1642 — Medium-Mu Twin Triode Vacuum Tube

The 1642 (also designated as the 2C21/1642) is an indirectly heated, medium-mu twin triode vacuum tube designed for use in amplifier applications, particularly as a Class A voltage amplifier. Featuring two independent triode sections within a single envelope, the device provides a compact solution for dual-stage amplification or independent signal processing.

Physical Construction and Basing

The 1642 is constructed with an indirectly heated cathode configuration, utilizing a common heater for both triode sections. The physical layout of the tube is designated by the 12-6 outline drawing.

The electrical connections are routed through a specialized 7-pin base configuration designated as 7BH (or 2C21). This basing arrangement features a unique asymmetrical layout where the plate of the first triode section is brought out to a top cap, while all other electrodes are connected to the base pins. The pinout configuration, viewed from the bottom of the base, is as follows:

This physical separation of the first plate via the top cap helps minimize capacitive coupling and provides excellent isolation between the two triode sections.

Heater and Maximum Ratings

The heater of the 1642 is designed for AC or DC operation with the following nominal ratings:

• Heater Voltage: 6.3 V
• Heater Current: 0.6 A
• Calculated Heater Power: 3.78 W

Under standard operating conditions, the tube is rated for a maximum plate voltage of 250 V.

Electrical Characteristics and Class A Operating Points

The 1642 is characterized by a moderate amplification factor and relatively low plate resistance, making it highly suitable for general-purpose Class A amplification. The typical operating parameters for each individual triode section under Class A amplifier service are detailed below:

• Plate Voltage: 250 V
• Negative Grid Bias Voltage: 16.5 V
• Plate Current: 8.3 mA
• Plate Resistance ($r_p$): 7,600 ̧
• Transconductance ($g_m$): 1,375 •mhos
• Amplification Factor (̧): 10.4

Parameter Relationship Verification

The fundamental relationship between the triode's amplification factor (̧), transconductance ($g_m$), and plate resistance ($r_p$) is expressed by the formula:

$\mu = g_m \times r_p$

Using the typical operating values as inputs for this calculation:

$\mu = 0.001375 \text{ S} \times 7,600 \ \Omega \approx 10.45$

This calculated value closely aligns with the published nominal amplification factor of 10.4, confirming the consistency of the electrical specifications.

Technical Summary

The 1642 (2C21/1642) is a versatile medium-mu twin triode featuring a 6.3 V, 0.6 A heater and an unusual 7BH basing configuration that utilizes a top cap for the first plate connection. With an amplification factor of 10.4, a transconductance of 1,375 •mhos, and a plate resistance of 7,600 ̧ per section at a plate voltage of 250 V, the tube delivers reliable performance in Class A voltage amplifier circuits. Its independent cathode connections and top-cap plate design provide circuit designers with flexible layout options and excellent section isolation.