1AH5 — Miniature Diode-Pentode Vacuum Tube

A directly heated, low-drain miniature diode-pentode vacuum tube designed for detector and audio-frequency (A.F.) amplifier service in battery-operated receivers.

Overview

The 1AH5 (often printed as IAH5) is a miniature, directly heated vacuum tube containing a combined single diode and a pentode section within a single glass envelope. It is electrically equivalent to the DAF96. Designed primarily for portable, battery-operated radio receivers, the 1AH5 features an extremely low-drain filament that can be operated from series or parallel direct-current (DC) supplies.

The pentode section is optimized for high-gain voltage amplification, while the independent diode section is situated at the negative end of the filament and is intended for signal detection or automatic volume control (AVC) functions. The tube is housed in a standard miniature 7-pin (B7G) all-glass envelope and can be mounted in any position.

Physical Specifications and Base Connections

The 1AH5 utilizes a standard T5 1/2 glass bulb and a miniature button 7-pin base with a JETEC 7DJ basing designation. Its physical dimensions are highly compact to suit portable equipment:

The bottom-view pin connections are assigned as follows:

• Pin 1: Filament Negative (-), Suppressor Grid ($g_3$)
• Pin 2: Internal Connection (I.C.) / Not Connected
• Pin 3: Diode Anode ($a_d$) / Diode Plate
• Pin 4: Screen Grid ($g_2$) / Grid No. 2
• Pin 5: Anode ($a_p$) / Plate
• Pin 6: Control Grid ($g_1$) / Grid No. 1
• Pin 7: Filament Positive (+)

Filament Ratings

The 1AH5 features a coated filament designed for highly efficient battery operation. The filament can be configured for either series or parallel DC supply systems:

• Series Supply: Nominal filament voltage ($V_f$) of 1.3 V at a current ($I_f$) of 24 mA.
• Parallel Supply: Nominal filament voltage ($V_f$) of 1.4 V at a current ($I_f$) of 25 mA. The permissible filament voltage range is 1.1 V to 1.6 V.

The diode plate is physically positioned at the negative end of the filament (near Pin 1) to minimize the negative bias effect of the filament voltage drop on the diode detection threshold.

Maximum Ratings

The absolute and design-center maximum ratings for the 1AH5 must be strictly observed to prevent premature failure or degradation of the tube.

Pentode Section

• Maximum H.T. Supply Voltage: 120 V
• Maximum Battery Voltage: 90 V (with an absolute permissible maximum of 110 V)
• Maximum Anode Voltage ($V_a$): 90 V
• Maximum Screen Grid Voltage ($V_{[g2](/tubes/G2)}$): 90 V
• Maximum Anode Dissipation ($P_a$): 30 mW (0.03 W)
• Maximum Screen Grid Dissipation ($P_{[g2](/tubes/G2)}$): 10 mW (0.01 W)
• Maximum Cathode Current ($I_k$): 0.25 mA
• Maximum Control Grid Circuit Resistance ($R_{g1-f}$):
  • 3 MΩ under standard conditions (with $I_k < 0.25$ mA)
  • Up to 10 MΩ or 22 MΩ when utilizing grid-current biasing, provided the cathode current is limited to 0.1 mA
• Grid Current Starting Point: Control grid voltage ($V_{g1}$) is a maximum of 0 V (or -0.2 V in some rating systems) when the control grid current ($I_{g1}$) reaches +0.3 µA.

Diode Section

• Maximum Peak Inverse Voltage (P.I.V.): 100 V
• Maximum Mean Diode Current: 0.2 mA
• Maximum Peak Diode Current: 1.2 mA

Electrical Characteristics

Under typical static operating conditions, the pentode section of the 1AH5 exhibits the following characteristics:

• Anode Voltage ($V_a$): 67.5 V
• Screen Grid Voltage ($V_{[g2](/tubes/G2)}$): 67.5 V
• Control Grid Voltage ($V_{g1}$): -1.5 V
• Anode Current ($I_a$): 170 µA (0.17 mA)
• Screen Grid Current ($I_{[g2](/tubes/G2)}$): 55 µA (0.055 mA)
• Mutual Conductance ($g_m$): 170 µA/V (170 µmhos)
• Inner Amplification Factor ($ext{inner } \mu$ or $\mu_{g1-g2}$): 16

Resistance-Coupled Audio Frequency Amplifier Operation

The 1AH5 is primarily employed as a resistance-coupled audio-frequency voltage amplifier. It can be operated in either a standard pentode configuration or a triode-strapped configuration (where the screen grid $g_2$ is connected directly to the anode).

The typical operating examples below are based on supply voltages of 64 V or 85 V (derived from nominal battery voltages of 67.5 V or 90 V, reduced by the negative bias required for the subsequent output stage).

Pentode Connection

In this configuration, the control grid resistor ($R_{g1-k}$) is 10 MΩ, the anode load resistor ($R_a$) is 1.0 MΩ, and the output voltage ($V_o$) is maintained at 5 V r.m.s.

Specifically, the 3.3 MΩ screen feed resistor is typically decoupled by a 0.47 µF capacitor.

Triode Connection

When triode-connected (screen grid $g_2$ tied to the anode), the control grid resistor ($R_{g1-k}$) is 10 MΩ, and the output voltage ($V_o$) is 5 V r.m.s.

Interelectrode Capacitances

The direct interelectrode capacitances of the 1AH5, measured without an external shield, are as follows:

• Input Capacitance (Grid No. 1 to all other elements except Plate): 1.8 pF to 2.1 pF
• Output Capacitance (Plate to all other elements except Grid No. 1): 2.7 pF to 2.9 pF
• Plate to Grid No. 1 Capacitance ($C_{a-g1}$): max. 0.3 pF
• Diode Plate to all other elements: 1.0 pF to 1.1 pF
• Diode Plate to Pentode Plate ($C_{ad-ap}$): max. 0.9 pF
• Diode Plate to Pentode Grid No. 1 ($C_{ad-g1}$): max. 0.03 pF

These low internal capacitances, particularly the extremely low coupling capacitance between the diode plate and the pentode control grid (0.03 pF), ensure excellent isolation between the detector and audio amplifier stages, minimizing RF feedback and unwanted signal leakage.

Qualitative Curve Observations and Microphony

Plate Characteristics

The plate current ($I_a$) versus plate voltage ($V_a$) curves, plotted at a nominal filament voltage of 1.4 V and a screen grid voltage ($V_{[g2](/tubes/G2)}$) of 67.5 V, demonstrate classic pentode behavior.

• For control grid bias ($V_{g1}$) steps from 0 V to -3 V (in 0.5 V increments), the curves exhibit a distinct "knee" at low plate voltages. At $V_{g1} = 0$ V, the plate current rises sharply below 20 V, reaching the knee and flattening to approximately 0.55 mA at a plate voltage of 100 V.
• At a moderate bias of $V_{g1} = -1.0$ V, the plate current saturates and flattens around 0.25 mA.
• As the bias is increased to $V_{g1} = -3.0$ V, the plate current is nearly cut off, approaching 0 mA.

Transfer Characteristics

The transfer curves (plotting $I_a$ and $I_{[g2](/tubes/G2)}$ against $V_{g1}$) at $V_a = 67.5$ V and $V_{[g2](/tubes/G2)} = 67.5$ V show a smooth, continuous decrease in both currents as the grid bias becomes more negative, with complete cutoff occurring between -3 V and -4 V.

Microphony

Due to its robust miniature construction, the 1AH5 does not require special anti-microphonic mounting precautions in typical receiver circuits, provided the input signal voltage is at least 20 mV for a corresponding output of 50 mW at the receiver's power output stage.

Technical Summary

The 1AH5 is a highly efficient, directly heated miniature diode-pentode designed for battery-operated portable receivers. Combining a high-gain pentode voltage amplifier and an independent detector diode in a single B7G glass envelope, it operates on a low-drain filament requiring only 24–25 mA at 1.3–1.4 V.

With a maximum plate voltage rating of 90 V and an anode dissipation limit of 30 mW, the pentode section provides a mutual conductance of 170 µA/V and an inner amplification factor of 16 under typical conditions. When configured as a resistance-coupled A.F. amplifier, the 1AH5 can deliver stage gains of up to 70 in pentode mode and up to 13.5 in triode-connected mode, with low harmonic distortion. Its low interelectrode capacitances and minimal microphonic sensitivity make it an exceptionally reliable choice for compact, low-power detector and preamplifier applications.