Heathkit AA-40 (1960–1961)

Forty watts per channel of tube power, built by hand and still praised by owners who crank it.

Overview

The Heathkit AA-40 was built during the short window of 1960-1961. It was Heathkit's 40-watt-per-channel stereo power amplifier, aimed at the serious audiophile and the hands-on builder. Many kits of the time leaned toward modest performance and easy assembly. The AA-40 did not. It was complex, powerful and demanding. It was not for someone dipping a toe into DIY audio. The builder had to deal with high-voltage circuits, bias adjustments and a parts count pushing 470, and the reward was an amplifier that could drive speakers with authority and grace.

Reviewers at the time rated it highly. One said: “It proved to be rock-stable under a variety of load conditions, with a clean, solid sound that places it in a select but widening group of outstanding power amplifiers.” That came from a measured review in its era, and owners today still agree. “Sounds awesome,” one reports. “Good sound tone, beautiful sound color.” Another puts it more bluntly: “If you own an AA-40, in my opinion you own gold.”

This is not a plug-and-play relic. It's a 60-year-old tube amp with selenium rectifiers, bias adjustments that require a voltmeter, and a chassis prone to sagging under the weight of its own transformers. It needs maintenance and a willingness to get your hands dirty.

Specifications

Manufacturer	Heathkit
Model	AA-40
Year Introduced	1960
Year Discontinued	1961
Type	Stereo power amplifier
Power Output	40 watts per channel
Power Output (Measured)	48 watts per channel at 1% harmonic distortion at mid-frequencies
Power Output (Mono)	80 watts monophonically (paralleled operation)
Power Bandwidth	Below 20 cps to over 20 kc at half power (24 watts) with 1% distortion
Intermodulation Distortion	A few tenths of a percent at listening levels under 10 watts, reaches 1% at 40 watts
Frequency Response	Within a few tenths of a dB from 20 cps to 10 kc, -2.5 dB at 20 kc
Hum Level	75 to 80 dB below 10 watts
Output Impedances (Stereo)	4, 8, 16, and 32 ohms
Output Impedances (Mono)	2, 4, 8, and 16 ohms
Tube Complement	4 x EL34/6CA7, 2 x 6AN8
Construction Time	About twelve hours
Parts Count	Approximately 470 parts
Dimensions	11 in. deep by 15 in. wide by 7¼ in. high
Fuse Rating	4 amp slo-blo

Key Features

Push-Pull Power with Precision Tubes

The circuit is classic push-pull. Each channel uses a pair of EL34 or 6CA7 output tubes, driven by a 6AN8, which combines a pentode and triode in one envelope. This gives high power and tight control over the output stage. The amplifier stays stable even with difficult speaker loads and refuses to “break up or sound mushy at the highest levels our ears could stand,” as one contemporary review put it.

Robust Power Supply with Surge Protection

The power supply uses four silicon diodes in a voltage-doubling rectifier circuit. That was unusual for the time. A selenium rectifier handles bias voltage for the output tubes. It's a known failure point now, but it was standard practice in 1960. There is also a “surgistor,” a current-limiting resistor in the primary power lead that acts like a time-delay relay and reduces surge voltage during warm-up. It protects both the tubes and the power transformer.

Hands-On Control and Flexibility

Each channel has its own level control for fine-tuning balance. A slide switch marked STEREO-MON connects both channel inputs for mono operation, useful for older recordings or critical listening. Channel A includes a phase-reversing switch, handy for checking speaker polarity. For multi-channel setups, there's a center channel output that sums the two signals, a rare feature in a stereo amp of this era.

The bias voltage and balancing adjustments require an external voltmeter. This isn't a set-it-and-forget-it amp. The bias needs to be measured and tweaked regularly, especially after tube changes.

Collectibility & Value

The Heathkit AA-40 is not a common sight, and a properly restored unit can command serious money. One eBay listing sold for US $1,049.00, described as used but restored. Even a bent-chassis AA-121, a model identical to the AA-40 except for cosmetics, sold for over $400 in a 2004 auction, suggesting strong and lasting demand.

A proper restoration takes more than new tubes. It requires replacing all electrolytic, paper, film, and oil capacitors, a full recap of all PCBs. One seller offers a restoration kit for US $145.00, but even that won't cover everything. The chassis is also a weak spot: the top can bend from the weight of the transformers.

Crosstalk is another potential issue, possibly caused by dirty switches or pots, though it's not universally reported. Some units show “very light hum” at startup, while others report “no hum” at all.

If you find one, expect to invest in its upkeep. But for those who do, the payoff is an amplifier that doesn’t just play music—it commands it.