Altec 458A
A vacuum-tube plug-in preamplifier built for the rack-mounted consoles used in broadcast and recording in the early 1960s.
Overview
The Altec 458A is a discrete, transformer-coupled plug-in amplifier from Altec's professional speech-input equipment line. It's the same modular system used in the 250 SU console, which Audio (October 1960, p.9) described as "the ultimate in control consoles for TV, AM, FM, recording studio or sound system use." The 458A was made to be interchangeable in mounting trays. It worked as a preamplifier, booster amplifier, or program amplifier, depending on the wiring of its tray and the external power supply feeding it. It is a vacuum-tube design with push-pull circuitry, running from a 6.3-volt DC heater supply at 0.7 amps and a B+ supply of 275 volts at 15 mA. The operating voltage comes from an external source, most commonly the Altec 5354 power supply. The 5354 delivered 275 VDC at up to 275 mA with ripple held to 0.02 V peak-to-peak, plus 6.3 VDC at 13 amps for heater chains. At 117 VAC, 50-60 Hz, it drew 245 watts at full load.
Altec took real care with impedance matching and termination on the 458A. Both 150-ohm and 600-ohm source and load impedances are available at the connector. No socket wiring changes or internal strapping are required, so any amplifier is correctly matched when inserted into a tray wired for its specific function. The internal output impedance equals the load impedance, so building-out resistors are unnecessary when feeding devices like equalizers that are sensitive to terminating impedance. The input transformer is left unterminated for microphone use. When termination is needed, a 150- or 600-ohm resistor connects to the receptacle on the mounting tray. Altec explicitly warns against terminating at the transformer secondary.
Design and Features
The 458A occupies a standard plug‑in chassis that fits the type 13401 Mounting Tray Assembly. Up to nine such trays fit into the 13225 rack‑mount assembly, which also accommodates the Altec 459A amplifier and the 13387 Utility Input Device. The amplifier includes push‑button metering switches wired to the connector for use with an external microammeter—Altec recommended the 5981 meter, calibrated to show tube space current. Pressing the button on any amplifier in a chain gives a reading; the needle sits at roughly 67 % of full scale on a 0‑to‑200 µA meter with about 1000 ohms internal resistance. This made it practical to monitor cathode current across a whole console without pulling modules, a genuine time‑saver in broadcast maintenance.
The B+ supply includes a 20 µF section of capacitor C₃ with resistor R₃ forming a hash filter, which keeps rectifier noise out of the audio path. The neutralizing capacitors C₁ and C₂ are set at the factory and are not to be touched. They control high-frequency response, and changing them shifts the amplifier's stability margins. Cathode bias resistors R₁ and R₂ are precision wire-wound types, selected to survive overload current without changing value if a tube fails. Altec's manual notes that tube shields fit tightly and must be removed carefully to avoid bending pins. Loosening the shield fit is discouraged because it impairs bulb-temperature reduction.
System Integration
The 458A was not sold as a standalone component. It belonged to a modular ecosystem built around the 5354 power supply and the 13225 rack frame. The power supply itself is a substantial unit—16 pounds, finished in dark green, measuring 7 ½ × 9 ½ × 7 inches—with a circuit breaker (push to reset), a power switch, and a four‑position tap switch that provides voltage adjustment via autoformer action to accommodate a 2‑to‑1 range of loads. It uses silicon rectifiers, a mark of the transition era between selenium and silicon in professional gear. The 458A's push‑pull circuitry means multiple units fed from a common power supply can be cascaded without coupling through the supply, a practical advantage in multi‑channel consoles.
Maintenance and Servicing
Altec's service approach for the 458A was simple: replace tubes when cathode current drops significantly. A severe imbalance between the push-pull pair will cause a loss of 20 Hz response, which is the tell-tale sign. The precision cathode resistors and the fixed neutralizing capacitors leave little else to drift. The hash filter capacitor and the tube shields are the only routine wear items. The unit runs from a regulated DC heater supply, so hum injection is minimal if the heater wiring in the tray is kept twisted and routed cleanly. AC heaters are possible but require careful rewiring of both the amplifier leads and the receptacle wiring.
The 458A has no built-in power supply and no on/off switch. It depends entirely on the 5354 or equivalent. Combined with the proprietary mounting tray system, that means surviving examples are almost always found still installed in original Altec console racks, or pulled from broadcast stations that upgraded to solid-state in the 1970s. Standalone units are uncommon because the amplifier was never sold by itself to home users.
Collectibility and Market Notes
The 458A was a professional OEM module, not a consumer product, so it does not appear in vintage hi-fi price guides. What market exists is among broadcast-equipment restorers and Altec console collectors who need spares for 250 SU or similar desks. A single 458A in good condition with its tube complement intact typically sells in the $75-$150 range. A full rack of nine with the 5354 power supply can fetch $800-$1,200, depending on condition and documentation. The manual does not specify tube types in the excerpts available, but the push-pull topology and current draw suggest a 12AX7 or 12AU7 family for the gain stage and possibly a 12AT7 or similar for the output. These are standard octal or noval types, none particularly rare. The harder part is finding the 13401 mounting trays and the 13225 rack frame, which are scarcer than the amplifiers themselves. The 5354 power supply, with its autotransformer tap switch and silicon rectifiers, is sturdy, but the electrolytic capacitors in the hash filter and B+ rail are sixty years old and should be reformed or replaced before extended use.
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