12B7 — Remote-Cutoff Pentode Voltage Amplifier

The 12B7 (also designated as the 14A7/12B7) is a remote-cutoff pentode voltage amplifier designed using lock-in construction. It is primarily intended for radio frequency (RF) or intermediate frequency (IF) amplification service in high-frequency circuits and AC/DC receivers. The tube features a coated unipotential cathode, a 12.6 V heater, and a compact glass envelope with an eight-pin locking base.

Construction, Physical Specifications, and Pinout

The 12B7 is constructed with a T-9 glass bulb and an 8-pin locking-in base (basing designation 8V). It is designed to operate in any mounting position. The physical dimensions of the tube are as follows:

• Maximum Overall Length: 2 25/32 inches (approx. 70.6 mm)
• Maximum Seated Height: 2 1/4 inches (approx. 57.2 mm)
• Maximum Diameter: 1 3/16 inches (approx. 30.2 mm)

The locking-in base provides secure mechanical retention and reliable electrical contact. The pin connections, viewed from the bottom of the base, are assigned according to the standard RMA numbering system:

Heater Characteristics

The 12B7 utilizes an indirectly heated, coated unipotential cathode. The heater is designed to operate on either AC or DC. The nominal heater characteristics are:

• Heater Voltage: 12.6 V
• Heater Current: 0.15 A (150 mA)
• Calculated Heater Power: 1.89 W (calculated as $12.6\text{ V} \times 0.15\text{ A}$)

Electrical Ratings

The maximum electrical ratings for the 12B7 vary slightly depending on the publication date and specific standard applied. Under the RMA Standard MB-210, the absolute limits are defined as follows:

Typical Operating Conditions and Characteristics

The 12B7 can be operated under different Class $A_1$ amplifier conditions depending on the available plate supply voltage. The published typical operating points illustrate the performance of the tube with both fixed bias and self-bias configurations:

Operating Point Comparison

When configured with a 100 V plate supply and a 60-ohm self-bias resistor, the control grid operates at a nominal -1 V bias, which increases the plate current to 13 mA and transconductance to 2350 $\mu$mhos, though plate resistance drops to approximately 0.12 megohm. At a higher plate voltage of 250 V, the plate resistance increases significantly to 0.8 megohm, providing high voltage gain in RF and IF stages.

The remote-cutoff characteristic is demonstrated by the control grid voltage required to reduce transconductance to approximately 10 $\mu$mhos, which is -35 V under both the 100 V and 250 V operating conditions. This wide grid-bias range allows effective automatic volume control (AVC) or automatic gain control (AGC) without introducing severe distortion.

Interelectrode Capacitances

The direct interelectrode capacitances of the 12B7 depend on the shielding configuration used during measurement. The published values under different shielding standards are:

• With RMA Shield #308 Connected to Cathode:

  • Grid to Plate ($G_1$ to $P$): 0.003 $\mu\mu$F
  • Input ($G_1$ to $H+K+G_2+G_3+IS$): 6.0 $\mu\mu$F
  • Output ($P$ to $H+K+G_2+G_3+IS$): 7.0 $\mu\mu$F

• With Standard External Shield Connected to Cathode:

  • Grid to Plate ($G_1$ to $P$): 0.005 $\mu\mu$F (maximum)
  • Control Grid to Cathode (Input): 5.5 $\mu\mu$F
  • Plate to Cathode (Output): 7.0 $\mu\mu$F

The extremely low grid-to-plate capacitance (0.003 to 0.005 $\mu\mu$F) is a critical feature of this pentode, minimizing Miller effect feedback and ensuring stable operation in high-frequency amplifier stages without the need for neutralization.

Characteristic Curves and Behavior

The published characteristic curves for the 12B7 illustrate its behavior across a range of operating conditions:

• Plate Characteristics ($I_b$ and $I_{c2}$ vs. $E_b$): At a constant screen grid voltage ($E_{c2}$) of 100 V, the plate current ($I_b$) rises sharply as plate voltage increases from 0 to approximately 50 V, after which it becomes nearly independent of plate voltage, exhibiting the high dynamic plate resistance characteristic of a pentode. The curves are plotted for control grid voltages ($E_{[c1](/tubes/C1)}$) ranging from 0 V to -15 V. The screen current ($I_{c2}$) curves show a similar but lower-magnitude profile, decreasing slightly as plate voltage increases.
• Transfer Characteristics ($I_b$, $I_{c2}$, $r_p$, and $g_m$ vs. $E_{[c1](/tubes/C1)}$): Plotted for plate voltages of 100 V and 250 V (with screen grid at 100 V and suppressor grid at 0 V), these curves show that transconductance ($g_m$) and plate current decrease smoothly as the control grid bias becomes more negative. The transconductance curve highlights the remote-cutoff design, showing a gradual tailing off rather than a sharp cutoff, which is essential for low-distortion gain control.

Technical Summary

The 12B7 (or 14A7/12B7) is a versatile lock-in remote-cutoff pentode designed for RF and IF voltage amplification. Operating with a 12.6 V, 150 mA heater, it features a unipotential cathode and an 8-pin locking base. It provides a transconductance of 1900 to 2350 $\mu$mhos and a high plate resistance of up to 0.8 M$\Omega$ under typical Class $A_1$ operating conditions. Its low grid-to-plate capacitance of 0.003 to 0.005 $\mu\mu$F and its smooth remote-cutoff characteristic make it highly effective for stable, gain-controlled high-frequency amplification in AC/DC radio receivers.