4-250A

The 4-250A is a radial-beam transmitting tetrode intended for amplification, oscillation and modulation. Eimac also documents it as 5D22; RCA additionally has a 6156/4-250A sheet. The 250 W figure identifies maximum plate dissipation, not output power: RF examples can deliver more because only part of the input energy becomes plate heat. The thoriated-tungsten filament operates at 5 V. Eimac and RCA list 14.5 A for 4-250A/5D22; RCA's separate 1957 6156 sheet gives 14.1 A. Connections and basic figures must therefore be read with the exact document version.

Low control-grid-to-plate capacitance and modest drive requirements favor RF amplifier use. Eimac gives full ratings up to 110 MHz; RCA's 6156/4-250A sheet instead specifies full input up to 75 MHz and reduced input up to 120 MHz. These limits must not be mixed into an undifferentiated table. Eimac's May 1970 sheet lists class-C CW or FM maxima of 4000 V plate, 600 V screen, 350 mA plate current and dissipations of 250 W plate, 35 W screen and 10 W control grid. For plate-modulated service, carrier limits fall to 3200 V, 275 mA and 165 W plate dissipation.

An Eimac class-C point below 110 MHz uses 2500 V plate, 500 V screen, −150 V bias and 300 mA plate current: plate input 750 W, RF output 575 W and calculated drive about 1.7 W. Power actually delivered to the load also depends on tank and coupling losses. For linear service the same sheet gives one AB1 tube up to 30 MHz at 2500 V plate and 600 V screen, producing 312 W single-tone output with 60 mA idle current. This is a different regime from class C, which does not become linear merely by using the same tube.

Audio and modulator applications are documented in push-pull. An Eimac AB1 pair at 2500 V plate and 600 V screen, −110 V bias and 11,400 Ω plate-to-plate load provides 625 W at the plates; total plate current rises from 120 mA idle to 430 mA at maximum signal. Other AB2 points draw grid current during part of the cycle and consequently demand driver power. Eimac explicitly distinguishes average from peak drive and notes output-transformer losses: tabulated plate output is not automatically the power available at the load.

Cooling uses radiation and forced air through base holes, around the bulb and toward the plate seal. The metal base shell participates in shielding and must be grounded; the socket must clear the glass exhaust tip. Eimac specifies vertical mounting, base up or down, flexible plate connection and HR-6 heat-dissipating connector; its 1970 sheet lists SK-400-series Air-System sockets and SK-406 chimney. Ventilation may be necessary with only the filament energized. Shielding, bypassing and neutralization need particular attention at higher frequencies; Eimac explains that screen-lead inductance may become significant above 45 MHz.

Bottom-view connections put filament on 1 and 5, screen on 2 and 4, control grid on 3 and plate on the top cap. RCA distinguishes the metal-shell Giant five-pin base from electrical diagram 5BK; its 1957 6156 uses Special-Button construction, with internal shield also connected to pin 4. Mechanical differences are real: Eimac in 1970 gives maximum length 161.93 mm, maximum diameter 90.50 mm and net weight 226.8 g; RCA's 1957 6156 instead gives maximum length 5²⁹⁄₃₂ inches, maximum diameter 3⁷⁄₁₆ inches and weight about 6 ounces. Manufacturer and edition references preserve these differences without turning them into universal dimensions.

Documentation consulted: Eimac 4-250A/5D22, effective 15 July 1955, two scans; Eimac 5D22/4-250A, 5 May 1970; RCA 6156/4-250A, tentative data August 1957; RCA 4-250A/5D22, tentative data 15 November 1948, September 1948 curves and January 1963 update.