1AF33 — Diode-AF Pentode Vacuum Tube

The 1AF33 is a directly heated, miniature diode-pentode vacuum tube designed primarily for use in portable and battery-operated radio receivers. It combines a single diode section for AM demodulation or detection with a high-gain audio-frequency (AF) pentode section for voltage amplification.

Mechanical Specifications and Pinout Configurations

The 1AF33 is constructed in a miniature glass envelope with a 7-pin base on a glass button. The physical dimensions of the tube are compact, featuring a maximum diameter of 19 mm, a body height of 49 mm, and an overall height ranging up to 56 mm (including a 7 mm pin length and an 8.5 mm top tip).

Two distinct pinout configurations are documented for this tube type. Designers must verify the specific physical layout of the device in use:

Pinout Configuration A

Pinout Configuration B

Filament and Heating

The 1AF33 features a directly heated oxide-coated cathode. The filament is designed for low power consumption, making it highly suitable for battery-operated equipment. It can be supplied with DC from dry cells or mains-operated series/parallel filament chains.

• Filament Voltage ($U_f$): 1.4 V (nominal)
• Filament Voltage Range: Minimum > 1.1 V, Maximum 1.6 V
• Filament Current ($I_f$): 25 mA

Electrical Characteristics

Static Pentode Characteristics

At typical operating potentials, the pentode section exhibits the following static characteristics:

• Anode Voltage ($U_a$): 67.5 V
• Screen Grid Voltage ($U_{[g2](/tubes/G2)}$): 67.5 V
• Control Grid Voltage ($U_{g1}$): -1 V to 0 V
• Anode Current ($I_a$): 1.4 mA (at $U_{g1} = -1$ V) or 1.6 mA (at $U_{g1} = 0$ V)
• Screen Grid Current ($I_{[g2](/tubes/G2)}$): < 0.4 mA (at $U_{g1} = -1$ V) or 0.4 mA (at $U_{g1} = 0$ V)
• Mutual Conductance ($S$): > 300 µA/V (measured at $U_{g1} = -0.5$ V) or 500 µA/V (at $U_{g1} = 0$ V)
• Internal Resistance ($R_i$): 0.6 MΩ
• Amplification Factor ($\mu$): 300

Diode Characteristics

The diode section is electrically independent of the pentode, sharing only the common filament.

• Diode Current ($I_d$): > 0.1 mA (measured at a diode voltage $U_d = 3$ V)

Interelectrode Capacitances

Measured with external shielding, the capacitive coupling between electrodes is minimal:

• Input Capacitance ($C_{g1}$): 2.4 pF
• Output Capacitance ($C_a$): 4.6 pF
• Grid-to-Anode Capacitance ($C_{a/g1}$): < 0.3 pF
• Diode-to-Filament Capacitance ($C_{d/f}$): 1.5 pF

Circuit-Specific Operating Examples

AF Resistance-Coupled Amplifier (Pentode Connection)

When operated as a resistance-coupled pentode amplifier, the tube provides high voltage gain at low operating currents across various supply voltages:

Note: Alternative operating configurations specify a screen grid series resistor ($R_{[g2](/tubes/G2)}$) of 3.0 MΩ with the control grid bias ($U_{g1}$) set to 0 V.

AF Resistance-Coupled Amplifier (Triode Connection)

For applications requiring lower distortion and lower output impedance, the pentode section can be triode-connected at a 90 V supply voltage:

Maximum Ratings

To ensure reliable operation and prevent premature failure, the following maximum limits must not be exceeded:

Pentode Section

• Anode Cold Voltage ($U_{ao}$): 250 V
• Anode Voltage ($U_a$): 90 V
• Anode Dissipation ($W_a$): 0.12 W
• Screen Grid Cold Voltage ($U_{g2o}$): 250 V
• Screen Grid Voltage ($U_{[g2](/tubes/G2)}$): 67.5 V to 90 V (depending on operating standard)
• Screen Grid Dissipation ($W_{[g2](/tubes/G2)}$): 0.03 W
• Control Grid Voltage ($U_{g1}$): 0 V
• Cathode Current ($I_k$): 4.5 mA
• Control Grid Resistor ($R_{g1}$):
  • Max 3 MΩ (fixed bias)
  • Max 10 MΩ to 22 MΩ (with grid-leak bias, where $U_{g1}$ is produced across $R_{g1}$)

Diode Section

• Peak Inverse Diode Voltage ($U_{d\text{ sp}}$): 50 V
• Diode Current ($I_d$): 0.2 mA
• Peak Diode Current ($I_{d\text{ sp}}$): 1.2 mA

Qualitative Curve Observations

The anode characteristic curves ($I_a$ vs. $U_a$) demonstrate typical pentode behavior:

• At $U_{[g2](/tubes/G2)} = 67.5$ V: The characteristic knee occurs between 15 V and 25 V. With the control grid voltage ($U_{g1}$) at 0 V, the anode current plateaus near 2.0 mA at an anode voltage of 100 V. Curves are typically plotted down to $-5$ V in steps of 0.5 V.
• At $U_{[g2](/tubes/G2)} = 45$ V: The characteristic knee shifts lower, occurring between 10 V and 20 V. At $U_{g1} = 0$ V, the anode current plateaus near 1.1 mA at an anode voltage of 100 V. Curves are typically plotted down to $-3$ V in steps of 0.5 V.

Related Designations and Substitutions

The 1AF33 is designated by TESLA and is directly equivalent to the foreign type 1S5T.

It can serve as a direct electrical substitute for the 1S5, DAF 91, or DAF 191, though it is distinguished from these types by having exactly half the filament power consumption (25 mA versus 50 mA). With minor mechanical modifications to the chassis or socket wiring, the 1AF33 can also replace older types such as the DAF 40 or DAF 41, though minor electrical differences exist and must be accounted for in the circuit design.