12BZ6 — Semi-Remote-Cutoff Pentode

The 12BZ6 is a miniature, semi-remote-cutoff pentode designed primarily for use as an automatic gain-controlled (AGC) intermediate-frequency (IF) amplifier in both color and black-and-white television receivers. It is specifically engineered for service in television receivers employing 150 mA series-connected heater strings.

Except for its heater ratings and controlled warm-up characteristics, the 12BZ6 is electrically identical to the 6BZ6. Its semi-remote-cutoff characteristic allows it to handle a wide range of signal amplitudes with reduced cross-modulation and distortion when AGC bias is applied.

Physical Construction and Pinout

The 12BZ6 is constructed in a miniature T-5 1/2 glass envelope with a 7-pin miniature button base (designated JEDEC E7-1, outline JEDEC 5-2). The tube utilizes the 7CM basing arrangement, which provides the following pin connections when viewed from the bottom:

The physical dimensions of the tube are standardized to ensure compatibility with compact high-frequency chassis layouts:

• Maximum Diameter: 0.750 inches
• Maximum Seated Height: 1.875 inches
• Maximum Overall Height: 2.125 inches

The tube is designed to operate reliably in any mounting position.

Heater Characteristics

The 12BZ6 features an indirectly heated, coated unipotential cathode. The heater is designed for series-string operation, and its thermal characteristics are tightly controlled to minimize voltage surges across the heater during the receiver's warm-up cycle. This protection is effective when the tube is operated in series with other heaters having similar controlled warm-up characteristics.

Heater Power Calculation: Using the nominal heater voltage (12.6 V) and nominal heater current (0.15 A): $\text{Heater Power} = 12.6\text{ V} \times 0.15\text{ A} = 1.89\text{ W}$

Heater-Cathode Voltage Ratings

To prevent insulation breakdown and minimize hum or leakage, maximum heater-cathode voltage limits must be strictly observed:

• Heater Negative with Respect to Cathode:
  • Maximum Total DC and Peak Voltage: 200 V
  • Maximum DC Voltage: 200 V
• Heater Positive with Respect to Cathode:
  • Maximum DC Voltage: 100 V
  • Maximum Total DC and Peak Voltage: 200 V

Maximum Ratings (Design-Maximum System)

The ratings of the 12BZ6 are established under the Design-Maximum Rating System. These are limiting values that must not be exceeded under the worst probable operating conditions, including supply-voltage variations, component tolerances, and environmental changes.

Maximum Circuit Values

To maintain bias stability and prevent thermal runaway, the maximum resistance in the Grid No. 1 circuit is restricted based on the biasing method:

• For Fixed-Bias Operation: 0.25 megohm maximum
• For Cathode-Bias Operation: 1.0 megohm maximum

Typical Operating Conditions and Characteristics

In typical Class A1 amplifier service, the 12BZ6 is operated with Grid No. 3 and its internal shield tied directly to the cathode at the socket. Under these conditions, the tube exhibits high transconductance and moderate plate resistance:

• Plate Supply Voltage: 125 V
• Grid No. 2 Supply Voltage: 125 V
• Grid No. 3 (Suppressor Grid): Connected to cathode at socket (Pin 7 to Pin 2)
• Cathode-Bias Resistor: 56 ohms
• Plate Resistance (Approximate): 0.26 megohm
• Transconductance: 8000 µmhos
• Plate Current: 14 mA
• Grid No. 2 Current: 3.6 mA
• Grid No. 1 Voltage (Approximate) for transconductance of 50 µmhos: -19 V
• Grid No. 1 Voltage (Approximate) for transconductance of 700 µmhos (at zero cathode resistance): -4.5 V

Direct Interelectrode Capacitances

The high-frequency performance of the 12BZ6 is heavily influenced by its internal capacitances. The tube's design includes an internal shield to minimize feedback capacitance between the control grid and the plate. The capacitances vary depending on whether an external shield is employed:

Note: External shield is connected directly to the cathode at the socket.

Characteristic Curves and Qualitative Analysis

The published characteristic curves for the 12BZ6 illustrate its performance under varying electrical conditions:

Plate Characteristics

The plate current versus plate voltage curves, plotted at a screen grid voltage of 150 V, display typical pentode behavior. Above a low-voltage knee (approximately 50 to 75 V depending on bias), the plate current curves flatten out, indicating high dynamic plate resistance. As the control grid voltage is stepped from 0 V down to -6 V, the plate current decreases in a controlled, non-linear fashion, demonstrating the semi-remote-cutoff characteristic.

Transfer Characteristics

The transfer curves plot plate current and screen grid current against control grid voltage at a plate voltage of 200 V and screen voltage of 150 V. The curves show that plate current falls gradually as the negative grid bias increases. The transition is smooth, allowing automatic gain control circuits to smoothly adjust the stage gain without abrupt cutoff. The plate current approaches near-zero levels (cutoff) at approximately -19 V.

Transconductance and Plate Resistance

Curves of transconductance and plate resistance versus control grid voltage show that transconductance falls from its nominal high value of 8000 µmhos down to 50 µmhos as the grid bias sweeps from 0 V to -19 V. Concurrently, the plate resistance rises significantly as the tube is biased toward cutoff, which is typical of remote-cutoff and semi-remote-cutoff RF pentodes.

Grid No. 2 Rating Curve

The screen grid rating curve defines the boundary of safe operation for Grid No. 2. It plots the permissible Grid No. 2 input (expressed as a percentage of the maximum 0.55 W rating) against the Grid No. 2 voltage (expressed as a percentage of the maximum 330 V supply voltage rating). The curve shows that full dissipation is permitted up to a certain voltage threshold, beyond which the allowable input power must be derated linearly to zero as the voltage approaches the maximum supply limit.

Related Designations and Comparisons

The 12BZ6 belongs to a family of identical pentodes that differ only in their heater configurations to accommodate different power supply topologies:

• 6BZ6 (and the dual-designated 6BZ6/6JH6): Designed for standard 6.3 V parallel heater circuits, drawing 0.3 A.
• 3BZ6: Designed for 3.15 V, 0.6 A series-string heater circuits with controlled warm-up.
• 4BZ6: Designed for 4.2 V, 0.45 A series-string heater circuits with controlled warm-up.

While these tubes share identical internal electrode structures, capacitances, and Class A1 electrical characteristics, they are not direct plug-in substitutes for one another due to these incompatible heater requirements.

Technical Summary

The 12BZ6 is a high-transconductance, semi-remote-cutoff pentode housed in a 7-pin miniature glass envelope. Operating with a 12.6 V, 150 mA heater, it is optimized for series-string television IF and AGC amplifier stages. At a typical operating point of 125 V on the plate and screen, it provides a transconductance of 8000 µmhos, a plate current of 14 mA, and an approximate plate resistance of 0.26 megohm. Its controlled heater warm-up characteristics, low grid-to-plate feedback capacitance (0.015 pF with shield), and smooth AGC control characteristics make it a highly effective component in classic television receiver designs.