1609 — Low-Microphonic Sharp-Cutoff Pentode

The 1609 is a sharp-cutoff pentode of the coated-filament type designed specifically for audio-frequency voltage amplification. It is engineered for use in battery-operated installations where susceptibility to microphonics is a critical design constraint.

Overview

The 1609 is optimized for low-noise, low-microphonic performance in sensitive preamplifier and audio-frequency stages. It features a filament designed for low-voltage DC operation, making it highly suitable for portable or battery-powered equipment. In later equipment designs, the manufacturer recommended the metal-envelope 1620 sharp-cutoff pentode (similar to the 6J7) as a replacement, which utilizes an octal base and a 6.3 V heater.

Construction, Filament, and Physical Specifications

The 1609 is constructed with a coated filament housed in an ST-12 glass bulb. The physical dimensions and mounting requirements are as follows:

• Maximum Overall Length: 4-3/16 inches
• Maximum Seated Height: 3-9/16 inches
• Maximum Diameter: 1-9/16 inches
• Base: Small 5-pin

Filament Ratings and Power Supply

The filament is designed to operate from dry-cells or a single lead storage cell.

The calculated filament power consumption of 0.275 W is derived from the nominal operating voltage of 1.1 V and current of 0.25 A (1.1 V multiplied by 0.25 A).

For multi-tube installations, the filaments of multiple 1609 tubes should preferably be connected in parallel. Series connection is permissible provided that the supply is regulated to maintain the rated filament current of 0.25 amperes across all series-connected filaments.

Pinout and Socket Connections

The tube fits a standard 5-contact socket. The pin connections, viewed from the bottom of the tube, are:

• Pin 1: Filament (+)
• Pin 2: Plate
• Pin 3: Grid No. 1 (Control Grid)
• Pin 4: Grid No. 2 (Screen Grid)
• Pin 5: Filament (-) and Grid No. 3 (Suppressor Grid)

The suppressor grid (Grid No. 3) is internally tied to the negative side of the filament at Pin 5.

Mounting Position

Early specifications dictate that the tube must be mounted in a vertical position. If horizontal operation is unavoidable, the tube must be oriented so that the plate lies in a vertical plane (on edge) to prevent filament sag. Later specifications permit mounting in any position.

Electrical Characteristics and Operating Points

The 1609 operates as a Class A1 audio-frequency amplifier. Typical operating parameters and characteristics under standard conditions are:

Grid Circuit Resistance Limits

When the input circuit of the 1609 is resistance-coupled, the DC resistance in the control-grid circuit must not exceed 0.5 megohm under fixed-bias conditions to ensure bias stability.

Maximum Ratings

The maximum ratings for the 1609 are absolute values and must not be exceeded under any operating conditions:

• Maximum D-C Plate Voltage: 135 V
• Maximum D-C Screen Voltage (Grid No. 2): 67.5 V

Interelectrode Capacitances

The direct interelectrode capacitances of the 1609, measured without an external shield, are:

• Grid to Plate (Maximum): 1.0 µµf (with some early tentative specifications listing up to 1.1 µµf)
• Input Capacitance: 7 µµf (with some early tentative specifications listing 6.7 µµf)
• Output Capacitance: 7 µµf

Average Characteristics and Curve Interpretation

The electrical behavior of the 1609 is illustrated by its average plate and transfer characteristics:

Plate Characteristics

At a constant screen voltage of 67.5 V and a filament voltage of 1.1 V DC, the plate current curves demonstrate classic pentode behavior. For a control-grid bias of -1.5 V, the plate current remains nearly constant at approximately 2.5 mA for plate voltages above 50 V, reflecting the high plate resistance (approximately 0.4 megohm). As the control-grid voltage is varied from 0 V down to -6.0 V, the plate current curves shift downward, showing a corresponding reduction in plate current.

Transfer Characteristics

With the plate voltage held at 135 V and the screen voltage at 67.5 V, the transconductance ($g_m$), plate current ($I_b$), and screen current ($I_{c2}$) vary with the control-grid voltage:

• At $E_{[c1](/tubes/C1)} = -1.5\text{ V}$, the transconductance is 725 µmhos, plate current is 2.5 mA, and screen current is 0.65 mA.
• As the control-grid bias becomes more negative, all three parameters decrease.
• The tube reaches cutoff at approximately -7 V, where the transconductance and plate current approach zero.

Related Designations and Recommendations

The 1609 belongs to a specialized class of low-microphonic amplifier tubes. For new equipment designs, the manufacturer recommended the 1620, which is a metal-type sharp-cutoff pentode similar to the 6J7. The 1620 features an octal 7-pin base, a miniature top cap, and a 6.3 V, 0.3 A heater, offering a more modern construction for applications requiring low microphonics.

Technical Summary

The 1609 is a specialized, low-microphonic, sharp-cutoff pentode featuring a 1.1 V, 0.25 A coated filament and a small 5-pin base. It is designed for Class A1 audio-frequency voltage amplification in battery-operated systems. Operating with a maximum plate voltage of 135 V and a maximum screen voltage of 67.5 V, it provides an amplification factor of approximately 300, a plate resistance of 0.4 megohms, and a transconductance of 725 µmhos. Its low-microphonic design and low filament power consumption make it highly suited for sensitive portable audio applications, though it was eventually superseded by the metal-cased 1620 for new equipment designs.