Voltage - Triode (with diode, heptode or hexode) ·Diode-Diode-Triode
Dual-diode high-mu triode providing detector diode sections and a Class-A voltage-amplifier triode for conventional receiving applications.
The 12S8GT (also designated as 12S8-GT) is a multi-unit, indirectly heated vacuum tube designed for use in combined AM/FM radio receivers. It integrates a single high-mu triode and three independent diode units within a single glass envelope. This specialized configuration provides all the necessary elements for detection and audio voltage amplification of both AM and FM signals without requiring additional switching circuitry.
The 12S8GT is engineered to simplify the detector and first audio stages of AM/FM receivers. By combining three separate diodes and a high-mu triode, the tube can perform multiple functions simultaneously, such as AM detection, FM detection (using two of the diodes in a balanced discriminator or ratio detector configuration), and subsequent audio pre-amplification.
The tube features coated unipotential cathodes and is housed in a T-9 glass bulb with an intermediate shell 8-pin octal base. It is designed to operate in any mounting position.
The internal structure of the 12S8GT consists of a high-mu triode and three separate diode plates. To facilitate its use in balanced detector circuits, the cathode arrangement is split:
This configuration allows Diode 1 to operate at a different cathode potential than the other elements. It is specifically recommended that Diode 1 and Diode 3 be utilized in balanced detector circuits to take advantage of this isolation.
The heater is designed to operate from either an AC or DC source.
| Heater Parameter | Value |
|---|---|
| Nominal Heater Voltage | 12.6 V |
| Nominal Heater Current | 0.15 A (150 mA) |
| Calculated Heater Power | 1.89 W (calculated as 12.6 V * 0.15 A) |
The 12S8GT utilizes an 8-pin octal base with an 8CB basing arrangement. The triode grid is connected to a skirted miniature top cap on the glass envelope. The pin connections, viewed from the bottom of the base, are as follows:
| Pin / Cap | Connection |
|---|---|
| Pin 1 | Plate of Diode 3 (P3) |
| Pin 2 | Common Cathode (K) for Diode 2, Diode 3, and Triode |
| Pin 3 | Plate of Diode 1 (P1) |
| Pin 4 | Separate Cathode (K) for Diode 1 |
| Pin 5 | Plate of Diode 2 (P2) |
| Pin 6 | Plate of Triode (P) |
| Pin 7 | Heater (H) |
| Pin 8 | Heater (H) |
| Top Cap | Grid of Triode (G) |
The maximum ratings for the 12S8GT are established under the RMA Standard M8-210 system. These limits represent absolute boundaries that must not be exceeded under any operating conditions.
| Rating Parameter | Maximum Limit |
|---|---|
| Triode Plate Voltage | 300 V |
| Triode Plate Dissipation | 0.5 W |
| Continuous Diode Current (Each Diode) | 1 mA |
| Heater-Cathode Voltage | 90 V |
The triode section of the 12S8GT is characterized by a high amplification factor (mu = 100), making it highly suitable for voltage pre-amplification. Typical Class A operating points for the triode unit are detailed below:
| Parameter | Operating Point A | Operating Point B |
|---|---|---|
| Plate Voltage | 100 V | 250 V |
| Grid Voltage | -1 V | -2 V |
| Grid Circuit Resistor | 0 M̦ | 0 M̦ |
| Plate Current | 0.4 mA | 0.9 mA |
| Plate Resistance (Approx.) | 110,000 ̦ | 91,000 ̦ |
| Transconductance | 900 ̦mhos | 1,100 ̦mhos |
| Amplification Factor | 100 | 100 |
Each of the three diode units is capable of rectification. With 10 Volts DC applied directly to any individual diode plate, the average diode current is typically 2.5 mA.
The high-mu triode section is frequently employed as a resistance-coupled audio amplifier. It can be configured using either conventional cathode-resistor bias or grid-leak bias.
The following table outlines representative operating conditions for the triode unit across various plate supply voltages (Ebb), load resistances (R1), and coupling configurations (Rc). The output voltage (Eo) is measured in RMS volts at 5% total harmonic distortion, and the voltage gain is measured at an output level of 2.0 Volts RMS.
| Plate Load (R1) | Grid Resistor (Rg1) | Coupling Resistor (Rc) | Supply (Ebb) | Cathode Resistor (Rk) | Voltage Gain | Output Voltage (Eo) |
|---|---|---|---|---|---|---|
| 0.1 M̦ | A | 0.1 M̦ | 90 V | 6,800 ̦ | 25 | 6 V |
| 0.1 M̦ | A | 0.1 M̦ | 180 V | 4,700 ̦ | 27 | 10 V |
| 0.1 M̦ | A | 0.1 M̦ | 300 V | 2,200 ̦ | 36 | 29 V |
| 0.27 M̦ | A | 0.27 M̦ | 90 V | 10,000 ̦ | 35 | 9 V |
| 0.27 M̦ | A | 0.27 M̦ | 180 V | 6,800 ̦ | 44 | 19 V |
| 0.27 M̦ | A | 0.27 M̦ | 300 V | 4,700 ̦ | 53 | 42 V |
| 0.47 M̦ | A | 1.0 M̦ | 90 V | 18,000 ̦ | 46 | 15 V |
| 0.47 M̦ | A | 1.0 M̦ | 180 V | 12,000 ̦ | 54 | 29 V |
| 0.47 M̦ | A | 1.0 M̦ | 300 V | 9,100 ̦ | 63 | 59 V |
| 0.27 M̦ | 10 M̦ | 0.27 M̦ | 90 V | Grounded (0 ̦) | 34 | 5 V |
| 0.27 M̦ | 10 M̦ | 0.27 M̦ | 180 V | Grounded (0 ̦) | 54 | 20 V |
| 0.27 M̦ | 10 M̦ | 0.27 M̦ | 300 V | Grounded (0 ̦) | 58 | 40 V |
| 0.47 M̦ | 10 M̦ | 1.0 M̦ | 90 V | Grounded (0 ̦) | 42 | 9.5 V |
| 0.47 M̦ | 10 M̦ | 1.0 M̦ | 180 V | Grounded (0 ̦) | 65 | 33 V |
| 0.47 M̦ | 10 M̦ | 1.0 M̦ | 300 V | Grounded (0 ̦) | 70 | 63 V |
Note: "A" indicates that the value of the grid resistor Rg1 is not critical for the associated cathode-biased operating point.
The capacitive coupling between the internal elements of the 12S8GT is minimized to prevent unwanted feedback and signal degradation, particularly at high frequencies.
The plate characteristic curves for the triode unit of the 12S8GT illustrate its behavior as a high-mu voltage amplifier. At typical operating plate voltages (between 100 V and 300 V), the plate current remains low (generally under 1.5 mA), which is characteristic of high plate resistance devices. The curves demonstrate that small changes in grid voltage produce significant variations in plate voltage, confirming the tube's high amplification factor of 100.
For the diode units, the rectification curves show the relationship between the rectified microamperes and the DC volts developed across various load resistors (ranging from 0.1 Megohm to 2 Megohms) under different RMS signal inputs (up to 30 Volts). These curves indicate that higher load resistances yield higher developed DC voltages for a given signal input, which is a critical consideration when designing detector and automatic volume control (AVC) circuits.
The 6S8GT is a closely related designation. It shares the same physical construction, triple-diode triode configuration, and electrical characteristics as the 12S8GT, with the sole exception of the heater requirements. The 6S8GT is designed for a nominal 6.3 V heater supply, whereas the 12S8GT requires a 12.6 V heater supply. These two types are not directly interchangeable without modifying the heater supply circuit.
The 12S8GT is an octal-based, triple-diode high-mu triode vacuum tube designed for AM/FM receiver applications. It features a 12.6 V, 150 mA heater, a T-9 glass envelope, and an 8CB pinout configuration. By incorporating three diodes (one with an independent cathode) and a high-gain triode (mu = 100) in a single envelope, it provides an efficient solution for detection and pre-amplification. Designers must adhere to its maximum plate voltage of 300 V and plate dissipation of 0.5 W to ensure reliable operation.
International Octal
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
Promoted from printed maximum/characteristic columns; additional published values retained in TUBE_SECTION_PARAMETER.
Section 2
No fixed ratings are recorded for this section.
Promoted from printed maximum/characteristic columns; additional published values retained in TUBE_SECTION_PARAMETER. The chart identifies the repeated detector-diode section(s) but does not publish separate diode electrical ratings on this 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
1.2 pF
Section1
Output capacitance
5 pF
Section1
Grid-to-plate capacitance
2 pF
Section1
Plate voltage
250 V
ClassA
Control-grid voltage
-2 V
ClassA
Plate current
0.9 mA
ClassA
Plate resistance
91000 Ω
Section1
Transconductance
1100 µS
Section1
Amplification factor
100
Section1
Data set 02
Published operating points and test conditions from this source.
Heater voltage
12.6 V
Nominal
Heater voltage
12.6 V
Nominal
Heater voltage
12.6 V
Nominal