Altec 128B (1969)

A rack-mount monobloc that Altec built for broadcast racks, not hi-fi shelves. The collector market has never quite decided what to do with it.

Overview

The 128B turns up in old studio inventory lists and almost nowhere else. It's a monobloc vacuum tube power amplifier, sold as a single rack-mount chassis. The eBay titles call it a "rack mount Thermeguard Amplifier Monobloc," and that's about as much as most listings ever say. The Thermeguard name (spelled "Thermegaurd" in some listings, because nobody at Altec seems to have been consistent about it) was Altec's branding for the protection circuitry built into the output transformer windings. The company's own literature for the 128B says it reduces line current proportionally when a tube shorts or a component fails, until the fault is corrected. In a broadcast rack, a blown output transformer means a dead channel and a service call, so this was a useful feature.

Internally, the 128B is a push-pull EL34 amplifier. One seller lists it as running "a pair of el34 6ca7 in output," and a Reverb listing for a restored unit notes new "JJ" EL34 power tubes and a rectifier tube installed. Power output is reported at 40-45 watts per monobloc, but that figure comes from a single Audiogon listing and hasn't been independently confirmed, so treat it as approximate. One seller describes the input and output stages as "a high-quality input stage and a powerful output stage," which is what you write when you don't have a schematic in front of you. A pair of 15095 line transformers is sometimes included with the amps. One listing calls them "15095a peerless" transformers, another just "ALTEC 15095." The suffix difference may reflect different production variants or just sloppy listing copy.

That's the whole of it. There are no published frequency response figures, no THD numbers, no dimensions, no weight, no confirmed input/output count. For an amplifier built for professional installation, the paper trail is thin.

Key Features

Push-Pull EL34 Output Stage

The 128B runs a pair of EL34s (or 6CA7s, the American equivalent) in push-pull. This is the classic British-style output topology that Altec adopted for its later tube amps. EL34s in push-pull give a warm, midrange-forward character that suits the broadcast and recording work the 128B was built for. The tube complement isn't documented beyond the output pair and a rectifier, so anyone buying a project unit should expect to trace the small-signal tubes themselves.

Thermeguard Transformer Protection

Altec's Thermeguard circuitry is built into the output transformer windings. It's designed to sense a fault, such as a shorted tube or a failed component, and automatically reduce the line current the amplifier draws until the problem is fixed. It's not a fuse. It's a proportional current-limiting scheme, and it's the single most distinctive thing about the 128B's design. In a studio rack running continuously, that's the difference between a blown transformer and a serviceable amp.

Rack-Mount Monobloc Chassis

Each 128B is a single-channel amplifier in a rack-mount chassis, one amp per channel, so stereo means two units. The eBay listings consistently describe it as a "Monobloc," and the rack-mount format points squarely at permanent installation rather than the living room. One Reverb seller reports a unit that served as the main amp in a Montreal auditorium from 1969 onward.

15095 Line Transformer Compatibility

The 128B is frequently paired with Altec's 15095 line transformer, and some listings include a pair of them with the amps. The transformer lets the amplifier interface with 150- and 600-ohm lines, which is what you'd need in a broadcast or recording facility. Buyers should confirm whether a given unit includes the transformers. They're not always present, and a pair of 15095s is worth having on its own.

Historical Context

The 128B dates to 1969, according to a Reverb listing that identifies the year and describes the unit's service history in a Montreal auditorium. Forum accounts describe it as "among Altec's last tube amplifiers" and as "a deluxe version of the 1568 amplifier." The 1568 was Altec's general-purpose PA amp, while the 128B was aimed at recording and television production studios. One Gearspace poster puts it more bluntly: "The 128b is a higher quality version of the 1568a." A hostboard account describes a theatre installation built around "a pair of A7s and a pair of 128b amps fed by a 1567," which shows how these were actually deployed, as the amplification backbone of an Altec theatre system rather than as standalone hi-fi.

That's the extent of the documented history. No production run figures, no end date, no original MSRP. The 128B exists in the record mostly as a model number on a rack panel and a handful of forum threads.

Collectibility & Value

The 128B is rare and under-documented, which makes the market thin and the pricing erratic. Japanese collector price guides list the 128B at roughly ¥200,000 to ¥250,000 per pair. Domestic asking prices are all over the map. One eBay listing for a single "Altec Lansing 128B rack mount Thermeguard Amplifier Monobloc" asks $2,000, while another listing bundles a 128B with an Altec 438C compressor amplifier for $4,200. That price is for the pair, not the 128B alone, so it can't be read as a 128B valuation.

Condition is the usual story. An AudioHeritage owner reports acquiring "a pair of Altec 128B amplifiers. Both are in decent condition, but are going to need some work." Restoration kits for filter capacitor rebuilds exist and are listed at around $95, which points to the electrolytics as the known weak point. One Audiogon listing notes consecutive serial numbers (391221-201 and 202), the sort of detail collectors look at when pairing monoblocs.

The verdict on sound comes from a Gearspace poster who calls it "a good amplifier but far from stellar. It is no sonic rival to the similar power Marantz 8b or McIntosh MC-40/MC-240." That puts the 128B as a competent, well-built professional amplifier that doesn't command the audiophile premiums of its better-known contemporaries. Buyers should expect to pay for rarity and provenance. Anyone shopping should verify the tube complement, confirm whether the 15095 transformers are included, and budget for a recap.