1620 — Sharp-Cutoff Pentode Detector Amplifier

The 1620 is a premium metal-shell, sharp-cutoff pentode designed specifically for low-noise audio amplification and detector applications where microphonics must be kept to an absolute minimum. It is classified as a low-microphonic amplifier tube and is highly optimized for applications critical as to microphonics.

Overview

The 1620 is a specialized receiving tube featuring a metal envelope, a miniature top cap, and a small wafer octal 7-pin base. It is electrically and physically similar to the standard 6J7 sharp-cutoff pentode but is manufactured and selected under stringent controls to ensure low-microphonic performance. In historical equipment design, the 1620 was recommended as a modern, low-microphonic replacement for older types such as the 1609 (a coated-filament type with a small 5-pin base).

The tube can be operated either in its primary pentode configuration or as a triode by connecting the screen grid (Grid No. 2) and suppressor grid (Grid No. 3) directly to the plate. It is designed to operate in any mounting position.

Physical Construction and Base Pinout

The 1620 is housed in an MT-8 metal shell with a miniature top cap for the control grid (Grid No. 1). It utilizes a small wafer octal 7-pin base (basing designation 7R). The pin connections, viewed from the bottom of the tube, are as follows:

Physical Dimensions

• Maximum Overall Length: 3-1/8 inches
• Maximum Seated Height: 2-9/16 inches
• Maximum Diameter: 1-5/16 inches

Heater Characteristics

The 1620 features an indirectly heated, coated unipotential cathode. The heater can be energized by either alternating current (AC) or direct current (DC).

• Nominal Heater Voltage: 6.3 V
• Nominal Heater Current: 0.3 A
• Calculated Heater Power: 1.89 W (calculated as 6.3 V * 0.3 A)

Electrical Characteristics and Operating Points

The 1620 can be operated as either a sharp-cutoff pentode or a triode. The typical operating conditions for both modes in Class A1 amplifier service are detailed below.

Pentode Connection

In the pentode configuration, the suppressor grid (Grid No. 3) is connected directly to the cathode at the socket.

Triode Connection

For triode operation, the screen grid (Grid No. 2) and suppressor grid (Grid No. 3) are tied directly to the plate.

Note on Triode Calculations: The relationship between amplification factor, transconductance, and plate resistance can be verified mathematically. For Operating Point II, the calculated amplification factor is 1900 µmhos * 10,500 Ω = 19.95 (calculated from transconductance and plate resistance), which closely aligns with the published nominal value of 20.

Maximum Ratings

The maximum ratings for the 1620 are established under the Design-Center system. These limits must not be exceeded under the worst-case operating conditions.

Pentode Connection Ratings

• Maximum Plate Voltage: 250 V
• Maximum Screen Voltage (Grid No. 2): 100 V
• Maximum Plate Dissipation: 0.75 W
• Maximum Screen Input: 0.1 W

Triode Connection Ratings

• Maximum Plate Voltage: 250 V
• Maximum Plate Dissipation: 1.75 W

Grid Circuit Resistance

• Maximum D-C Grid Circuit Resistance: Under maximum rated conditions, the d-c resistance in the grid circuit must not exceed 1.0 MΩ.

Hum Reduction and Cathode-Bias Operation

Because the 1620 is designed for high-gain, low-noise, and low-microphonic applications, minimizing hum is a critical design consideration. The following guidelines apply to cathode-bias operation:

• Cathode Bypass Condenser: A minimum cathode-resistor bypass condenser of 25 µF is recommended to minimize hum, especially in circuits where the 1620 is immediately followed by high-gain stages.
• Heater-to-Cathode Voltage (with 25 µF or larger bypass): When a bypass condenser of 25 µF or larger is used, the voltage difference between the heater and the cathode is not highly critical, though it should still be kept as low as possible.
• Heater-to-Cathode Voltage (with less than 25 µF bypass): If a bypass condenser smaller than 25 µF is used, positive or negative biasing of the heater with respect to the cathode is required. The bias voltage must be adjusted to minimize hum and must fall within the range of +5 V to +50 V or -5 V to -50 V.

Interelectrode Capacitances

The direct interelectrode capacitances of the 1620 are measured with the metal shell connected directly to the cathode.

Pentode Connection Capacitances

• Grid to Plate (Control Grid to Plate): 0.005 pF (maximum)
• Input Capacitance: 7.0 pF
• Output Capacitance: 12.0 pF

These low capacitances, particularly the extremely low grid-to-plate feedback capacitance of 0.005 pF, are characteristic of a high-performance sharp-cutoff pentode and help prevent unwanted oscillations in high-gain RF or audio stages.

Related Types and Designations

The 1620 is closely related to several other vacuum tubes in construction and electrical behavior:

• 6J7: The 1620 is a low-microphonic variant of the standard 6J7 metal pentode. The characteristic curves published for the 6J7 apply directly to the 1620.
• 1612: The physical outline dimensions of the 1620 are identical to those of the 1612 pentagrid mixer.
• 1609: The 1620 was recommended as a superior alternative to the older 1609 low-microphonic triode/pentode for new equipment designs, offering the advantages of a modern metal envelope and an octal base.

Technical Summary

The 1620 is a premium, low-microphonic, sharp-cutoff metal pentode designed for sensitive detector and preamplifier applications. Featuring a 6.3 V, 0.3 A heater, an octal 7-pin base, and a miniature top cap, it provides highly stable Class A1 amplification. In pentode operation, it delivers a transconductance of up to 1225 µmhos with a plate resistance exceeding 1.0 MΩ. When triode-connected, it operates with an amplification factor of 20 and a plate resistance of approximately 10.5 kΩ. By incorporating specific cathode-bypass and heater-biasing techniques, designers can achieve exceptionally low hum levels, making the 1620 an ideal choice for high-gain audio circuitry where microphonics and noise must be minimized.