15 — Sharp-Cutoff RF Pentode Vacuum Tube
The 15 is an indirectly heated, sharp-cutoff radio-frequency (RF) pentode vacuum tube designed primarily for use in battery-operated radio receivers. It features a unipotential cathode with a low-power heater, allowing for independent cathode biasing—a significant design advantage over directly heated filament tubes of its era. The tube is housed in an ST-12 glass envelope and utilizes a small 5-pin base with a top-cap connection for the control grid.
Construction, Heater, and Base Configuration
The 15 is constructed with a unipotential (indirectly heated) coated cathode. This design allows the cathode to be electrically isolated from the heater, enabling independent biasing and minimizing the introduction of hum when operated under various circuit configurations.
Heater Ratings
The heater is designed for low power consumption, making it highly suitable for battery-operated equipment.
- Heater Voltage (DC): 2.0 V
- Heater Current: 0.22 A (220 mA)
- Calculated Heater Power: 0.44 W
Physical Dimensions and Bulb
- Bulb Type: ST-12 (also designated as ST-12C or ST 12-D)
- Maximum Overall Length: 4-17/32 inches (approx. 115 mm)
- Minimum Overall Length: 4-9/32 inches
- Maximum Seated Height: 3-25/32 inches
- Maximum Diameter: 1-9/16 inches
- Top Cap: Small metal cap for the control grid (G1)
- Mounting Position: Any (vertical or horizontal)
Base and Pin Connections
The tube uses a small 5-pin base (often referred to as a U.X. base or 5-F configuration). The control grid (G1) is connected to the top cap to maximize isolation from the other electrodes and minimize input capacitance. The suppressor grid (G3) is internally connected to the cathode (K).
Note: While some historical drawings contain a "7 prong" label in the outline diagram, the actual socket layout and all manufacturer specifications confirm a standard 5-pin base.
Electrical Characteristics and Class A Operating Points
The 15 is characterized by high plate resistance and high amplification factor, typical of sharp-cutoff RF pentodes. Manufacturers document two primary Class A operating points:
Brimar documents a slightly different mutual conductance of 0.7 mA/V (equivalent to 700 µmhos) at the 135 V operating point, while other manufacturers specify 750 µmhos.
Heater-to-Cathode Voltage Limits
Because the 15 utilizes an indirectly heated cathode, a potential difference can exist between the heater and the cathode. To prevent insulation breakdown and minimize noise injection, this voltage must be kept as low as possible.
- Maximum Heater-Cathode Potential: 22.5 V (some European specifications list a maximum of 22 V).
Interelectrode Capacitances
The physical separation of the control grid (via the top cap) and the plate, combined with internal shielding, results in extremely low grid-to-plate capacitance. This is essential for stable high-gain RF amplification without neutralisation.
- Grid-to-Plate Capacitance (with shield-can): 0.01 µµF (max)
- Input Capacitance (Grid to Cathode): 2.35 µµF
- Output Capacitance (Plate to Cathode): 7.80 µµF
Applications and Circuit Design
The 15 is a versatile tube designed for several roles in receiver circuits:
- RF and IF Amplifier: Its primary application is as a high-gain radio-frequency or intermediate-frequency amplifier. The sharp-cutoff characteristic ensures stable operation when bias is fixed.
- First Detector / Mixer-Oscillator: In superheterodyne receivers, the 15 can function as the first detector, mixing the incoming RF signal with a local oscillator signal.
- Second Detector: It can be configured as a second detector, where its pentode characteristics and automatic bias capability are highly advantageous.
- Automatic Bias Circuits: Unlike directly heated battery tubes, the separate cathode connection of the 15 allows the use of a cathode resistor to provide automatic self-bias, simplifying power supply design.
- AC/DC Operation: Although optimized for 2.0 V battery operation, the indirectly heated cathode permits operation from AC heater supplies, provided the heater-to-cathode voltage limits are strictly observed and low power consumption is a priority.
The application of the 15 is highly similar to that of the type 32 vacuum tube, sharing comparable circuit topologies and operating philosophies.
Analysis of Characteristic Curves
The published average plate characteristics for the 15 illustrate classic pentode behavior.
- Plate Current vs. Plate Voltage: At a fixed screen voltage of 67.5 V, the plate current rises sharply as plate voltage increases from 0 V to approximately 40 V. Beyond this knee, the curves become nearly horizontal, demonstrating very high plate resistance. For a control grid bias of -1.5 V, the plate current remains virtually constant at 1.85 mA from 60 V up to 400 V.
- Control Grid Cutoff: The transfer characteristics show that plate current and transconductance decrease smoothly as the control grid bias is made more negative. At a grid bias of -1.5 V, transconductance is approximately 750 µmhos. The plate current approaches zero (cutoff) at approximately -6.0 V to -8.0 V, confirming the sharp-cutoff nature of the grid design.
- Screen Current: The screen current remains low and stable at approximately 0.3 mA across the normal operating range, representing a small fraction of the total cathode current.
Technical Summary
The 15 is a sharp-cutoff RF pentode featuring a 2.0 V, 0.22 A indirectly heated cathode, housed in an ST-12 glass bulb with a small 5-pin base and a top cap for the control grid. It is optimized for battery-operated receivers, offering an amplification factor of up to 600, a transconductance of 750 µmhos, and a high plate resistance of 0.8 MΩ at its maximum rated plate voltage of 135 V. Its separate cathode connection enables automatic bias configurations and flexible circuit designs, including RF amplification, first and second detection, and low-power AC/DC receiver applications.
