18HB8 — Triode-Pentode Vacuum Tube

The 18HB8 is a miniature triode-pentode vacuum tube designed primarily for audio applications in monaural and stereo sound equipment. It combines a high-mu triode section and a power pentode (or beam power amplifier) section within a single glass envelope. The tube features a coated unipotential cathode and is designed for versatile installation in any mounting position. Except for its heater ratings, the 18HB8 is electrically identical to the 35HB8.

Mechanical Specifications and Physical Dimensions

The 18HB8 is constructed in a miniature T-6 1/2 glass bulb. It utilizes a miniature button 9-pin base, designated as an E9-1 base with a JEDEC 6-3 outline. The physical dimensions of the tube are constrained by the following maximum limits:

• Maximum Overall Length / Height: 2 5/8 inches (66.7 mm)
• Maximum Seated Height (Base to Top): 2 3/8 inches (60.3 mm)
• Maximum Diameter: 7/8 inch (22.2 mm)

The tube's compact miniature construction allows it to be integrated into space-constrained chassis designs typical of compact stereo and monaural audio systems.

Heater and Cathode Characteristics

The 18HB8 features an indirectly heated, coated unipotential cathode structure. The heater is designed for series-string operation with the following nominal ratings:

• Heater Voltage: 18.0 V
• Heater Current: 0.30 A (300 mA)

To minimize hum and potential differences in series-connected heater strings, the heater must be wired with Pin 4 positioned closest to the ground end of the heater string.

The heater-to-cathode insulation is rated under the Design-Maximum System. The limits are identical for both the triode and pentode sections:

Basing and Pinout Configurations

The 18HB8 is assigned a 9ME basing arrangement. However, the technical documentation contains three distinct, conflicting descriptions of the pin connections. Designers must verify the physical layout of the specific device being utilized, as the pin assignments for the triode electrodes, pentode grids, and internal shields vary significantly between these documented configurations.

Pinout Variation A

In this configuration, the triode cathode is connected to Pin 3, while the pentode cathode is tied to Pin 8 along with Grid No. 3 and the internal shield:

• Pin 1: Triode Plate ($P_T$)
• Pin 2: Triode Grid ($G_T$)
• Pin 3: Cathode ($K$)
• Pin 4: Heater ($H$)
• Pin 5: Heater ($H$)
• Pin 6: Pentode Grid No. 1 ($G_1$)
• Pin 7: Pentode Grid No. 3 / Internal Shield ($G_3/I.S.$)
• Pin 8: Pentode Grid No. 2 ($G_2$)
• Pin 9: Pentode Plate ($P_P$)

Pinout Variation B

In this configuration, the triode grid and plate pins are swapped relative to Variation A, and the pentode grid connections are rearranged:

• Pin 1: Triode Grid ($G$)
• Pin 2: Triode Cathode ($K$)
• Pin 3: Triode Plate ($P$)
• Pin 4: Heater ($H$)
• Pin 5: Heater ($H$)
• Pin 6: Pentode Grid No. 2 ($G_2$)
• Pin 7: Pentode Plate ($P$)
• Pin 8: Pentode Grid No. 3, Cathode, Internal Shield ($G_3, K, IS$)
• Pin 9: Pentode Grid No. 1 ($G_1$)

Pinout Variation C

This configuration shares the triode pinout of Variation A but uses the pentode pinout of Variation B:

• Pin 1: Triode Plate
• Pin 2: Triode Grid
• Pin 3: Triode Cathode
• Pin 4: Heater
• Pin 5: Heater
• Pin 6: Pentode Grid No. 2
• Pin 7: Pentode Plate
• Pin 8: Pentode Cathode, Grid No. 3, Shield
• Pin 9: Pentode Grid No. 1

Maximum Ratings

The maximum ratings of the 18HB8 are established under the Design-Maximum System. These limits must not be exceeded under the worst-case operating conditions of supply voltage, signal input, and environmental factors.

Grid Circuit Resistance Limits

The maximum allowable resistance in the grid circuit depends on the bias method employed. For the triode section, the limits are:

• Fixed Bias: 0.1 Megohm Max
• Cathode Bias: 0.47 Megohm Max

For the pentode section, some specifications leave the grid circuit resistance limits unspecified, while others explicitly apply the same limits as the triode section (0.1 Megohm maximum for fixed bias and 0.47 Megohm maximum for cathode bias).

Typical Operating Conditions and Electrical Characteristics

At a nominal plate voltage of 115 V, the triode and pentode sections exhibit the following typical operating characteristics in Class A1 service:

The triode section provides a high amplification factor of 74, making it highly suitable for preamplification and voltage gain stages. The pentode section is optimized as a power amplifier, capable of delivering 1.0 Watt of audio output into a 3500 Ohm load with approximately 8% total harmonic distortion.

Circuit-Specific Audio Amplifier Example

A practical application of the 18HB8 is demonstrated in a two-stage, single-ended triode-pentode audio amplifier circuit operating from a single 115 V supply ($B+$). This circuit utilizes the high-mu triode section as a voltage preamplifier to drive the power pentode output stage.

Circuit Configuration and Component Values

• Input Coupling: The input signal of 0.1 V is applied to the triode grid through a 0.1 µF coupling capacitor.
• Triode Stage:
  • Grid Leak Resistor: 10 Megohms, providing grid leak bias for the triode section.
  • Plate Load Resistor: 0.1 Megohm (100 kΩ), connected between the triode plate and the 115 V supply.
  • Interstage Coupling: A 0.1 µF capacitor couples the amplified signal from the triode plate to the pentode grid.
• Pentode Stage:
  • Grid Resistor: 470 kΩ (0.47 Megohm), referencing the pentode grid to ground.
  • Cathode Bias: A 150 Ohm cathode resistor, bypassed by a 100 µF capacitor, provides self-bias for the pentode section.
  • Grid No. 2 (Screen Grid): Connected directly to the 115 V supply.
  • Output Load: An output transformer presenting a primary load impedance of 3500 Ohms is connected to the pentode plate.

Performance Characteristics

When operated with a 115 V supply and a 0.1 V input signal, this specific circuit configuration achieves the following performance:

• Power Output ($P_O$): 1.0 Watt
• Total Harmonic Distortion: Approximately 8%

This circuit demonstrates the efficiency of the 18HB8, allowing a complete, functional low-power audio amplifier stage to be built using a single active envelope and a minimal number of external components.