1623 — Power Triode Transmitting Tube

The 1623 is a thoriated-tungsten-filament power triode designed for use as an audio-frequency (AF) power amplifier and modulator, as well as a radio-frequency (RF) power amplifier and oscillator. It is particularly well-suited for self-excited oscillator circuits subject to wide variations in plate load, as its operation is not critical to changes in grid excitation voltage. The 1623 is primarily utilized for renewal purposes in existing equipment.

Mechanical Construction, Mounting, and Pinout

The 1623 features a glass envelope with a top cap anode terminal and a medium four-pin base. The tube is designed to operate without its plate showing color when run at its maximum Continuous Commercial Service (CCS) ratings.

Physical Dimensions and Weight

• Maximum Overall Length: 6-13/32 inches (with a tolerance of ± 5/32 inch)
• Seated Height: 5-25/32 inches (with a tolerance of ± 5/32 inch)
• Maximum Diameter: 2-1/16 inches
• Anode Cap Diameter: 0.566 inch (with a tolerance of ± 0.007 inch)
• Net Weight: Approximately 3 ounces
• Shipping Weight: Approximately 7 pounds

Installation and Mounting

The tube requires a small four-contact socket (specifically described as an A4-10 medium 4-pin base with a bayonet). It can be mounted in two permissible orientations:

1. Vertical position with the base down.
2. Horizontal position provided that pins 1 and 4 are aligned in a vertical plane (keeping the plane of the electrodes vertical).

Pin Connection Configuration

The base connections, viewed from the bottom of the tube, are assigned as follows:

---

Filament and Electrical Characteristics

The 1623 incorporates a thoriated-tungsten filament. Its basic electrical and interelectrode capacitance characteristics are:

• Filament Voltage (AC/DC): 6.3 V
• Filament Current: 2.5 A
• Calculated Filament Power: 15.75 W (calculated as 6.3 V * 2.5 A)
• Amplification Factor: 20

Direct Interelectrode Capacitances

• Grid to Plate: 6.7 µµF (micromicrofarads)
• Grid to Filament: 5.2 µµF
• Plate to Filament: 0.9 µµF

---

Maximum Ratings (CCS vs. ICAS)

The 1623 is rated under both Continuous Commercial Service (CCS) and Intermittent Commercial and Amateur Service (ICAS) systems. The absolute maximum ratings vary depending on the class of service:

Class B Audio-Frequency Power Amplifier and Modulator

Class B Radio-Frequency Power Amplifier

Carrier conditions per tube for use with a maximum modulation factor of 1.0:

Class C Telephony Plate-Modulated RF Power Amplifier

Carrier conditions per tube for use with a maximum modulation factor of 1.0:

Class C Telegraphy RF Power Amplifier and Oscillator

Key-down conditions per tube without modulation:

---

Typical Operating Conditions

Class B Audio-Frequency Power Amplifier and Modulator (Two Tubes)

Unless otherwise specified, values represent typical operation for a pair of tubes:

Class B Radio-Frequency Power Amplifier (Single Tube)

Carrier conditions per tube for use with a maximum modulation factor of 1.0:

Class C Telephony Plate-Modulated RF Power Amplifier (Single Tube)

Carrier conditions per tube for use with a maximum modulation factor of 1.0:

Class C Telegraphy RF Power Amplifier and Oscillator (Single Tube)

Key-down conditions per tube without modulation:

---

High-Frequency Operation and Derating

The maximum ratings for the 1623 are fully applicable up to a frequency of 60 megacycles (Mc). When operating at higher frequencies, both the maximum plate voltage and plate power input must be reduced to protect the tube.

The required percentage of maximum rated plate voltage and plate input for elevated frequencies is specified as follows:

At these high frequencies, special attention must be paid to providing adequate ventilation for the glass bulb to prevent overheating.

---

Circuit Design and Application Notes

Break-In Operation

When the 1623 is employed in the final amplifier stage or a preceding stage of a transmitter designed for break-in operation and oscillator keying, a small amount of fixed bias must be incorporated into the circuit. This fixed bias is necessary to limit the plate current to a safe value when the excitation is removed. For example, with a plate voltage of 1000 volts, a fixed bias of at least -35 volts must be applied.

Amplitude Modulation Limits

For Class C Telegraphy and RF Power Amplifier applications, amplitude modulation that is essentially negative may be utilized, provided that the positive peak of the audio-frequency envelope does not exceed 115% of the carrier conditions.

---

Characteristic Curves and Interpretation

The typical characteristics of the 1623 are illustrated by its grid and plate curves:

• Grid Characteristics: Plotting grid current against plate voltage reveals how grid current behaves under positive grid potentials ranging from +20 V to +140 V. At lower plate voltages, the grid current rises significantly, particularly at higher positive grid voltages, before tapering off as plate voltage increases.
• Average Plate Characteristics: These curves show plate current in amperes plotted against plate voltage in volts for grid voltages ranging from -30 V to +140 V. The curves demonstrate a highly linear relationship across the typical operating regions, confirming the tube's capability to deliver clean power amplification and stable modulation.

---

Technical Summary

The 1623 is a versatile thoriated-tungsten-filament power triode designed for transmitting and modulator applications. Operating with a 6.3 V, 2.5 A filament supply, it features an amplification factor of 20 and a robust construction that allows up to 25 W of plate dissipation under CCS conditions (30 W under ICAS). It is highly capable across multiple modes of operation—including Class B AF modulator service, Class B RF amplification, and Class C telegraphy/telephony—at frequencies up to 60 Mc with full ratings, and up to 100 Mc with appropriate derating and bulb ventilation.