1621 — Power Amplifier Pentode

The 1621 is a ruggedized power amplifier pentode designed specifically for applications requiring high reliability and continuity of service. It is structurally and electrically similar to the standard 6F6 power pentode but is manufactured to tighter tolerances and quality standards to ensure dependable operation in critical industrial and communications equipment. Housed in a metal envelope with an octal base, the 1621 can be operated as either a pentode or a triode-connected power amplifier, primarily in push-pull Class A1 configurations.

Construction, Physical Dimensions, and Base Pinout

The 1621 features a robust construction utilizing a metal shell envelope designated as an MT-8 bulb. It is designed to be mounted in any physical position. The physical dimensions of the tube are compact, matching the outline of the 12A6 beam power tube:

• Maximum Overall Length: 3-1/4 inches
• Maximum Seated Height: 2-11/16 inches
• Maximum Diameter: 1-5/16 inches

The tube is equipped with a Small Wafer Octal 7-Pin base. The electrical connections, viewed from the bottom of the tube, correspond to the 7S basing diagram:

Heater and Cathode Characteristics

The 1621 utilizes an indirectly heated, coated unipotential cathode. The heater is designed for operation from either an alternating current (AC) or direct current (DC) supply.

• Nominal Heater Voltage: 6.3 V
• Nominal Heater Current: 0.7 A

In circuits where the cathode is not directly connected to the heater, the potential difference between the heater and cathode must be kept as low as possible to prevent insulation breakdown and minimize hum injection.

Interelectrode Capacitances

The direct interelectrode capacitances of the 1621 are measured with the metal shell connected directly to the cathode. The approximate values are:

• Grid to Plate (Input to Output): 0.20 µµF
• Input (Grid to Cathode, Heater, and Screen): 7.5 µµF
• Output (Plate to Cathode, Heater, and Screen): 11.5 µµF

Maximum Ratings (Design-Center Values)

The maximum ratings for the 1621 are established under the Design-Center Rating System. These limits must not be exceeded under the worst-case operating conditions. The ratings differ depending on whether the tube is operated in a triode connection (with the screen grid connected directly to the plate) or in a standard pentode connection.

Typical Push-Pull Class A1 Operating Conditions

The 1621 is primarily specified for push-pull Class A1 amplifier service. The typical operating parameters are published for a two-tube configuration.

Push-Pull Triode Connection

In this configuration, the screen grid (Grid No. 2) of each tube is connected directly to its respective plate. Cathode-bias operation is the only recommended biasing method for this mode.

• Plate Supply Voltage: 327.5 V
• Cathode Resistor: 500 Ω (shared by both tubes)
• Peak A-F Grid-to-Grid Voltage: 54 V
• Zero-Signal Plate Current: 55 mA (combined for both tubes)
• Maximum-Signal Plate Current: 59 mA (combined for both tubes)
• Load Resistance (Plate-to-Plate): 5000 Ω
• Total Harmonic Distortion (THD): 1%
• Power Output: 2 W

Note on Cathode Bias: The actual operating voltage between the cathode and the plate is equal to the plate-supply voltage minus the voltage drop across the cathode resistor. For example, with a zero-signal plate current of 55 mA flowing through the 500 Ω cathode resistor, the voltage drop is calculated as:

$V_{\text{drop}} = 0.055\text{ A} \times 500\ \Omega = 27.5\text{ V}$

Thus, the actual plate-to-cathode voltage under zero-signal conditions is:

$V_{\text{plate-to-cathode}} = 327.5\text{ V} - 27.5\text{ V} = 300\text{ V}$

Push-Pull Pentode Connection

In standard pentode operation, the screen grid is operated at a fixed potential, independent of the plate voltage.

• Plate Voltage: 300 V
• Screen Voltage: 300 V
• D-C Grid Voltage (Grid No. 1): -30 V
• Peak A-F Grid-to-Grid Voltage: 60 V
• Zero-Signal Plate Current: 38 mA (combined for both tubes)
• Maximum-Signal Plate Current: 69 mA (combined for both tubes)
• Zero-Signal Screen Current: 6.5 mA (combined for both tubes)
• Maximum-Signal Screen Current: 13 mA (combined for both tubes)
• Load Resistance (Plate-to-Plate): 4000 Ω
• Total Harmonic Distortion (THD): 3%
• Power Output: 5 W

Grid Circuit and Input Coupling Requirements

To maintain stable operation and prevent thermal runaway or bias shifting, the input coupling circuit must be carefully designed to limit the resistance in the control grid circuit.

• Coupling Methods: Transformer- or impedance-coupling devices are highly recommended because they present low DC resistance to the grid.
• Fixed Bias Operation: Fixed bias may only be used if the grid circuit resistance does not exceed 0.05 MΩ (50,000 Ω).
• Cathode Bias Operation: If the grid circuit resistance is higher than 0.05 MΩ, cathode bias is strictly required.
• Maximum Grid Resistance: Under cathode-bias conditions, the grid circuit resistance may be as high as, but must not exceed, 0.5 MΩ (500,000 Ω). This maximum limit is only permissible provided that the heater voltage is tightly regulated and not allowed to rise more than 10% above its nominal rated value (6.3 V) under any operating condition.

Relationship to Other Tube Types

The 1621 is closely related to several standard receiving tubes:

• Type 6F6: The 1621 is classified as a "Special 6F6" power pentode. The electrical characteristics and family of curves published for the standard 6F6 apply directly to the 1621.
• Type 12A6: While electrically distinct due to heater and power ratings, the 1621 shares the exact same physical outline dimensions as the 12A6 beam power tube.

Technical Summary

The 1621 is a specialized, high-reliability power amplifier pentode designed for industrial and communication applications where continuity of service is paramount. Featuring a 6.3 V, 0.7 A heater, an MT-8 metal envelope, and an octal 7-pin base, it provides robust performance in push-pull Class A1 amplifier circuits. When operated in a triode connection, a pair of 1621 tubes can deliver 2 W of output power with an exceptionally low total harmonic distortion of 1%. In a standard pentode connection, the same pair can deliver up to 5 W of output power at 3% distortion. By adhering to strict grid circuit resistance limits and utilizing recommended coupling methods, designers can ensure long-term stability and reliable operation in demanding environments.