1630 — U-H-F Orbital-Beam Hexode

The 1630 (also known by its developmental designation A-5588 and military designation VT-128) is a specialized vacuum-tube orbital-beam amplifier designed for ultra-high-frequency (UHF) applications in the order of 500 MHz. Utilizing an acorn-type envelope and incorporating a secondary-electron multiplier stage, the 1630 is engineered for high transconductance and low lead inductance, making it highly suitable for detector or radio-frequency (r-f) amplifier service in UHF receivers, particularly in early radar systems.

Construction and Operating Principles

The 1630 features a compact acorn-type construction designed to minimize lead resistance and inductance. Its operation is based on the orbital-beam principle, which employs focused electron beams to isolate the input and output electrodes.

To significantly augment the transconductance of the input electrodes, the tube incorporates a stage of secondary-emission multiplication via a dynode (secondary-electron emitter). The electron stream is focused into an orbital path, striking the dynode to release secondary electrons, which are then collected by the output anode (plate).

To minimize input loading caused by common cathode lead inductance at ultra-high frequencies, the 1630 is constructed with two separate cathode leads. This allows separate returns for the input and output circuits. Additionally, double leads are provided for several other electrodes to facilitate the reduction of lead inductance and resistance.

Physical Specifications and Installation

The 1630 is housed in a T-8 glass bulb with an acorn-style envelope.

Socketing and Mounting

The tube can be held by a special 12-contact socket, such as the RCA stock No. 9916. It may be mounted in a vertical position, or horizontally with terminals No. 2 and 8 positioned in a horizontal plane.

Installation Precautions

Due to the delicate nature of the acorn-type glass seal, connections must not be soldered directly to the terminal pins. The heat of soldering can weaken or crack the bulb seal, destroying the vacuum. For general installation guidelines, reference is made to the installation practices of the type 955 acorn tube.

Heater Ratings

The 1630 is equipped with a heater-cathode structure designed for either alternating or direct current operation.

• Heater Voltage (A.C. or D.C.): 6.3 V
• Permissible Voltage Variation: ±10% maximum under any operating condition
• Heater Current: 0.3 A

Pinout Configurations

Two distinct terminal connection layouts are recorded for the 12-terminal base (viewed from the bottom). Designers must verify the specific pinout configuration of the device in use.

Pinout Configuration A

• Pin 1: Grid No. 1 ($G_1$)
• Pin 2: Heater ($H$)
• Pin 3: Cathode ($K$)
• Pin 4: Focus (outer) [Focus 4]
• Pin 5: Focus (inner) [Focus 5]
• Pin 6: Dynode (Secondary-electron emitter)
• Pin 7: Anode Focus (inner)
• Pin 8: Plate / Anode ($P$)
• Pin 10: Grid No. 2 ($G_2$)
• Pin 11: Heater ($H$)
• Pin 12: Grid No. 1 ($G_1$)

Pinout Configuration B

• Pins 1, 12: Heater ($H$)
• Pins 2, 8: Cathode ($K$)
• Pin 5: Grid No. 1 ($G_1$)
• Pin 11: Grid No. 2 / Screen ($G_2$)
• Pins 3, 9: Inner Focusing Electrode ($G_3$)
• Pins 4, 10: Outer Focusing Electrode ($G_4$)
• Pin 7: Plate ($P$)
• Pin 6: Dynode ($DY$)

Electrical Characteristics and Typical Operating Points

The 1630 operates with high transconductance, achieved through its secondary-emission multiplier stage. Two typical operating points are documented, reflecting developmental and standard operating conditions:

Interelectrode Capacitances

Direct interelectrode capacitances measured without an external shield are recorded in two variants:

Capacitance Set 1

• Input Capacitance: 6.5 µµF
• Output Capacitance: 3.5 µµF

Capacitance Set 2

• Input (Grid No. 1 to all other electrodes except Plate): 6 µµF
• Output (Plate to all other electrodes except Grid No. 1): 3.2 µµF
• Grid No. 1 to Plate: 0.0005 max. µµF

The extremely low grid-to-plate capacitance (0.0005 µµF maximum) highlights the excellent isolation between the input and output circuits provided by the orbital-beam design.

Absolute Maximum Ratings

The following absolute maximum limits must not be exceeded under any operating conditions:

• Plate Voltage: 375 max. V
• Dynode Voltage: 250 max. V
• Outer Focusing Electrode Voltage: ±25 max. V
• Inner Focusing Electrode Voltage: 300 max. V
• Screen (Grid No. 2) Voltage: 100 max. V
• Screen Supply Voltage: 375 max. V
• Inner Focusing Electrode Dissipation: 0.75 max. W
• Screen Dissipation: 0.2 max. W

Dissipation Calculations

The maximum plate and dynode dissipations are calculated using specific voltage and current relationships:

• Plate Dissipation (1.5 max. Watts): Calculated as the plate current multiplied by the difference between the plate voltage and the dynode voltage: $P_D = I_b \times (E_b - E_{dynode})$
• Dynode Dissipation (0.6 max. Watt): Calculated as the dynode voltage multiplied by the sum of the plate current and the dynode current (noting that dynode current is inherently negative): $P_{dynode} = E_{dynode} \times (I_b + I_{dynode})$

Circuit-Specific Operating and Application Guidelines

UHF Tank Circuits

For operation at frequencies around 500 MHz, a practical tank circuit consists of two concentric cylindrical transmission lines placed on opposite sides of the tube socket. These lines are connected directly to the grid and plate terminals and are tuned using sliding short-circuiting bars.

By-passing and R-F Grounding

To ensure stable operation, every electrode terminal except the control grid and plate must be bypassed to an r-f ground using low-inductance condensers. At very high frequencies, these bypass condensers should be supplemented by r-f chokes placed as close as possible to the condenser terminals.

Biasing and Power Supplies

• Screen Supply: The screen (Grid No. 2) voltage is typically obtained from the high-voltage supply through a 1-megohm voltage-dropping resistor.
• Other Electrodes: Direct-current voltages for the remaining electrodes (excluding the screen) should be derived from a voltage divider connected across the high-voltage supply. The design of this voltage divider must account for the inherently negative dynode current resulting from secondary-electron emission.

Qualitative Curve Observations

Average Plate Characteristics

The plate current of the 1630 is highly dependent on the dynode potential. Below the dynode voltage (approximately 240 V to 250 V), the plate current remains extremely low, near zero. As the plate voltage rises above the dynode potential, the plate current increases sharply (typically between 250 V and 350 V) and then levels off, exhibiting a flat, pentode-like saturation characteristic above 350 V.

Average Volume-Control Characteristics

When plate current is plotted against the Outer Focusing Electrode ($G_4$ or $J_2$) voltage, it forms a symmetrical, bell-shaped curve.

• In some operating conditions, this curve peaks at an outer focusing voltage of 0 V, with peak plate currents of approximately 28 mA (for a screen voltage of 100 V) and 14 mA (for a screen voltage of 50 V), dropping to near zero at ±80 V.
• In other conditions, the curve peaks near +10 V to +20 V and drops off sharply toward both negative and positive potentials.

This characteristic allows effective gain (volume) control by varying the outer focusing electrode potential. This method of gain control does not alter the cathode current or affect the input capacitance and inductance, preserving circuit tuning and stability.

Transfer Characteristics

The transfer characteristic curves (plate current versus control-grid $G_1$ voltage) show a sharp rise from cutoff near -5 V up to 0 V. At a plate voltage of 360 V, an inner focusing voltage of 300 V, and a dynode voltage of 240 V, the plate current reaches approximately 48 mA when the screen voltage is 100 V, and approximately 24 mA when the screen voltage is 50 V.

Technical Summary

The 1630 is an acorn-type, indirectly heated orbital-beam hexode designed for UHF receiver applications up to 500 MHz. By combining focused electron beams with a secondary-emission dynode, the tube achieves an exceptionally high transconductance of 15,000 µmhos and an extremely low grid-to-plate capacitance of 0.0005 µµF maximum. Operating with a 6.3 V, 0.3 A heater, the 1630 requires careful attention to its dual-cathode returns, negative dynode current, and delicate glass seals. It provides a unique, high-performance solution for UHF amplification and detection where minimizing lead inductance and input loading is critical.