1852 — Television Amplifier Pentode

Overview

The 1852 (also designated as the 6AC7/1852 or RCA-1852) is a heater-cathode metal pentode designed for high-gain amplification in radio frequency (RF) and intermediate frequency (IF) stages. It is characterized by a high ratio of transconductance to plate current, low interelectrode capacitances, and close electrode spacing. The tube is particularly suited for the RF and IF stages of picture amplifiers in experimental television receivers, the first stages of multi-stage video amplifiers, mixers, and low-voltage oscillators.

Construction and Physical Specifications

The 1852 is constructed with a metal shell envelope (MT8 outline) and a small wafer 8-pin octal base. It features a coated unipotential cathode.

The electrode assembly of the 1852 is identical to that of the 1851, but the 1852 incorporates a special shielded lead construction. This design brings the control-grid lead directly to a base pin rather than an external top cap. This modification maintains a very low grid-to-plate capacitance while simplifying circuit wiring and reducing the input inductance loop.

The tube can be mounted in any position, though a vertical orientation (base up or down) is preferred. Horizontal mounting is permissible provided that pins 2 and 7 are aligned in a vertical plane.

Physical Dimensions

• Maximum overall height / length: 2-5/8 inches
• Maximum diameter: 1-5/16 inches

Pinout and Connections

The 1852 utilizes a standard 8-pin octal base. The bottom-view pin connections are assigned as follows:

Heater and Interelectrode Capacitances

The 1852 operates with a nominal heater voltage of 6.3 Volts (AC or DC) with an allowable voltage deviation of plus or minus 10 percent. The heater current is 450 mA (0.45 A).

When the metal shell is connected directly to the cathode, the direct interelectrode capacitances are:

• Grid to Plate (G1 to P): 0.015 uuf (maximum)
• Input (G1 to H + K + G2 + G3 + S): 11 uuf
• Output (P to H + K + G2 + G3 + S): 5 uuf

Absolute and Design-Maximum Ratings

The maximum ratings for the 1852, established under the RMA Standard MB-210, define the safe operating boundaries of the device:

Typical Operating Conditions and Characteristics

The 1852 can be operated as a Class A1 amplifier under two primary configurations depending on the screen supply method. Condition I utilizes a fixed screen supply to achieve a sharp cut-off characteristic, while Condition II uses a series screen resistor to provide an extended cut-off characteristic.

Installation and Operating Considerations

Suppressor Grid Connection

To minimize feedback in RF and IF stages, the suppressor grid (Grid #3) should be connected directly to ground.

Screen Voltage Supply

When operating under Condition I, the screen voltage should be obtained from a potentiometer or a low-resistance bleeder. Under Condition II, which is suitable for variable-gain applications, the screen voltage is supplied from the B-supply through a 60,000 Ohm series resistor. If screen supply voltages exceed 150 Volts, a series-dropping resistor must be used to limit the screen voltage to 150 Volts when the plate current is at its normal operating value of 10 mA.

Cathode Bias and Degeneration

The cathode-bias resistor must be adjusted to maintain a nominal plate current of 10 mA, with a minimum value of 160 Ohms.

Because of the tube's high transconductance, changes in plate current cause pronounced variations in input capacitance and input conductance. In RF or IF amplifiers, these variations can be minimized by leaving a portion of the cathode-bias resistor unby-passed. While this sacrifices some effective transconductance and slightly increases the effective grid-plate capacitance, it stabilizes the input characteristics. Alternatively, the associated circuits can be heavily loaded with resistance and capacitance.

In video amplifier stages, leaving the cathode-bias resistor unby-passed introduces degeneration, which is desirable for reducing distortion. If maximum stage gain is required without degeneration, the cathode resistor should be bypassed with a very large capacitor (350 uF).

Audio Applications

The 1852 can function as a high-gain audio amplifier, but this application is not recommended unless the heater is operated from a battery supply to minimize hum.

Circuit-Specific Examples and Components

In practical circuit designs, the following component values are utilized for optimal performance:

• Capacitors: C1 = 0.1 uF, C2 = 350 uF, C3 = 1.0 uF, C4 = 12 uF, C5 = 0.25 uF, C6 = 0.05 uF
• Resistors: Rk = 160 Ohms (with a tap located approximately 35 Ohms from the cathode end), R1 = 0.25 Megohm, R2 = 160 Ohms, R3 = 60000 Ohms, R4 = 5000 Ohms, R5 = 2000 Ohms, R6 = 1.0 Megohm, R7 = 1000 Ohms, R8 = 9000 Ohms, R9 = 250000 Ohms

Additionally, the 1852 can be configured as an impedance transformer to couple high-impedance circuits to low-impedance circuits, yielding an effective output impedance of 110 Ohms.

Qualitative Curve Observations

Plate Characteristics

At a screen grid voltage of 150 V, the plate current rises sharply when the plate voltage is increased from 0 to 50 V. Above 100 V, the plate current flattens out, demonstrating typical pentode saturation. With the control grid bias set to 0 V, the plate current reaches approximately 30 mA at a plate voltage of 100 V and increases only slightly to approximately 32 mA at a plate voltage of 300 V.

Control Grid Characteristics

The method of screen supply significantly alters the control grid transfer characteristics:

• Fixed Screen Supply (150 V): The tube exhibits a sharp cut-off characteristic, with plate current and transconductance dropping to zero near a grid bias of -4.5 V.
• Series Screen Resistor (300 V supply with 60,000 Ohm resistor): The tube exhibits an extended cut-off characteristic. The plate current and transconductance decrease much more gradually, approaching zero near a grid bias of -10 V. This extended cut-off is highly beneficial for automatic volume control (AVC) or variable-gain applications.

Technical Summary

The 1852 is a high-transconductance metal pentode featuring a 6.3 V, 450 mA heater, an 8-pin octal base, and a unipotential cathode. It is designed to deliver a transconductance of 9000 umhos with a plate current of 10 mA and a plate resistance of 0.75 to 1.0 Megohm. By bringing the control grid to a base pin instead of a top cap, the 1852 achieves low grid-plate capacitance (0.015 uuf max) and reduced input inductance, making it highly effective for high-gain RF, IF, and video amplification in television and high-frequency receiver designs.