Altec 421-8H Musical Instrument Loudspeaker (Years Not Documented)
A 15-inch bass driver with a 17½-pound magnet structure and a 150-watt pink-noise rating. Altec built it for cabinets that lived on the road.
Overview
Altec's own 1976 catalog copy is specific about what the 421-8H is for: a low-frequency loudspeaker designed for electronically amplified musical instruments, or as the bass driver in two-way musical sound reinforcement. It is not a hi-fi woofer or a bookshelf component. It sits in an instrument cabinet and absorbs whatever a bass amp sends it. Altec's pitch leans on high sensitivity, power handling, and "designed-in dependability," which is the language of a working part.
The numbers back that up. Altec rated the 421-8H at 150 watts, with a frequency response of 35 Hz to 8000 Hz, a pressure sensitivity of 98 dB SPL, and a nominal impedance of 8 ohms. The frame measures 16 inches across and the complete driver weighs 20.7 lbs (9.4 kg). The same catalog lists the 411-8A, 414-8C, 416-8B, and 515B as bass drivers. The 421-8H is the only one rated at 150 watts, double the 75-watt figure carried by the 416-8B and 515B. It is the high-power option in Altec's low-frequency lineup.
The 98 dB sensitivity figure comes with conditions. Altec measured it at four feet on axis with one watt of pink noise, band-limited from 100 to 1000 Hz, inside a five-cubic-foot laboratory enclosure. That means the driver makes serious output from modest amplifier power, which counted for a lot when a bass head was lucky to deliver 100 clean watts.
The record doesn't say when any of it happened. There is no introduction date, no discontinuation date, and no original retail price in the available documentation. The 421-8H appears in a 1976 Altec catalog, and that's the only firm date anchor. Anything more specific is speculation.
Key Features
150 Watts, Measured with Pink Noise
Read the power rating's footnote carefully. The 150-watt figure was measured with pink noise from 20 to 1000 Hz in the recommended enclosure. That's a band-limited test rather than a single-frequency stunt, and it fits the driver's role as a bass and instrument speaker. A separate Altec spec sheet adds a caveat: the driver may be used with amplifiers having up to an equal continuous RMS power rating. So 150 watts is the ceiling, and a 300-watt head is a bad idea.
The 17½-Pound Magnet Structure
In the Series II spec sheet, the 421-8H's magnet structure is listed at 17½ lbs, against 11½ lbs for the 12-inch 417-8H, the 15-inch 418-8H, and the 10-inch 425-8H. That's the clearest sign of how Altec built this driver differently from its siblings: more magnet, more motor, more control over a cone that has to move a lot of air down low. The complete driver weighs 22 lbs in that sheet, heavier than the 418-8H's 20 lbs.
35 Hz to 8000 Hz — Wide for a Bass Driver
The 421-8H's response is listed as 35 Hz to 8000 Hz in the bass-driver table. That's an unusually wide band for a driver described as a bass unit, and it suggests the cone reaches up into the midrange rather than handing off early to a horn. Altec's own application note supports that reading: the driver is intended either as a bass unit in a two-way system or as a standalone instrument speaker, and the second use only makes sense if it covers enough spectrum to stand alone.
The Series II spec sheet tells a slightly different story. It lists the 421-8H II at 35-3500 Hz with "Bass" as its application, while the bass-driver table gives 35-8000 Hz. The two documents appear to describe different things. The Series II sheet is a family comparison where the 421 is explicitly the bass-only member, while the bass-driver table appears to quote the driver's own response. Both agree on the 35 Hz bottom end.
16-Inch Frame Diameter
The 421-8H is a 15-inch driver with a 16-inch frame diameter. That dimension decides whether a replacement drops into an existing baffle cutout or whether the saw comes out. It's a standard-size frame for the era, and these drivers turn up in cabinets built for other 15-inch speakers as often as in Altec's own enclosures.
Historical Context
The 421-8H's documented working life is more interesting than its documented history. In Recording-Engineer (December 1975, p.47), the magazine described a large theatrical sound installation, the "Sound Towers," assembled with rigid tubular steel and anchored in concrete. Its low-frequency section was a modified 816 reflex horn loaded with a 421-8H 15-inch loudspeaker. The same system used a modified 511 horn with a 291-16A driver for mids and an 811B horn with an 802-8D driver up top, crossed at 800 Hz and 6 kHz. That's a 421-8H doing bass duty in a purpose-built professional installation, not a garage rig.
The Series II designation appears in Altec's own spec sheet, but the documentation doesn't explain what changed between the 421-8H and the 421-8H II, or when the II suffix was applied. The Series II sheet lists the driver at 150 watts, 102 dB SPL sensitivity at one watt and three feet, 35-3500 Hz, 8 ohms, and 22 lbs. Those figures don't line up cleanly with the bass-driver table's 98 dB at four feet and 35-8000 Hz. It could be a measurement-condition difference, a genuine revision, or a documentation inconsistency. The sources don't resolve it.
A classified ad lists a pair of 421-8H bass speakers, heat-treated with magnetic fluid, at $275 for the pair in excellent condition. That's a snapshot of what the driver was worth on the secondary market while it was still a current product.
Collectibility & Value
The 421-8H is a working driver rather than a showpiece, so prices depend heavily on whether the cone is intact and the voice coil is clean. The classified evidence from the period is $95 for a single unit and $275 a pair for heat-treated, fluid-cooled units in excellent condition. That is the only pricing data the available documentation provides, and it dates from the driver's active years rather than from today's collector market.
What the record does establish is that the 421-8H was built to survive. The 150-watt pink-noise rating, the heavy magnet structure, and the driver's documented use in a professional theatrical install all point to a speaker designed for hard service. Whether a given example has actually survived that service is the only question that matters at the point of sale, and the documentation doesn't offer a checklist of known failure modes to work from.
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