for the 14Q7 must be strictly observed to prevent damage and ensure reliable operation:

* This minimum bias limit applies when operating with a self-excited oscillator.

Typical Operating Conditions and Converter Performance

The 14Q7 is typically operated as a converter with an oscillator-grid resistor of 20,000 ohms. The performance of the tube is documented at two distinct plate voltage operating points:

Total Cathode Current Calculation

The total cathode current is the sum of the individual currents flowing through the active electrodes: the plate current, the screen current, and the oscillator-grid current.

• Calculation for Operating Point A: 3.3 mA + 8.5 mA + 0.5 mA = 12.3 mA
• Calculation for Operating Point B: 3.5 mA + 8.5 mA + 0.5 mA = 12.5 mA

These calculated values align precisely with the documented total cathode current specifications of 12.3 mA and 12.5 mA, respectively.

Conversion Transconductance

The conversion transconductance of the 14Q7 is highly dependent on the bias voltage applied to the control grid (Grid #3). This characteristic allows for effective automatic volume control (AVC) or gain control in receiver circuits:

• At a control grid voltage of -2 V, the conversion transconductance is 525 µmhos (at 100 V plate) or 550 µmhos (at 250 V plate).
• At a control grid voltage of -6 V, the conversion transconductance drops to 275 µmhos (at 100 V plate) or 300 µmhos (at 250 V plate).
• At a control grid voltage of -35 V, the conversion transconductance is reduced to an approximate minimum of 2 µmhos under both plate voltage conditions.

Interelectrode Capacitances

The interelectrode capacitances of the 14Q7 are measured with an external shield connected to the base shell. These values determine the high-frequency performance and stability of the oscillator and mixer stages:

These low coupling capacitances (such as the 0.20 pF maximum between the signal grid and the plate or oscillator grid) help minimize unwanted feedback and oscillator radiation into the antenna circuit.

Technical Summary

The 14Q7 is a locking-in, 8-pin pentagrid converter designed for frequency conversion service. It features an indirectly heated unipotential cathode with a heater rated at 12.6 V (0.15 A) under typical operating conditions or 14.0 V (0.16 A) under nominal ratings. The tube combines an oscillator and mixer in a single T-9 bulb, operating with a maximum plate voltage of 300 V and a maximum screen voltage of 100 V. It delivers a conversion transconductance of up to 550 µmhos at a plate voltage of 250 V, which can be varied down to approximately 2 µmhos with control grid bias for effective gain control.