Altec 421-8H Series II

At 80 ounces of magnet, this 15-inch driver has deep, compliant bass and a cast aluminum frame built for serious enclosures.

Overview

The Altec 421-8H Series II is a 15-inch loudspeaker driver built for high-output bass in professional and high-fidelity use. Altec marketed it as a "high compliance" woofer, tuned for large vented cabinets where low-frequency extension and air-moving capability count most. It has a long-excursion paper cone on a cloth surround, a 3-inch voice coil and a massive 80-ounce magnet structure. Those features point to power handling and dynamic headroom. Precise production dates remain undocumented, but the technical profile fits late-era Altec driver development, likely aimed at both studio and instrument amplifier markets.

Owners report using the 421-8H Series II as a replacement in Ampeg B-15 N(C) amplifiers, where it was noted as "a good match for the Ampeg’s Double Baffle Reflex Cabinet." That suggests it works in complex bass-reflex designs that demand careful driver alignment and compliance. Anecdotal commentary often brings up the 421A in folded horn systems, described by one user as part of "some of the best (hifi) speakers I ever heard." The 421-8H Series II appears to be a distinct variant, optimized for different loading and power regimes. It is not a mere rebrand. Its electrical and mechanical parameters indicate a deliberate evolution for higher continuous power and broader application.

There are no formal reviews or manufacturer documentation in the public record, so field reports are what exist. Units are described as "SOUNDS GREAT" on Worthpoint and "they get loud and sound very good" on Reverb. These are not lightweight drivers. Sellers call them "heavy beasts, with big, big magnets," which fits the substantial cast aluminum basket and dense magnetic assembly. The combination of high efficiency (97 dB at 1 watt/1 meter) and high compliance (Cms: 0.25 mm/N) makes it responsive to amplifier control. That matters most in vented alignments, where low Fs (30 Hz) and low Qts (0.22) are critical for tight, extended bass.

Specifications

Manufacturer	Altec
Product type	15-inch loudspeaker / driver / bass speaker / musical instrument speaker
Size	15 inches
Nominal Impedance	8 ohms
Power Handling	75 watts program
Power rating	150 watts RMS / continuous power rating of 150 watts
Voice Coil Diameter	3 inches
Magnet Weight	80 oz
Cone Material	Paper
Suspension	Cloth surround
Basket	Cast aluminum
Frequency Response	30 Hz to 2 kHz
Frequency response	35 Hz to 8000 Hz
Resonance Frequency (Fs)	30 Hz
Total Q (Qts)	0.22
Equivalent Volume (Vas)	11.5 cubic feet
Efficiency	97 dB (1 watt/1 meter)
Cone Weight	85 grams
Moving Mass (Mms)	110 grams
Compliance (Cms)	0.25 mm/N
Force Factor (BL)	18.5
DC Resistance (Re)	5.5 ohms

Key Features

High Compliance Design for Extended Bass

The compliance (Cms) is 0.25 mm/N and the resonance frequency (Fs) is 30 Hz. The 421-8H Series II is explicitly engineered for deep bass in large enclosures. Its low Qts of 0.22 indicates a well-damped system, suited to vented alignments where control and extension are balanced. The high compliance allows significant cone movement with minimal input, which gives high efficiency and responsiveness in reflex-loaded cabinets.

Cast Aluminum Basket with 80 oz Magnet

The structural core is a rigid cast aluminum basket, chosen for its strength and resistance to deformation under high magnetic and mechanical stress. Paired with an 80-ounce magnet, it delivers a strong force factor (BL: 18.5) and tight control over the 3-inch voice coil. Users consistently note the heft, calling them "heavy beasts." That comes straight from the big magnetic assembly.

15-inch Paper Cone with Cloth Surround

The cone is paper, a material valued for its favorable stiffness-to-mass ratio and natural damping. At 85 grams, it contributes to a total moving mass (Mms) of 110 grams, a value optimized for low-frequency output without sacrificing transient response. The cloth surround gives long-term durability and consistent excursion behavior, essential for both musical instrument and high-fidelity use.

Rated for 150 Watts Continuous Power

The driver is listed with a 75-watt program rating, and it is also specified at 150 watts RMS or continuous. That suggests strong thermal handling, likely from the large 3-inch voice coil and heat dissipation through the cast basket. Such ratings imply suitability for sustained high-level operation in demanding environments like bass amplifiers or studio monitors.

Designed for Large Vented Enclosures

The 421-8H Series II is not intended for sealed boxes or small cabinets. Its Vas of 11.5 cubic feet demands a large enclosure volume, particularly in vented configurations. That lines up with user reports of successful use in the Ampeg B-15 N(C)’s Double Baffle Reflex Cabinet, a design known for its complex tuning and deep bass response.

Collectibility & Value

Current market data is limited to individual listings rather than formal valuation trends. Units appear on Reverb and Worthpoint with subjective condition notes: "good working condition" with no rips, tears, or voice coil rubbing, and others described as having "very minimal hours and wear, excellent condition." No original pricing or widespread failure patterns are documented, and repair guidance is sparse beyond general suggestions like "re-cone the speaker" for vintage Altecs. As with many professional drivers of this era, value appears tied to condition and completeness rather than collectible premium.