12BY7 — High-Transconductance Pentode
The 12BY7 and 12BY7-A are miniature, high-transconductance pentode tubes designed primarily for service as video amplifiers. They are engineered to deliver large output voltages across low-value load resistances while operating under relatively low supply voltages.
The 12BY7-A variant is specifically optimized for television receivers and other equipment utilizing series-connected heater strings. In these applications, the tube's heater is wired in series with other heaters, such as that of the picture tube. When integrated into a series circuit with other devices rated for a 600 mA heater current, the 12BY7-A minimizes heater voltage surges across individual tubes due to its controlled heater warm-up characteristic.
Heater and Cathode Characteristics
The tube features a coated unipotential cathode. The heater is designed for versatile operation, allowing for either series or parallel connections depending on the system requirements.
The approximate heater warm-up time is 11 seconds. This warm-up time is defined by a specific test condition: the time required for the voltage across the heater to reach 80% of its rated value after applying four times the rated heater voltage to a circuit consisting of the tube heater in series with a resistance. This series resistance is calculated as three times the rated heater voltage divided by the rated heater current.
Heater-to-cathode voltage ratings must be carefully observed to prevent insulation breakdown:
- Heater Negative with Respect to Cathode: The maximum allowable total DC plus peak voltage is 200 V.
- Heater Positive with Respect to Cathode: The maximum allowable DC component is 100 V, and the maximum total DC plus peak voltage is 200 V.
Physical Construction and Pinout
The 12BY7 and 12BY7-A are housed in a T-6 1/2 glass bulb with a miniature 9-pin base (JEDEC E9-1). The physical outline is designated as 6-3, and the tube can be mounted in any position.
The physical dimensions of the envelope are:
- Maximum Diameter: 7/8 inch
- Maximum Overall Length: 2 5/8 inches
- Maximum Seated Height: 2 3/8 inches
The tube utilizes the 9BF basing arrangement. The pin connections, viewed from the bottom of the base, are assigned as follows:
Note that Grid 3 (the suppressor grid) is internally connected to both Pin 3 and Pin 9. Pin 6 provides access to the heater midpoint tap, enabling the dual-voltage series/parallel heater configurations.
Design-Center Maximum Ratings
The following maximum ratings are established under the design-center system. Operating the tube beyond these limits may impair its performance or longevity.
- Plate Voltage: 300 V max.
- Screen Voltage (Grid 2): 180 V max.
- Plate Dissipation: 6.5 W max.
- Screen Dissipation: 1.1 W max.
- Control Grid Voltage (Grid 1):
- Negative Bias: 50 V max.
- Positive Bias: 0 V max.
- Control Grid Resistance:
- With Fixed Bias: 0.25 MΩ max.
- With Cathode Bias: 1.0 MΩ max.
Electrical Characteristics and Typical Operation
Under typical operating conditions as a class A1 amplifier, the 12BY7 and 12BY7-A exhibit high transconductance and a high amplification factor. The standard operating point and resulting characteristics are detailed below:
- Plate Voltage: 250 V
- Screen Voltage: 180 V
- Cathode Bias Resistor: 100 Ω
- Transconductance: 11,000 µmhos
- Plate Resistance: 93,000 Ω
- Plate Current: 26 mA
- Screen Current: 5.75 mA
- Grid 1 Cutoff Voltage: -11.6 V (defined at a plate current of 20 µA)
- Amplification Factor (Pentode): 1035
- Amplification Factor (Triode-Connected): 28.5
The direct interelectrode capacitances of the tube, measured without an external shield, are:
- Grid to Plate (Input to Output): 0.063 µµF
- Input Capacitance: 10.2 µµF
- Output Capacitance: 3.5 µµF
Technical Summary
The 12BY7 and 12BY7-A are high-transconductance pentodes designed for high-performance video amplification. Featuring a miniature 9-pin construction, they provide a transconductance of 11,000 µmhos and a pentode amplification factor of 1035 under typical operating conditions. The 12BY7-A variant incorporates a controlled 11-second heater warm-up characteristic, making it highly suitable for series-string heater designs in television receivers. With flexible series/parallel heater options (12.6 V / 6.3 V) and robust design-center ratings, these tubes deliver reliable voltage amplification across low-impedance loads.
