12Z8 — High-Frequency Pentode with Short Characteristic Curve

The 12Z8 (also designated in Cyrillic as 12Ж8) is a high-frequency pentode with a short characteristic curve, designed for the amplification of intermediate- and high-frequency voltages. It features an indirectly heated oxide-coated cathode, a metal envelope, and an octal base.

Construction, Physical Dimensions, and Environmental Ratings

The 12Z8 is enclosed in a metal case (envelope) that provides robust mechanical protection and built-in electrostatic shielding. The metal envelope is internally connected to pin 1 of the octal base.

Physical Dimensions

• Maximum overall height: 67 mm
• Maximum envelope diameter: 27 mm
• Maximum base diameter: 33 mm
• Maximum weight: 42 g

Environmental and Mechanical Durability

The tube is designed to operate reliably under demanding environmental conditions:

• Ambient temperature range: -60 °C to +70 °C
• Relative humidity: 95% to 98% at an ambient temperature of +20 °C
• Mechanical vibration resistance: Capable of withstanding vibration loads up to 6 g
• Service life guarantee: Minimum of 2000 hours of operation

Heater and Pinout Configuration

The 12Z8 features an indirectly heated, oxide-coated unipotential cathode. The heater is designed for a nominal 12.6 V supply, making it suitable for series heater strings or 12 V electrical systems.

Heater Specifications

• Nominal heater voltage ($U_h$): 12.6 V
• Permissible heater voltage range: 11.3 V to 13.9 V
• Nominal heater current ($I_h$): 150 ± 15 mA
• Calculated nominal heater power: 1.89 W (calculated as 12.6 V multiplied by 0.150 A)

Pin Connection (Octal Base)

The tube uses a standard octal base with the following pin assignments (viewed from the bottom of the tube):

• Pin 1: Metal envelope / shield
• Pin 2: Heater
• Pin 3: Grid 3 (Suppressor Grid)
• Pin 4: Grid 1 (Control Grid)
• Pin 5: Cathode
• Pin 6: Grid 2 (Screen Grid)
• Pin 7: Heater
• Pin 8: Plate (Anode)

Electrical Characteristics and Operating Points

The typical electrical characteristics of the 12Z8 are measured under specific Class A amplifier operating conditions.

Typical Operating Conditions

• Plate voltage ($U_a$): 250 V
• Grid 2 (screen) voltage ($U_{[g2](/tubes/G2)}$): 100 V
• Grid 3 (suppressor) voltage ($U_{g3}$): 0 V
• Grid 1 (control grid) bias ($U_{g1}$): -3 V

Measured Characteristics at Typical Operating Point

• Plate current ($I_a$): 3 ± 1 mA
• Grid 2 current ($I_{[g2](/tubes/G2)}$): 0.8 ± 0.4 mA
• Transconductance ($S$): 1.65 ± 0.35 mA/V
• Plate cutoff current ($I_{az}$): $\le 30\ \mu\text{A}$ (measured at a control grid bias $U_{g1} = -10\text{ V}$)

Interelectrode Capacitances

The 12Z8 exhibits low interelectrode capacitances, which is critical for stable high-frequency operation:

• Input capacitance (Grid 1 to cathode, $C_{g1k}$): 6 ± 1.1 pF
• Output capacitance (Plate to cathode, $C_{ak}$): 7 ± 1.8 pF
• Feedback capacitance (Grid 1 to plate, $C_{g1a}$): $\le 0.005\text{ pF}$

The extremely low grid-to-plate feedback capacitance ($\le 0.005\text{ pF}$) is a key feature of this pentode, minimizing Miller effect and preventing unwanted oscillations in high-frequency amplifier stages.

Maximum and Minimum Permissible Ratings

To ensure the guaranteed service life of 2000 hours, the operating conditions must be kept within the following absolute limits:

Characteristic Curves and Behavior

The behavior of the 12Z8 is illustrated by several characteristic curves.

Pentode Operation

Under standard pentode operation ($U_h = 12.6\text{ V}$, $U_{[g2](/tubes/G2)} = 100\text{ V}$, $U_{g3} = 0\text{ V}$), the plate current ($I_a$) and screen grid current ($I_{[g2](/tubes/G2)}$) curves demonstrate typical pentode knee characteristics. Above a plate voltage of approximately 50 V, the plate current becomes highly independent of plate voltage, confirming a high dynamic plate resistance. The control grid voltage ($U_{g1}$) regulates the plate current from its maximum at $0\text{ V}$ down to near-cutoff around $-8\text{ V}$ to $-10\text{ V}$.

Triode-Connected Operation

When the 12Z8 is triode-connected, the plate current ($I_a$) versus plate voltage ($U_a$) curves show typical triode behavior. At $U_h = 12.6\text{ V}$, positive control grid voltages (up to $+6\text{ V}$) and negative control grid voltages (down to $-18\text{ V}$) show a progressive shift in the conduction threshold and plate resistance.

Transfer Characteristics

The transfer curves ($I_a, I_{[g2](/tubes/G2)}$ vs. $U_{g1}$) at $U_a = 250\text{ V}$ and $U_{g3} = 0\text{ V}$ show the effect of varying the screen grid voltage ($U_{[g2](/tubes/G2)}$) at $75 ext{ V}, \100 ext{ V}, and \125 ext{ V}$. Higher screen grid voltages shift the transfer curve to the left, increasing both plate current and transconductance for a given control grid bias.

Suppressor Grid Control

The relationship of plate current to suppressor grid voltage ($I_a = f(U_{g3})$) is plotted at $U_a = 250\text{ V}$, $U_{[g2](/tubes/G2)} = 100\text{ V}$, and $U_{g1} = -3\text{ V}$. The curve shows that the suppressor grid can be used to control plate current, with the plate current dropping from approximately 3 mA at $U_{g3} = 0\text{ V}$ down to zero as the suppressor grid is biased highly negative, reaching cutoff near $-80\text{ V}$. This characteristic indicates potential application in dual-control or modulating circuits.

Technical Summary

The 12Z8 is a robust, metal-envelope high-frequency pentode featuring a short characteristic curve, an octal base, and a 12.6 V heater. It is designed for intermediate- and high-frequency voltage amplification under demanding environmental conditions, including temperatures from -60 °C to +70 °C and vibration loads up to 6 g. With a transconductance of 1.65 mA/V, a low plate current of 3 mA, and an exceptionally low grid-to-plate capacitance of $\le 0.005\text{ pF}$, it provides stable, high-gain amplification. The tube's suppressor grid also offers a wide control range, adding versatility to its application profile.