104D — Filamentary Power Amplifier Triode

The 104D is a filamentary power amplifier triode designed primarily for telephone communication systems and precision testing applications. It serves as a direct replacement for the older D-86327 tube, retaining similar electrical characteristics while introducing structural and mechanical improvements. These enhancements include an improved filament design, transverse mica supports, and a dome-type glass envelope.

Mechanical Construction, Base, and Mounting

The 104D features a dome-type glass bulb with transverse mica supports to secure the internal electrode structure. The tube is fitted with a medium, four-pin bayonet-type base. The pins are finished with a special contact metal at their ends to ensure reliable electrical connections.

Mounting and Spacing

• Mounting Position: The tube can be mounted in either a vertical or horizontal position. When mounted horizontally, the plane of the filament must be oriented vertically.
• Ventilation: To ensure adequate heat dissipation and ventilation, a minimum spacing of 2 5/8 inches between tube centers must be maintained when operating multiple tubes in proximity.
• Socket Compatibility: The base is designed to fit Western Electric 100L, 100R, or similar socket types, preferably those equipped with matching contact-metal contacts.

Physical Dimensions

• Maximum Overall Height: 4 1/2 inches
• Maximum Bulb Diameter: 1 13/16 inches
• Maximum Base Diameter: 1.474 inches

Filament Ratings and Characteristics

The 104D utilizes an oxide-coated filament designed to operate on a current-regulated basis.

The filament must be operated as close to its rated current of 1.00 A as practicable. During the first 2000 hours of service, the filament resistance increases slightly; the nominal voltage rating of 4.5 V represents the stabilized value after this initial burn-in period. Grid and plate voltages are referenced from the negative terminal of the filament.

Electrical Characteristics and Interelectrode Capacitances

The average direct interelectrode capacitances of a based 104D tube, measured without a socket, are as follows:

• Grid to Plate: 4.9 ΔΔf (pF)
• Grid to Filament: 4.1 ΔΔf (pF)
• Plate to Filament: 3.4 ΔΔf (pF)

Class A Operating Characteristics

At a typical Class A operating point (130 V plate, -20 V grid), the tube exhibits the following average characteristics:

• Amplification Factor (Δ): 2.5
• Plate Resistance (r₅): 2100 Δ
• Transconductance (gₘ): 1180 Δmhos
• Plate Current (I₅): 25.0 mA
• Power Output: 150 mW into a matching 2100 Δ load, or 125 mW into a 5000 Δ load.

Operating Conditions and Performance

The permissible operating region for plate and grid voltages is bounded by a specific operating area. Operating the tube within recommended limits is preferred to maximize service life, as operation at maximum ratings accelerates filament depletion and shortens tube life.

The following table details the recommended and maximum operating conditions, along with the corresponding output power and harmonic distortion levels. The performance data is specified for two load conditions: a load resistance equal to the plate resistance ($R_L = r_p$) and a standard load of 5000 Δ.

Operating Points Table

Harmonic Distortion Calculations

The harmonic levels ($F_{2m}$ and $F_{3m}$) and power output ($P_m$) listed in the table correspond to a peak sinusoidal input voltage ($E_{gm}$) that is numerically equal to the applied grid bias. For smaller input signal voltages ($E_g$), the operating parameters can be calculated using the following mathematical relationships:

• Power Output ($P$): $P = P_m \left(\frac{E_g}{E_{gm}}\right)^2$
• Second Harmonic Level ($F_2$ in dB below fundamental): $F_2 = F_{2m} + 20 \log_{10}\left(\frac{E_{gm}}{E_g}\right)$
• Third Harmonic Level ($F_3$ in dB below fundamental): $F_3 = F_{3m} + 40 \log_{10}\left(\frac{E_{gm}}{E_g}\right)$

Microphonic Noise and Applications

Microphonic Performance

The 104D is characterized for microphonic noise under specific test conditions: a plate voltage of 130 V, a grid bias of -20 V, and a load resistance of 100,000 Δ.

• Mean Microphonic Output: 31 dB below 1 volt, as measured in a standard laboratory reference test set.
• Range of Individual Tubes: Individual production tubes typically exhibit microphonic levels ranging from 20 to 40 dB below 1 volt, depending on the intensity and nature of the external mechanical disturbance.

Applications

The electrical and mechanical design of the 104D makes it suitable for several specialized roles in telecommunications and instrumentation:

1. Telephone Repeater Amplifiers: Used as a voice-frequency and carrier-frequency amplifier in telephone repeater equipment where higher power outputs are required than can be delivered by 101D or 101F triodes.
2. Volume Limiters: Employed in carrier telephone systems to regulate signal levels.
3. Testing Apparatus: Used as an active amplifier element in various laboratory and field testing instruments.

Technical Summary

The 104D is a filamentary, low-mu power amplifier triode featuring an oxide-coated filament, transverse mica supports, and a dome-type glass envelope. Operating on a 4.5 V, 1.00 A DC filament supply, it provides reliable Class A amplification with an amplification factor of approximately 2.3 to 2.6, transconductance ranging from 810 to 1430 Δmhos, and a plate resistance between 1800 and 2900 Δ depending on the chosen operating point. With its robust mechanical construction and detailed characterization for harmonic distortion and microphonic noise, the 104D is optimized for high-reliability service in telephone repeaters, volume limiters, and precision testing equipment.