12AD5 — Remote Cut-Off R.F. Pentode

The 12AD5 is an indirectly heated, remote cut-off pentode designed primarily for use as a radio frequency (R.F.) or intermediate frequency (I.F.) amplifier. Featuring a miniature construction, the tube is engineered to provide variable-mu (remote cut-off) characteristics, allowing for effective automatic volume control (AVC) or automatic gain control (AGC) in receiver circuits without introducing excessive distortion.

Mechanical Design and Pinout

The 12AD5 is constructed in a T6-1/2 glass bulb envelope and utilizes an E9-1 miniature 9-pin base. The tube is designed to operate in any mounting position. Its physical dimensions include a maximum overall diameter of 7/8 inches, a maximum overall length of 2 5/8 inches, and a seated height ranging from 2 inches to a maximum of 2 3/8 inches (2" ± 3/32").

The tube utilizes the RETMA basing designation 9AZ. The pin connections, viewed from the bottom of the base, are assigned as follows:

Pins 7 and 8 are designated for internal connections and must not be used for external circuit wiring, while Pin 9 has no internal connection. The integration of Grid No. 3 (suppressor grid) and the internal shield with the cathode on Pin 3 simplifies external circuitry and aids in minimizing parasitic feedback at high frequencies.

Heater and Interelectrode Capacitances

The 12AD5 features a coated unipotential cathode heated by a nominal 12.6-volt supply.

Heater Specifications

• Heater Voltage: 12.6 V
• Heater Current: 0.1 A
• Calculated Heater Power: 1.26 W (calculated as $12.6\text{ V} \times 0.1\text{ A}$)

Direct Interelectrode Capacitances

The direct interelectrode capacitances of the device, measured without an external shield, are:

• Input Capacitance (Grid No. 1 to all other electrodes except Plate): 5.1 µµF
• Output Capacitance (Plate to all other electrodes except Grid No. 1): 8.1 µµF
• Grid No. 1 to Plate Capacitance: 0.002 µµF (maximum)
• Grid No. 1 to Heater Capacitance: 0.05 µµF (maximum)

The extremely low grid-to-plate capacitance (0.002 µµF maximum) is a critical parameter for stable R.F. and I.F. amplification, preventing unwanted Miller-effect feedback and oscillation at high operating frequencies.

Maximum Ratings (Design Center Values)

The maximum ratings for the 12AD5 are established under the Design Center system. Operating the tube beyond these limits may impair its performance and longevity.

The maximum allowable Grid No. 2 (screen grid) voltage is dynamically linked to the plate current. When the plate current is low (less than 4 mA), the screen grid can withstand up to 250 V. However, under typical full-conduction operating conditions where plate current reaches 7.2 mA, the maximum screen grid voltage rating is reduced to 150 V to prevent exceeding the 0.3-watt screen dissipation limit.

Operating Characteristics and Circuit Configurations

The 12AD5 can be operated as an R.F. or I.F. amplifier using either a sliding (series-resistor fed) screen grid voltage or a fixed screen grid voltage.

A. Operation with Sliding Grid No. 2 Voltage

In this configuration, Grid No. 2 is supplied from the plate voltage source through a 40,000-ohm series resistor. A 325-ohm cathode resistor provides self-bias. As the negative bias on Grid No. 1 increases (for example, via AVC action), the plate and screen currents decrease. This reduction in screen current decreases the voltage drop across the 40,000-ohm series resistor, causing the screen grid voltage to "slide" upward toward the supply voltage, which extends the cut-off region.

B. Operation with Fixed Grid No. 2 Voltage

Alternatively, the tube can be operated with a fixed screen grid voltage of 100 V, using a 325-ohm cathode resistor for self-bias.

• Plate Voltage: 100 V
• Grid No. 2 Voltage: 100 V
• Cathode Resistor: 325 Ω
• Grid No. 1 Voltage Range: -2.5 V to -16.5 V
• Plate Current (at -2.5 V): 6.0 mA
• Grid No. 2 Current (at -2.5 V): 1.75 mA
• Transconductance: 2200 µmhos (at -2.5 V) decreasing to 22 µmhos (at -16.5 V)
• Plate Resistance: 0.6 MΩ (at -2.5 V) increasing to >10 MΩ (at -16.5 V)
• Grid No. 2 to Grid No. 1 Amplification Factor ($\mu_{g2-g1}$): 18
• Equivalent Noise Resistance (at -2.5 V): 6500 Ω

Comparing the two methods, the sliding screen voltage configuration allows the tube to handle much larger negative grid bias swings (up to -34 V with a 200 V supply) before reaching the 22 or 23 µmhos transconductance level, compared to the fixed screen configuration which cuts off to 22 µmhos at -16.5 V.

Analysis of Characteristic Curves

The relationship between the control grid (Grid No. 1) voltage and the resulting plate current or transconductance highlights the remote cut-off design of the 12AD5.

At low negative bias values, the transconductance is high (typically 1900 to 2300 µmhos), providing high stage gain for weak signals. As the negative bias increases, the transconductance drops smoothly and logarithmically. Under sliding screen conditions, the screen voltage rises as plate current drops, which softens the cut-off characteristic and prevents cross-modulation or clipping of strong incoming signals.

The screen grid current characteristics show that when using a 40,000-ohm series resistor, the operating path of the screen grid remains safely below the 0.3-watt Grid No. 2 dissipation curve across the entire range of plate and screen voltages (from 100 V to 200 V supply levels).

Technical Summary

The 12AD5 is a high-performance, remote cut-off R.F. pentode featuring a 9-pin miniature construction (E9-1 base) and a 12.6 V, 0.1 A heater. It is optimized for variable-gain R.F. and I.F. amplifier stages in communication receivers. With a low feedback capacitance of 0.002 µµF, a maximum plate dissipation of 2.0 W, and a screen grid amplification factor of 18, the 12AD5 offers robust and stable gain control characteristics. Its ability to operate with either a fixed screen voltage or a sliding screen voltage via a 40,000-ohm series resistor provides circuit designers with flexibility in tailoring the AGC/AVC control range and signal-handling capabilities.