7588 — High-Mu Metal-Ceramic Planar RF Amplifier Triode

An industrial broadband radio-frequency triode designed for amplification up to 500 MHz, with high transconductance, low-noise operation, and controlled mechanical and electrical reliability.

Overview

The 7588 is a high-mu, ceramic-and-metal planar triode manufactured by General Electric for broadband radio-frequency amplification at frequencies up to 500 megacycles (500 MHz). The manufacturer documents a compact high-frequency electrode structure, very high transconductance, low-noise performance, and extensive electrical and mechanical qualification testing.

Unlike a conventional general-purpose receiving triode, the 7588 is designed for demanding RF amplifier service. Its principal characteristic point gives an amplification factor of approximately 175 and transconductance of 45,000 micromhos (45 mA/V). These values identify a high-gain, high-slope device intended for broadband amplification rather than audio power output.

Construction, Heater, and Mounting

The tube has a ceramic-and-metal planar construction, a coated unipotential cathode, and separately accessible heater, cathode, grid, and plate connections. General Electric permits mounting in any position and supplies a detailed dimensional and mounting drawing.

The mechanical sheet illustrates a T-bolt mounting arrangement. General Electric specifies that a stainless-steel T-bolt may be inserted into the tube's mounting-base slot, rotated 90 degrees, and fastened to a chassis or circuit board using a 4-40 nut and lock washer. The tightening torque must not exceed 3 inch-pounds.

This mounting method is part of the manufacturer's documented high-frequency mechanical construction and is relevant to maintaining reliable contact and alignment.

Typical Electrical Characteristics

General Electric provides the following average characteristic point:

The positive grid-supply specification is part of the manufacturer's particular characteristic test arrangement. It does not override the published zero maximum positive DC grid-to-cathode voltage rating for general operation.

The electrical relationship μ ≈ gₘ × rₚ is consistent with the nominal figures: 0.045 S × 3,900 Ω ≈ 175.5.

Production Characteristic Limits

General Electric gives initial characteristic acceptance ranges:

The noise figure is measured in a grounded-grid amplifier at 200 MHz, corrected for second-stage noise and diode temperature. It should not be treated as a universal noise figure at every frequency or source impedance.

The manufacturer also specifies a maximum 20% transconductance change when heater voltage is reduced from 6.3 V to 5.7 V under the defined test conditions.

Absolute-Maximum Ratings

General Electric explicitly describes these as absolute-maximum values that must not be exceeded under worst probable operating conditions. The equipment designer must allow for variations in supply voltage, component values, load, signal, environment, and tube characteristics.

Interelectrode Capacitances

The production tolerance ranges include 2.1–3.5 pF grid-to-plate, 5.1–8.3 pF input, 0.05–0.10 pF output, and 1.9–3.3 pF heater-to-cathode capacitance.

These capacitances are significant in a broadband RF amplifier because they influence input and output loading and the amount of signal coupling between electrodes.

Grid Recovery and Pulse Behavior

General Electric includes a specialized grid-recovery test. The sheet explains that a grid driven positive by a pulse of signal or noise can exhibit recovery behavior resembling an unwanted RC network in series with the grid, causing temporary changes in plate current.

The specified test applies pulses that drive the grid approximately 3 V positive relative to the cathode, at 60 pulses per second and approximately 0.12% duty cycle. The manufacturer limits the change in average plate current to 1.0 mA and the peak plate-current backswing to 2.0 mA.

These are test criteria for identifying undesirable recovery behavior, not general permission to exceed the normal grid-voltage ratings.

Mechanical, Vibration, and Altitude Qualification

The 7588 is supported by extensive special-performance testing.

General Electric specifies low-frequency vibration testing at 40 Hz and 15 g peak acceleration, with an output-noise limit of 25 mV RMS. Its variable-frequency vibration test covers 100–2,000 Hz at 10 g, with a 75 mV RMS output limit under the stated circuit.

A low-pressure breakdown test simulates approximately 100,000 feet altitude, using 300 V RMS at 60 Hz between plate and grid at an air pressure of approximately 8 mm Hg. The tube must not exhibit flashover or corona under the specified test.

Shock qualification subjects statistical samples to approximately 450 g impacts in multiple mounting orientations, followed by electrical evaluation.

Life and Stability Testing

The General Electric documentation includes several qualification programs:

• Early-hour stability testing, evaluating transconductance change after 2 and 20 hours.
• Survival-rate testing at approximately 100 hours.
• Intermittent life testing over 1,000 hours, with periodic electrical evaluation.
• Interface-life testing over 1,000 hours with heater-only operation.
• Heater-cycling testing for at least 2,000 cycles.

These are manufacturer qualification procedures, not a single unconditional guaranteed operating-life rating. They demonstrate the emphasis on stable transconductance, insulation integrity, heater-cathode behavior, and resistance to early failure.

Characteristic Curves

The six-page General Electric sheet includes average plate-characteristic curves and plots of transconductance and amplification factor against plate current. These illustrate the dependence of the tube's high-gain characteristics on operating point and provide a basis for analyzing broadband RF amplifier stages.

Technical Summary

The 7588 is a 6.3 V / 400 mA high-mu ceramic-and-metal planar triode for broadband RF amplification up to 500 MHz. Its nominal characteristics include μ ≈ 175, 45 mA/V transconductance, 3.9 kΩ plate resistance, and approximately 24 mA plate current.

With 5.5 W maximum plate dissipation, a 3 dB typical noise figure at 200 MHz, compact planar construction, and extensive mechanical and electrical qualification tests, it is a specialized high-performance industrial RF amplifier triode rather than a conventional low-frequency receiving valve.