1A4P Remote-Cutoff RF Pentode

The 1A4P is a remote-cutoff (variable-mu) RF pentode vacuum tube designed primarily for high-frequency and intermediate-frequency amplification in battery-operated receivers. It features a coated filament, a glass envelope, and a design optimized for automatic volume control (AVC) applications with minimal cross modulation.

Physical Characteristics and Pinout

The 1A4P is constructed with an ST-12 (or ST-12C) glass bulb and a small 4-pin base (base designation 4-M or 4M). The control grid (G1) is connected to a small metal top cap at the apex of the envelope. The suppressor grid (G3) is internally connected to the negative end of the filament.

Physical Dimensions

• Maximum Overall Length: 4-15/32 inches
• Maximum Seated Height: 3-29/32 inches
• Maximum Diameter: 1-9/16 inches
• Maximum Base Diameter: 1.165 inches

Mounting Position

The tube is designed to be mounted vertically with the base down. Horizontal operation is also permitted, provided that pins 1 and 4 are aligned on a vertical plane.

Pin Connections

The small 4-pin base uses two larger pins for the filament connections and two smaller pins for the plate and screen grid. The pinout configuration (bottom view) is as follows:

Electrical Ratings and Typical Operation

The 1A4P operates with a low-current DC filament supply, making it highly efficient for battery-powered equipment.

Filament Ratings

• Filament Voltage: 2.0 V (DC, or AC/DC)
• Filament Current: 0.060 A (60 mA)

Maximum Ratings

• Maximum Plate Voltage: 180 V
• Maximum Screen Voltage: 67.5 V

Typical Operating Conditions (Class A Amplifier)

The tube can be operated under several typical plate voltage conditions while maintaining a constant screen voltage of 67.5 V and a minimum control grid bias of -3 V:

Interelectrode Capacitances

The direct interelectrode capacitances remain identical whether measured with or without an external shield:

• Control Grid to Plate (Max): 0.007 µµF
• Input (G1 to G2, G3, F): 5.0 µµF
• Output (P to G3, G2, F): 11.0 µµF

Applications and Circuit Design

The 1A4P is specifically tailored for high-frequency (H.F.) and intermediate-frequency (M.F.) amplifier stages. Its remote-cutoff characteristic allows for effective automatic volume control (AVC) with low cross modulation.

Stability and Shielding

To ensure stable operation and prevent high-frequency instability, the input and output circuits of the amplifier stage must be properly separated. Complete shielding of each individual stage and adequate filtering of power supply lines are required.

Screen Voltage Supply

The screen grid voltage of 67.5 V can be supplied via a direct battery tap or a voltage divider. The use of a simple series dropping resistor is not recommended for screen supply, except in circuits utilizing self-bias (polarisation automatique).

Qualitative Curve Analysis

The operating behavior of the 1A4P is characterized by distinct performance curves under specific electrode potentials:

• Plate Characteristics ($I_p$ vs. $E_p$): At a screen voltage ($E_{sg}$) of 67.5 V, the plate current curves exhibit a typical pentode knee below 50 V. Above 50 V, the plate current remains substantially flat across grid voltages ranging from $E_g = 0$ V to $-7$ V.
• Screen Voltage Characteristics: At a plate voltage ($E_p$) of 180 V and grid bias ($E_g$) of $-3$ V, the transconductance ($S_m$), plate current ($I_p$), and screen current ($I_{sg}$) all rise monotonically as the screen voltage ($E_{c2}$) is increased from 25 V to 70 V.
• Cut-off Characteristics: At a plate voltage ($E_p$) of 180 V and screen voltage ($E_{sg}$) of 67.5 V, the transconductance ($S_m$), plate current ($I_p$), and screen current ($I_{sg}$) exhibit a smooth, remote cut-off characteristic as the negative control grid voltage ($E_g$) is increased toward $-15$ V, where transconductance drops to approximately 15 µmhos.

Designations and Equivalents

The "P" suffix in the 1A4P designation indicates pentode construction, distinguishing it from the "T" suffix which denotes tetrode construction (as seen in the 1A4T).

• Direct Equivalents: The "G" type equivalents for the 1A4P are the 1D5GP and 1D5GT.
• Interchangeability: The 1A4P is generally interchangeable with the 1A4T.
• Similar Types: The tube is highly similar to the type 34 tube, but is distinguished by its variable-mu (remote-cutoff) characteristic.