Specifications

6. A very good Damping Factor,
7. Relative insensitivity to fluctuations in load, and
8. Credible performance with both channels driven
The ST-70 delivered a ton of performance for very little money, from a young upstart company that had no business producing
such a good product -- all of which made the big boys blink, and set standards that many amplifiers manufactured today could
only dream about achieving.
The biggest failing this amplifier has, is that it was too good for the price. As a result, there were more sold than by any other
product of any other manufacturer -- by quite a large margin -- which is to say then, that it became all too commonplace. Once
that happened, then just like the equivalent of the small block Chevy that it is, everybody and their brother's uncle started
producing modifications to make it "better" at what it's supposed to do.
What's interesting though is that those producing the modifications say it is to address the 70's rather "average" performance,
diminished top end, or weak bass presentation. Some claim the driver stage is "ineffective", and so on, while they all say that
their modification turns the ST-70 into the best amplifier they've ever heard -- but offering precious little evidence to backup their
claims that its performance is actually any "better".
Truth is, many people today have little understanding for just how good the original design really is. With many having little
concern for specifications -- or accuracy for that matter --their ears then become the sole judge, trial, and jury of all things audio
in the creation of good sound.
But the ST-70 wasn't designed to "create" anything at all. If it did, that would mean it was altering the original source in some
way. In fact, ST-70 was designed to create as little as possible on its own, while faithfully REcreating as much of the input signal
as possible, and no more. THAT is the standard by which the original design should be judged.
Likely then, those who feel that a properly operating ST-70 alters or hinders sound may not really be familiar with what truly
accurate sound is at all, but rather, have their own definition of it. However, with nothing to base their definition on, it becomes a
slippery slope when measured data is disregarded, and accuracy becomes solely based on perceived sonic performance -- if
accuracy is to matter at all. Against this, the proven performance capabilities of the original design, and the unmatched sales
record of the ST-70 provides overwhelming evidence for just how well it did in fact accomplish its goals.
That being said however, there are simple things that can help adapt the ST-70's original purpose to today's audio reality:
1. The rectifier tube can be replaced as discussed for more efficient, and cooler operation, and eliminates the exorbitant cost of
NOS and less dependable modern replacement tubes.
2. The line voltage should be adjusted as necessary to maintain the original goal of high performance, coupled with very
conservative operation.
3. As discussed, an input coupling cap should be installed as necessary to prevent the possibility of any dc component within
the signal source from upsetting LF stability.
4. Choose your 7199s carefully. I have found that those employing a carbonized coating on the side of the glass envelope
containing the pentode section to be quite consistent in delivering proper performance.
Beyond these measures then, any modifications changing the active circuitry to alternate designs will likely change
performance capability, both measured, and perceived. That may be your goal which is fine, but if not, realize the possibility
exists.
As for those interested in installing EFB™ in the ST-70, it can be done, and will result in reduced distortion and quiescent
operating currents. But because of how well the ST-70 was originally designed, the gains it will produce will be greatly limited, as
compared to other installations documented, making the return on the effort rather small.
In addition to providing EFB™ action for distortion reduction, previous efforts have also allowed EFB™ to correct for
inappropriate bias configurations used for the loading conditions offered, reduced output tube dissipation levels that were
operating at or very near maximum ratings, or even change operating conditions that were inappropriate for the load conditions
offered. With multiple benefits then, adding EFB™ to those scenarios is highly recommended.
But as the original design of a unit becomes optimized from the get-go, adding EFB™ produces ever diminishing returns -- as it
should. A properly designed amplifier operating from regulated power supplies would not benefit from EFB™ at all.
12