Don't mess with the Stereo 70...pass it on, or pass on it.

The first really popular component stereo amplifier was the Dynaco Stereo 70. Something like a quarter of a million were made over a roughly fifteen year period, and many still exist. Every day, people find them in closets, garages, basements and other places and put them up for sale or decide to fix them up (or have it done for them) and use them.

Approximately twenty firms offer upgrades, replacement parts and services related to the venerable little amp. In my opinion, investing good time and money in them is a bad idea. If you find one, my advice is pass it on to someone else and start from scratch, unless you have a fixation on assembling an early-sixties-era college dorm setup as a nostalgia trip.

Why is something so popular something I think should be avoided? Because the Stereo 70 isn't a jewel in the rough, but rather a case of a product that was designed to shave costs so thoroughly that making a good unit out of it means replacing everything. It makes little sense to replace everything and still wind up with a undersized, crummy old chassis and a product that you can't resell very easily for nearly what you have in it anyway.

Let's look at any effort to upgrade the amplifier. Inevitably, the original circuit board has to be replaced, either with a recreation of the original circuit or one of a dozen others on new board material. We also must replace the filter capacitors, and usually all the wiring and connectors. This means we are reusing the chassis, the three transformers and the choke, and that's about it.

But let's look at those transformers. They're between 35 and 50 years old, and insulated with old materials that have a habit of breaking down. In addition, both the power and output transformers have flaws that mean we'd rather have new ones anyway. The power transformer is undersized, for one thing, and runs quite hot. It also has a primary meant for 115 volts when the line voltage in the US has increased by five to ten volts in most areas over the past half century. If we replace the tube rectifier with a solid state diode pair, as one might want to to reduce load on the transformer, both the HT primary and the heater voltage will increase. We will have to increase the voltage rating on the filter caps, which is a good idea anyway, and we might put the 5 volt heater winding in series with the primary to drop the voltages a little, but still, the HT voltage will be more than we really want.

Replacing the power transformer definitely helps: we want one with more lams, more copper, designed for current utility voltages, and perhaps with a primary set up for solid state rectifiers. But then we still have the old output transformers, which are both arguably the best thing about the old warhorse and the limiting factor on what can be done with it. They were wound according to a Hafler patent, which allows for perfect AC balance at the expense of DC symmetry in the primary. Why suppliers so proudly offer "perfect reproductions" of these compromised parts is beyond me.

Replacing the output transformers on the Dyna chassis is problematic due to the space available.

The bottom line is that if you find an old Dyna Stereo 70, the best move you can make is to pass it on to someone else and put the money into a new clean build. Buy or fabricate a decent chassis and a good pair of output transformers and go from there.