Eico ST-70 (1957–mid-1960s)
Eico's 80-watt stereo control center, sold as a kit for $99.95 or wired for $149.95, was one of the first ways an ordinary hobbyist could put real stereo power on the shelf.
Overview
The Eico ST-70 is a complete high-fidelity stereophonic control center and a pair of 35-watt amplifiers on one chassis. That's the manufacturer's own description, and it covers most of what the thing does. You feed it any stereo or mono source, select it, preamplify it, shape it with tone controls and filters, and push it through the self-contained dual amplifiers to a stereo pair of speakers. Eico built it in Long Island City, New York, and sold it both as a factory-wired unit and as a kit. Kits were the company's whole business: reliable, well-documented electronics that a motivated hobbyist could assemble at the kitchen table.
The source material says 1957, though several collector databases place it around 1962, and production clearly ran into the mid-1960s. Either way, the ST-70 landed when stereo was just emerging from mono and home audio was moving out of furniture-style consoles and onto component shelves. Eico sold over 25,000 of them, which is a huge number for kit-built hi-fi and explains why they still surface regularly. It became one of the company's most enduring products, and for a lot of people it was the first serious amplifier they ever owned, because they built it themselves.
The headline number is 35 watts per channel into 8Ω, confirmed across multiple sources. Some Eico advertising billed the ST70 as an "80-watt" stereo amplifier (the sum of both channels), and at least one period print ad rated each channel at 40 watts. Conservative ratings were common in this era, and Eico themselves were inconsistent about which figure they printed. Output impedance taps cover 8Ω with compatibility for 4Ω and 16Ω. Distortion is rated under 1% at full power. Most sources quote frequency response as 18Hz to 43kHz. One spec sheet gives 20Hz - 20kHz ±1dB, and a period measurement describes response flat within +0/−1 dB from 5 cps to 47 kc, down only 3 dB at 100 kc. Treat the low-frequency extension as approximate. That it goes well past 20kHz is settled.
Damping factor is listed at 6, which is low by modern standards and part of why the ST-70's bass has a characteristic looseness with some speakers. Input sensitivity is 4mV for moving-magnet phono and 480mV for line. The unit draws 400 watts from a 117V AC, 60Hz supply. It measures 19" W × 10" H × 10.5" D and weighs 40 lbs. You notice that when you slide it onto a shelf, and the output transformers inside are clearly doing real work.
Key Features
Kit or Factory-Wired — Your Choice
Eico sold the ST-70 two ways: as a kit for $99.95 or factory-wired for $149.95. That $50 spread was the whole Eico proposition. The kit shipped with all components, sockets, chassis, transformers, and hardware, plus a 100-plus-page assembly manual that walked builders through the job step by step. The manual is good. It explains resistor color codes, capacitor types, and soldering technique before it ever asks you to pick up an iron, and the construction steps are keyed to numbered lugs on switches and terminal strips so you can verify every connection. Owners report that a careful first-timer could get one running over a series of evenings. It's one reason the ST-70 is remembered as one of the first high-fidelity stereo tube amplifiers available as a build-it-yourself kit.
Four 7591 Output Tubes in Push-Pull
The output stage uses four 7591 power tubes, two per channel, in a fixed-bias push-pull configuration. The 7591 is a beam power tetrode similar in performance to the 6L6 but with higher efficiency, which is how Eico got 35 watts per channel out of a relatively compact chassis. Some sources describe the output stage as using 6L6GC tubes instead, and at least one claims a 12-tube circuit overall. The 7591 complement is what the factory documentation specifies, but buyers should verify what's actually installed in any unit they're considering, since tube substitutions were common.
Dual-Mono Construction
Each channel gets its own power supply and signal path, with separate filament windings and hum-balance controls, so each channel can be optimized independently rather than compromised to a shared setting. That keeps crosstalk down and keeps one channel's hum problem from becoming both channels' hum problem. The preamplifier section uses a dual-triode stage with equalization in a feedback loop around both stages for minimum distortion. The tone controls are a variable-inflection-point feedback type that doesn't interact with each other and doesn't shift the midrange when you boost or cut the extremes.
Adjustable Bias with Front-Panel Metering
Each output tube has its own bias test point and adjustment pot, and a front-panel meter lets you monitor bias current and troubleshoot tube performance without hauling out a bench multimeter. This separates the ST-70 from a lot of its contemporaries. You can match and re-bias your output tubes yourself, which is handy on a 60-year-old amplifier where tube drift is a fact of life. The manual specifies the procedure: set both bias and balance pots to mid-rotation before first power-up, then adjust each channel in turn using a VOM or VTVM with at least 3% DC accuracy and a lowest range no greater than 3 volts full scale. Eico says to redo the adjustment any time you replace an output tube or suspect DC unbalance.
Input Flexibility and the Center-Channel Provision
The ST-70 includes two magnetic phono inputs, tape-head equalization, multiple tuner and tape inputs, tone controls, and filters. The two phono pairs present different loading, one at 47kΩ and one at 100kΩ, so you can run a turntable and a record changer without a separate switcher, and both get RIAA equalization. Tape-head inputs offer selectable NARTB equalization for 7½ ips or standard equalization for 3¾ ips. There are also high-level input pairs for a multiplex adaptor, preamplified tape, FM tuner, and AM tuner. Provision is made for operating a center speaker directly from the ST-70, a useful feature in the transitional stereo era when plenty of households still had a mono speaker worth keeping in the middle. Unused inputs are grounded by the selector switch to kill crosstalk. The tape inputs are the exception, since some tape machines misbehave if their playback outputs get grounded during recording.
Filters, Loudness, and Phase Switching
Switched high and low frequency filters let you knock down rumble or scratch without touching the tone controls. There's a switched loudness compensation circuit, a tape monitor switch, and a speaker phase reversal switch. The manual is honest about the loudness control: it provides bass boost at low listening levels, and Eico explicitly warns against leaving it on at normal volume because the boost becomes excessive and unmusical. They even note that some listeners skip loudness compensation entirely on the grounds that it doesn't correspond to any natural listening condition.
Historical Context
Eico had built its reputation on reliable, well-documented electronic kits for radio, test equipment, and audio gear, and the ST-70 was the company's answer to the growing demand for stereo amplification. It arrived as home audio was transitioning away from furniture-style consoles toward component-based setups, and it quickly became one of Eico's most enduring products. The ST-70 was one of the first high-fidelity stereo tube amplifiers available as a build-it-yourself kit.
Within Eico's own lineup, the ST-40 integrated amp and the ST-84 preamp both use the same phono preamp design as the ST-70, so there's real circuit commonality across the range even though the line stages and power amps differ. The ST-70 was also commonly paired with Eico's ST96 AM/FM tuner as a matching combo, and complete untouched pairs still turn up for sale. Original pricing was $99.95 as a kit or $149.95 factory-wired. One source lists $524.99, which does not square with the period advertising and should be disregarded.
Contemporary coverage treated the ST-70 as a serious value. HiFi-Stereo (July 1961, p.40) covered the amplifier in a laboratory report roundup that also included the Lafayette LA-250A, PACO SA-40W, Heath AAW-100, Harman-Kardon A500, and Knight KN-775, noting the 35-watts-per-channel rating and the $149.95 wired price including a metal cabinet, with the kit version designated ST-70K at $94.95. High Fidelity (February 1963, p.55) ran the ST-70 in its consumer equipment reports section, describing it as an integrated stereo amplifier offering preamp-control facilities with a null-type balance check circuit. And Audio (June 1963, p.13) ran a full-page advertisement asking whether buyers could find another kit offering so much for $99.95. That's an ad, not a review, but it shows how Eico was positioning the price.
Collectibility & Value
Fully restored, working ST-70s typically sell in the $600 - $1,200 range depending on cosmetics, tube complement, and provenance. That's the top of the market. Actual completed eBay sales run considerably lower, roughly $149 to $490 over the past twelve months, with a median around $258 and volume of only about two units a month. Recent data points include $163 and $144 in February, $408 in April, and $249 in June. The high water mark in the recent chart was $578. A lot of what trades on the open market is unrestored, and the price reflects that.
Original kits with manuals and unused parts can command higher prices, especially when documented as un-built. An untouched kit is rare and appeals to a different buyer than someone hunting a working amplifier. There's also an active modification scene. An "ST-70A Modifications Package" exists, and owners report that the ST-70 has a couple of plain dumb glitches introduced by the manufacturer, with the major fixes well documented online and reportedly fun to implement. Accessories trade separately and cheaply: a tone control upgrade PEC runs about $40, and knob dots about $8.
Maintenance is the real cost of ownership. Electrolytic and paper capacitors, especially in the power supply and coupling positions, degrade over time and should be replaced to prevent hum, distortion, or outright component failure. Carbon composition resistors drift in value, affecting bias and gain, and should be checked against spec. The 7199 and 7591 tubes are still available as NOS or new production, but the 7199 is becoming scarce. Some builders substitute 6CG7 or 6FQ7 with minor circuit tweaks. Transformers should be tested for shorts or insulation breakdown before powering up a long-dormant unit, and the bias circuit can drift if neglected, leading to red-plating or premature tube failure. This amplifier operates at lethal voltages, over 500V DC in the power supply, and should only be serviced by experienced technicians using an isolation transformer and proper discharge procedures.
Sonically, the ST-70 is celebrated for a rich, warm, detailed tube sound. The 35 watts per channel are ample for efficient speakers, think 90dB-plus sensitivity, and it's been paired successfully with AR-3a, Klipsch Heresy, and Advent models. Bass is tight and controlled for a tube amp of its era, though not as deep or punchy as modern solid-state designs. The soundstage is described as organic and live, with instruments and vocals rendered with palpable presence and spatial depth. Audiophiles often call the tonal balance romantic or engaging, which makes it a natural fit for jazz, classical, and vocal recordings. Some listeners prefer its raw, unpolished character over more refined but less engaging contemporary designs.
Market Value
┌──────────────────────────────────────────────────────────────────────────┐ │ EICO ST-70 Nov 2026 │ ├──────────────────────────────────────────────────────────────────────────┤ │ High $578 Latest $119 │ │ Median $206 Trend — │ │ Low $107 Volume ~2/mo │ ├──────────────────────────────────────────────────────────────────────────┤ │ $600 ┤ │ │ │ . │ │ │ . │ │ $386 ┤ . │ │ │ │ │ │. . . │ │ │ │ │ $100 ┤. . ..│ │ ├─────────────────────────────────────────────────────────────────│ │ vol │█ █ ▄ ▄ ▄ ▄ ▄ █▄│ │ └─────────────────────────────────────────────────────────────────│ │ 2026-02 2026-05 2026-08│ └──────────────────────────────────────────────────────────────────────────┘
eBay completed sales. Excludes accessories, parts, and non-working units. Hover points for details.
eBay Listings
Recently sold on eBay: $119–$490 (median $206, based on 12 completed sales, past 12 months)
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