Voltage Radio Detector Converter ·Diode-Diode-Pentode
Double-diode pentode combining detector and control-voltage rectification with a separate voltage-amplifier pentode. High plate resistance and low feedback capacitance support stable receiver signal amplification.
The 12C8 is a multi-unit vacuum tube of metal construction containing a semi-remote-cutoff pentode and two independent diode plates sharing a common unipotential cathode. This combination of a high-gain radio-frequency pentode and dual detector diodes in a single envelope makes the 12C8 highly suited for simultaneous detection, automatic volume control (AVC) signal generation, and voltage amplification.
The 12C8 is housed in an MT-8 metal shell envelope with a small wafer 8-pin octal base. To minimize input capacitance and provide high isolation between the input and output circuits, the control grid (Grid #1) of the pentode section is terminated at a miniature top cap.
The physical dimensions of the device are:
The tube utilizes the 8E basing arrangement. Viewed from the bottom of the base, the pin connections are assigned as follows:
| Pin | Connection |
|---|---|
| Pin 1 | Metal Shell |
| Pin 2 | Heater |
| Pin 3 | Pentode Plate |
| Pin 4 | Diode Plate #2 |
| Pin 5 | Diode Plate #1 |
| Pin 6 | Grid #2 (Screen Grid) |
| Pin 7 | Heater |
| Pin 8 | Cathode and Grid #3 (Suppressor Grid) |
| Top Cap | Grid #1 (Control Grid) |
Pin 1 connects directly to the metal shell, allowing the envelope to be grounded for effective electrostatic shielding. Pin 8 internally connects the unipotential cathode to Grid #3 (the suppressor grid), simplifying external circuitry and ensuring proper suppression of secondary emission.
The 12C8 features an indirectly heated unipotential cathode designed for operation from either an AC or DC power source. The heater ratings are:
The interelectrode capacitances of the pentode section, measured with the metal shell connected directly to the cathode, are:
The extremely low grid-to-plate capacitance of 0.005 µµF is a critical design feature of the pentode section, preventing regenerative feedback and ensuring stable operation in high-gain intermediate-frequency (IF) and radio-frequency (RF) amplifier stages.
The 12C8 has specific maximum ratings and typical operating parameters established for Class A1 amplifier service and diode detection.
The two diode plates are designed for detection and AVC service. Under test conditions, each diode plate exhibits:
When operated as a Class A1 voltage amplifier, the typical operating points and electrical characteristics of the pentode section are:
| Parameter | Value |
|---|---|
| Plate Voltage | 250 V |
| Screen Voltage (Grid #2) | 125 V |
| Control Grid Voltage (Grid #1) | -3.0 V |
| Zero-Signal Plate Current | 10 mA |
| Zero-Signal Screen Current | 2.3 mA |
| Approximate Plate Resistance | 0.6 MΩ (600,000 Ω) |
| Transconductance | 1,325 µmhos |
| Control Grid Voltage for Cathode Current Cut-off (Approx.) | -21 V |
The semi-remote-cutoff characteristic of the pentode is highlighted by the control grid voltage required for cathode current cut-off. At approximately -21 V, the plate current is cut off, allowing the tube to handle large signal swings and respond smoothly to automatic volume control bias voltages without introducing excessive distortion.
The 12C8 is an octal-based, metal-envelope duplex-diode semi-remote-cutoff pentode. It features a 12.6 V, 0.150 A heater and combines a high-gain pentode amplifier with two independent diode plates. Under typical Class A1 operating conditions with a plate voltage of 250 V and screen voltage of 125 V, the pentode section provides a transconductance of 1,325 µmhos and a plate resistance of approximately 0.6 MΩ, drawing 10 mA of plate current. Its physical design incorporates a miniature top cap for the control grid and utilizes the metal shell (connected to Pin 1) for shielding, making it an efficient and compact solution for detection, AVC, and amplification in radio receiver designs.
General specifications
Properties common to every published source and operating point.
Electrical assignments shared by every tube using this base designation.
No verified pinout is recorded for this base designation.
3 physical sections, each with unique specifications.
Section 1
Published ratings/characteristics promoted where semantically applicable.
Section 2
No fixed ratings are recorded for this section.
Two diode sections are represented by one repeated-section row.
Section 3
No fixed ratings are recorded for this section.
Source-dependent data
Each block preserves the test conditions and published values from one reference. Values from different sources are intentionally kept separate.
Data set 01
Published operating points and test conditions from this source.
Input capacitance
6 pF
Output capacitance
9 pF
Grid-to-plate capacitance
0.005 pF
Plate voltage
250 V
ClassA
Screen voltage
125 V
ClassA
Grid voltage
-3 V
ClassA
Plate current
10 mA
ClassA
Screen current
2.3 mA
ClassA
Data set 02
Published operating points and test conditions from this source.
Heater voltage
12.6 V
Nominal
Heater current
150 mA
Nominal
Plate voltage
300 V
Typical operating conditions
Screen voltage
125 V
Typical operating conditions
Plate current
10 mA
Typical operating conditions
Heater voltage
12.6 V
Nominal
Heater current
150 mA
Nominal
Plate voltage
300 V
Typical operating conditions
Screen voltage
125 V
Typical operating conditions
Plate current
10 mA
Typical operating conditions
Heater voltage
12.6 V
Nominal
Heater current
150 mA
Nominal
Plate voltage
300 V
Typical operating conditions
Screen voltage
125 V
Typical operating conditions
Plate current
10 mA
Typical operating conditions