Dynaco ST-70-II
A kilobuck tube amp that carried the most famous name in kit audio - and never escaped its own shadow.
Overview
The Dynaco ST-70-II should be easy to research, and it isn't. It's a tube amplifier built in the United States by Dynaco. The original Stereo 70 is so common that restoring and modifying it is a hobby of its own, and next to it the Series II barely registered. Collectors run into the same wall: tons of documentation on the original, almost nothing on the II. One forum regular noted that the original ST-70 amps are everywhere while "very little info on the S2" survives.
What survives is oddly specific in places and thin in others. The amplifier sold for $995 under the Panor/Dynaco banner. That put it in what Stereophile's Corey Greenberg called his "quest for the Ultimate Kilobuck Tube Amp," a review where the ST-70-II was measured against a $1,695 Nobis Cantabile. This wasn't a budget kit like the ones Dynaco built its name on. It was a premium-priced tube amp arriving in the early 1990s, when the market had largely moved on to solid state.
Owners describe a machine that is "similar to the original ST70" with "a few differences," the biggest being the way bias is adjusted. That theme runs through every account. The Series II is close enough to the original to invite comparison, and different enough that the comparison often goes against it. One owner who acquired an ST-70-II while already owning and loving an original admitted preferring the original outright.
The Siemens output valves, produced in East Germany and branded Dynaco, are the one tube detail that surfaces for the Series II. The original used Mullard valves, also Dynaco-branded. Beyond that, the specification record is essentially blank. No power output figure, no frequency response, no distortion numbers, no dimensions or weight appear in the available documentation.
Key Features
A Different Bias Adjustment Scheme
The one design difference owners and forum posters agree on is the bias adjustment. Where the original ST-70 required getting under the hood, the Series II changed the method. Owner accounts call it the main practical difference between the two amplifiers. Anyone who's owned an original knows the ritual of periodic bias checks, so it's a real change. It's also the one area where the Series II moved the design forward rather than simply reissuing it. No documentation in the available record describes the specific mechanism, only that it differs.
Siemens Output Valves, Dynaco-Branded
The Series II shipped with Siemens output valves manufactured in East Germany and branded Dynaco. That's a departure from the original ST-70's Mullard valves, which were likewise Dynaco-branded. Tube provenance matters to collectors, but the available record doesn't say whether the Siemens valves were a deliberate voicing decision or a supply-chain necessity. It does confirm that the Series II's tube complement was not simply carried over from the original.
Built in the United States
The amplifier was manufactured in the United States, but Dynaco was a different company by then. Tandy Corporation (RadioShack) had acquired the brand in 1983, well before the Series II appeared. The ST-70-II is a product of that later era, not the Philadelphia years when David Hafler's company sold amplifiers primarily as kits that customers assembled themselves to save 30-40% off factory-built pricing. The Series II was a finished, premium-priced component.
Historical Context
The original Dynaco Stereo 70 was introduced in 1959, ran on four EL34 output tubes in a push-pull ultralinear configuration, delivered 35 watts per channel, and sold over 350,000 units across a 17-year production run ending in 1976. It remains the most popular vintage tube amplifier for restoration and modification. The Series II was born under that shadow.
The ST-70-II arrived around 1992 - a forum user refers to a "1992 version" - and Stereophile's Corey Greenberg reviewed it the following year, first published June 9, 1993, as part of his kilobuck tube amp survey. The timing was bad. By the early 1990s, the market's attention had shifted toward solid state, and the Series II never found the audience its predecessor had. The consensus explanation, repeated across owner discussions, is blunt: the ST-70-II "just never caught on like the original 70 did." One forum assessment offers two possibilities: audiophiles by 1990 were too committed to solid state, or the sound simply lacked the "magic" of the original. The reviewer, paraphrased in that discussion, apparently leaned toward the latter.
The result was a rare amplifier. Forum users describe seeing only one or two examples over years of watching the market. An eBay listing for one carried the word "Rare!!"
Collectibility & Value
The ST-70-II is rare and Dynaco-badged, yet poorly documented and less desired than its far more common ancestor. Scarcity usually drives collector interest, but here it's paired with a documented preference for the original among owners who've compared them directly. The amplifier that "never caught on" when new hasn't been rescued by the collector market the way some overlooked gear has.
Some documentation does exist. An owner's manual circulates, a schematic is archived, and the AK Database hosts an owners PDF. That's more than many rare amplifiers have, but it doesn't include the performance specifications a buyer would need to evaluate the unit on paper.
No current market prices or common failure points surface in the available record. Anyone buying an ST-70-II is buying a model with a thin repair-knowledge base compared to the original. The tube complement is at least partially known (Siemens output valves), but rectifier and driver tube types for the Series II aren't documented in the sources reviewed. Treat any specific claim about the amplifier's internals with skepticism until you've seen the manual or the schematic.
Similar items on eBay
No exact Dynaco ST-70-II listings are live right now — these searches show current Dynaco and similar vintage audio gear on eBay:
As an eBay Partner, we earn from qualifying purchases. This helps support our independent vintage technology research.