1229 — Filamentary Tetrode Electrometer Tube

The 1229 is a specialized filamentary tetrode electrometer tube designed for DC voltage amplification. It is characterized by exceptionally low grid current, making it highly suitable for measuring extremely small currents or operating in high-impedance circuits. To maintain its high input impedance and stable operation, the tube features electrostatic shielding and light shielding, and must be shielded from all light and external electrical influences during operation.

Mechanical Design and Basing

The 1229 is constructed in an ST-12 glass envelope with a small 4-pin phenolic base (designated as the A4-5 base). The control grid (Grid No. 1) is terminated at a top cap to maximize isolation and minimize leakage currents.

Physical Dimensions

• Bulb: ST-12
• Outline: 12-6
• Top Cap: C1-1
• Maximum Diameter: 1-9/16 inches
• Maximum Overall Height: 4-17/32 inches
• Seated Height: 3-29/32 inches

Pinout and Electrode Connections

The tube utilizes the 4K basing configuration. When viewed from the bottom of the base, the pin connections are assigned as follows:

Filament Characteristics

The 1229 features a directly heated filament designed for low-power DC operation.

Typical DC Voltage Amplifier Operation

When operated as a DC voltage amplifier, the 1229 is configured with a high-value plate load resistor to achieve significant voltage gain at extremely low operating currents. The typical operating parameters are defined under shielded conditions:

The DC voltage gain of 125 is defined as the ratio $\Delta E_b / \Delta E_{[c1](/tubes/C1)}$, which is calculated from the change in Grid No. 1 voltage required to produce a 2.0-volt increase in plate voltage ($E_b$).

Electrometer Characteristics and Linearity

As an electrometer tube, the primary performance metrics of the 1229 are its extremely low control grid current and its transfer linearity.

Grid No. 1 Current

The maximum Grid No. 1 current is rated at $\pm 1.0$ µµA (micromicroamperes, or picoamperes). This rating is established under the following specific test conditions:

• Filament Voltage ($E_f$): 0.8 to 1.0 V
• Plate Supply Voltage ($E_{bb}$): 45 V
• Plate Voltage ($E_b$): 12 V
• Grid No. 2 Voltage ($E_{c2}$): 16.5 V
• Plate Current ($I_b$): 0.33 µA
• Grid No. 1 Resistor ($R_{g1}$): 10,000 MΩ
• Grid No. 1 Voltage ($E_{[c1](/tubes/C1)}$): Adjusted

Linearity

The linearity of the tube is rated at a maximum of 10 percent. This is defined as the departure of the $E_{[c1](/tubes/C1)}$ versus $E_b$ curve from a straight line between the operating points of $E_b = 12$ Volts and $E_b = 14$ Volts. This departure is measured along the $E_b$ axis and expressed as a percentage of 2 volts under typical operating conditions.

Technical Summary

The 1229 is a specialized filamentary tetrode electrometer tube housed in an ST-12 glass envelope with a small 4-pin phenolic base and a top cap for the control grid. Operating with a low-power DC filament supply of 0.90 V at 35 mA, it is optimized for high-gain, low-current DC voltage amplification. With a typical DC voltage gain of 125 and an exceptionally low control grid current limit of $\pm 1.0$ µµA, the 1229 provides the high input impedance and sensitivity required for precision electrometer applications, provided it is fully shielded from light and external electrical influences.