1853 — Remote-Cutoff RF Pentode

The 1853 (also designated under RMA designation 6AB7) is a heater-cathode type metal pentode designed primarily for high-frequency amplification. It is particularly suited for use in the radio-frequency (r-f) and intermediate-frequency (i-f) stages of picture amplifiers in experimental television receivers, especially those employing automatic gain control (AGC). Additionally, the 1853 can function as a mixer or as a low-voltage oscillator.

Construction, Physical Dimensions, and Mounting

The 1853 features a metal shell envelope with a Small Wafer Octal 8-pin base (utilizing the 8N base connection). Physically, the tube has a maximum overall length of 2-5/8 inches and a maximum diameter of 1-5/16 inches.

A key construction feature of the 1853 is its special shielded lead design. This construction allows the control-grid lead to be brought out directly to a base pin (Pin 4) rather than to a top cap. This design choice provides several technical advantages:

• It maintains a very low grid-to-plate capacitance.
• It simplifies circuit wiring.
• It reduces the size of the input inductance loop by placing the control-grid pin in close proximity to the cathode pin, which significantly improves high-frequency stability.

The tube is preferably mounted in a vertical position, either base up or base down. Horizontal mounting is also permissible, provided that pins 2 and 7 are aligned in a vertical plane.

Heater Ratings and Pinout

The 1853 is equipped with an indirectly heated cathode. The heater is designed for either alternating current (A.C.) or direct current (D.C.) operation. The cathode-to-heater potential should always be kept as low as possible.

The bottom view of the socket connections (8N base) is configured as follows:

Electrical Characteristics and Maximum Ratings

The direct interelectrode capacitances of the 1853, measured with the metal shell connected to the cathode and without an external shield, are:

• Input Capacitance: 8.0 pF (8 μμf)
• Output Capacitance: 5.0 pF (5 μμf)
• Grid-to-Plate Capacitance ($G_1$ to P): 0.015 pF (0.015 μμf) Maximum

The absolute maximum ratings established for the device must not be exceeded:

Typical Operation and Circuit Guidelines

The 1853 can be operated under different screen supply configurations depending on the desired cutoff characteristics. Two typical Class A amplifier operating conditions are defined:

Circuit-Specific Guidelines

• Control-Grid Bias: Grid bias can be obtained from a fixed source or via a cathode-bias (self-bias) resistor. When using self-bias, a minimum resistor value of 190 Ohms is required to establish the minimum -3 V bias.
• Grid Circuit Resistance: Under fixed bias conditions, the grid circuit resistance must not exceed 0.25 Megohm. If full cathode bias is used in conjunction with a series screen resistor, the grid circuit resistance may be increased up to a maximum of 0.5 Megohm.
• Input Stabilization: Because of the tube's high transconductance, changes in plate current cause corresponding variations in input capacitance and input conductance. To minimize these variations and stabilize operation, a portion of the cathode-bias resistor can be left unby-passed. While this sacrifices a small amount of transconductance and slightly increases the effective grid-plate capacitance, it improves overall stability. Alternatively, the associated circuits can be heavily loaded with resistance and capacitance.
• Screen Voltage Supply: The screen voltage can be supplied via a potentiometer, a bleeder, or a series resistor. The series resistor method (Condition II) provides a more extended cutoff characteristic because the screen voltage rises as the grid bias increases and the plate and screen currents drop.
• Suppressor Connection: In r-f and i-f stages, the suppressor grid ($G_3$) should be connected directly to ground to minimize feedback.

Typical R-F or I-F Amplifier Circuit Values

For a typical r-f or i-f amplifier stage, the following component values are recommended:

• Cathode Resistor ($R_k$): 190 Ohms total, with a tap located approximately 70 Ohms from the cathode end to leave a portion unby-passed.
• Grid Resistor ($R_1$): 0.25 Megohm (connected to the AGC voltage source).
• Screen Series Resistor ($R_2$): 30,000 Ohms.
• Plate Filter Resistor ($R_3$): 1000 Ohms.
• Bypass Capacitor ($C$): 0.05 μf.

Qualitative Curve Observations

An analysis of the characteristic curves of the 1853 reveals distinct operating behaviors:

• Plate Characteristics: At a screen voltage ($E_{c2}$) of 200 V, the plate current curves exhibit a sharp knee below 40 to 80 plate volts. Above this range, the curves become highly flat, demonstrating typical pentode behavior. The screen current curves remain at a characteristically low magnitude.
• Transconductance vs. Grid Bias: The choice of screen supply significantly alters the transconductance cutoff profile. A fixed screen supply of 200 V yields a relatively sharp cutoff, with transconductance dropping to 50 μmhos at a grid bias of -15 V. Conversely, utilizing a 30,000 Ohm series resistor from a 300 V supply extends the cutoff region significantly, requiring a grid bias of approximately -22.5 V to reduce transconductance to the same 50 μmhos level.

Technical Summary

The 1853 (RMA 6AB7) is a high-transconductance, remote-cutoff metal pentode featuring an octal base and a 6.3 V, 0.45 A heater. Its unique shielded lead construction eliminates the need for a top cap, bringing the control grid to a base pin close to the cathode to optimize high-frequency stability and minimize grid-plate capacitance (0.015 pF maximum). Under typical Class A operating conditions, the tube delivers a transconductance of 5000 μmhos and a plate resistance of approximately 0.7 Megohm. By utilizing either a fixed screen supply or a series screen resistor, designers can tailor the cutoff characteristic from a sharper -15 V cutoff to an extended -22.5 V cutoff, making the 1853 highly versatile for AGC-controlled r-f and i-f television picture amplifier stages, mixers, and low-voltage oscillators.