12SR7 — Duplex-Diode Medium-Mu Triode

An indirectly heated, multi-unit vacuum tube containing two diodes and a medium-mu triode in a single envelope, designed for combined detector, audio amplifier, and automatic-volume-control service.

Overview

The 12SR7 (along with its glass-envelope counterpart, the 12SR7-GT) is a multi-unit receiving tube designed for radio receiver applications. It combines a medium-mu triode with two independent diode units sharing a common, coated unipotential cathode. This configuration allows a single tube to perform the functions of AM detection, automatic volume control (AVC), and low-frequency audio voltage amplification.

The tube features a nominal 12.6 V heater and utilizes an eight-pin octal base. The standard 12SR7 is housed in an MT-8 metal shell, while the 12SR7-GT is housed in a glass envelope.

Construction, Basing, and Pin Connections

The 12SR7 is constructed with a coated unipotential cathode that is common to both the triode and the two diode units. In the metal-envelope version, an internal shield is connected directly to the cathode. The tube is designed to operate in any mounting position.

Heater Ratings

Octal Base Pinout (RMA 8Q)

Using the standard octal base configuration, the pin connections viewed from the bottom of the tube are as follows:

• Pin 1: Metal Shell
• Pin 2: Triode Grid
• Pin 3: Cathode and Internal Shield
• Pin 4: Diode Plate Number 2
• Pin 5: Diode Plate Number 1
• Pin 6: Triode Plate
• Pin 7: Heater
• Pin 8: Heater

Direct Interelectrode Capacitances

Capacitance values vary slightly depending on the envelope construction and shielding conditions:

• Metal Envelope (12SR7) (with shell connected to cathode and internal shield):
  • Input: 2.4 µµf
  • Output: 3.6 µµf
  • Plate to Cathode: 2.8 µµf
• Metal Envelope (12SR7) (without external shield):
  • Input: 3.6 pF
  • Output: 2.8 pF
  • Grid-to-Plate: 2.4 pF
• Glass Envelope (12SR7-GT) (without external shield):
  • Input: 3.5 pF
  • Output: 3.8 pF
  • Grid-to-Plate: 2.3 pF

Electrical Characteristics and Typical Operation

At typical Class A operating points, the triode section of the 12SR7 provides moderate amplification with a relatively low plate resistance. Under typical operating conditions, the triode exhibits the following characteristics:

• Plate Voltage: 250 V
• Grid Bias Voltage: -9 V
• Amplification Factor (μ): 16
• Plate Resistance ($R_p$): 8,500 Ω
• Transconductance ($G_m$): 1,900 µmhos (1.9 mA/V)
• Plate Current: 9.5 mA (Note: Some specifications list typical plate current as 6.5 mA under these same voltage conditions)
• Optimum Load Resistance: 10,000 Ω
• Power Output: 0.30 W (300 mW)

When designing circuits for the triode section, the d-c resistance in the grid circuit must not exceed 1.0 megohm under rated maximum conditions.

Maximum Ratings

To ensure reliable operation, the operating potentials and currents must be kept within the manufacturer's specified limits. The ratings are defined under both Design Center and Absolute Maximum systems:

Resistance-Coupled Amplifier Service

The triode section of the 12SR7 is highly suited for resistance-coupled audio amplifier stages. Operating parameters and performance vary depending on the plate supply voltage ($E_{bb}$), plate load resistance ($R_p$), and grid bias method:

Cathode-Bias Operation

Under typical cathode-bias conditions with a grid resistor ($R_{g1}$) of a non-critical value and a subsequent stage grid resistor ($R_s$) equal to the plate load resistor:

• With $R_p = 0.10\text{ M}\Omega$ and $R_s = 0.10\text{ M}\Omega$:
  • At $E_{bb} = 90\text{ V}$: $R_k = 3,300\ \Omega$, Voltage Gain = 10, Peak Output ($E_o$) = 15 V
  • At $E_{bb} = 180\text{ V}$: $R_k = 3,000\ \Omega$, Voltage Gain = 10, Peak Output ($E_o$) = 32 V
  • At $E_{bb} = 300\text{ V}$: $R_k = 2,700\ \Omega$, Voltage Gain = 10, Peak Output ($E_o$) = 46 V
• With $R_p = 0.25\text{ M}\Omega$ and $R_s = 0.51\text{ M}\Omega$:
  • At $E_{bb} = 90\text{ V}$: $R_k = 8,200\ \Omega$, Voltage Gain = 11, Peak Output ($E_o$) = 20 V
  • At $E_{bb} = 180\text{ V}$: $R_k = 7,500\ \Omega$, Voltage Gain = 11, Peak Output ($E_o$) = 36 V
  • At $E_{bb} = 300\text{ V}$: $R_k = 6,800\ \Omega$, Voltage Gain = 11, Peak Output ($E_o$) = 53 V

Zero-Bias (Grid-Leak Bias) Operation

When operating with a high-value grid resistor ($R_{g1} = 10\text{ M}\Omega$) and zero applied d-c bias voltage, the cathode resistor is omitted ($R_k = 0\ \Omega$):

• With $R_p = 0.24\text{ M}\Omega$ and $R_s = 0.24\text{ M}\Omega$:
  • At $E_{bb} = 90\text{ V}$: Voltage Gain = 12, Peak Output ($E_o$) = 7.8 V
  • At $E_{bb} = 180\text{ V}$: Voltage Gain = 13, Peak Output ($E_o$) = 16 V
  • At $E_{bb} = 300\text{ V}$: Voltage Gain = 13, Peak Output ($E_o$) = 27 V
• With $R_p = 0.51\text{ M}\Omega$ and $R_s = 1.0\text{ M}\Omega$:
  • At $E_{bb} = 90\text{ V}$: Voltage Gain = 13, Peak Output ($E_o$) = 11 V
  • At $E_{bb} = 180\text{ V}$: Voltage Gain = 14, Peak Output ($E_o$) = 20 V
  • At $E_{bb} = 300\text{ V}$: Voltage Gain = 15, Peak Output ($E_o$) = 28 V

Note: The peak output voltage ($E_o$) represents the maximum RMS voltage output for five percent (5%) total harmonic distortion. The voltage gain is measured at an output level of 2.0 V RMS.

Technical Summary

The 12SR7 is an octal-based, duplex-diode medium-mu triode featuring a 12.6 V, 0.15 A heater and a common unipotential cathode. Its triode section provides an amplification factor of 16, a transconductance of 1,900 µmhos, and a plate resistance of 8,500 Ω under typical Class A operating conditions. Designed primarily for combined detector, AVC, and audio preamplifier service, the tube offers reliable performance in both transformer-coupled and resistance-coupled circuits. For detailed characteristic curves and further operating parameters, reference is made to the electrically similar 6R7G vacuum tube.