Specifications

To do a good job of mimicking the voltage drop of a GZ34 tube, there should be a 50 ohm 10 watt resistor installed BETWEEN
the output of the SS rectifiers, and the FIRST filter cap. Installing a resistor at this location will control the peak currents drawn
and improve regulation, providing nearly the same B+ level that a GZ34 would under any condition of operation. Installing a
resistor after the first filter cap, would hurt regulation, and NOT limit the peak currents drawn from the power transformer.
When using this rectifier approach with a buck transformer to provide 117 vac, the initial B+ rises to 500 volts, but then falls to
nearly the same B+ level as produced by a GZ34 in the same scenario, yet is easily able to handle the maximum current needs
of the amplifier without concern. The tube heaters operate from 6.4 vac using a 117 volt line, the bias controls barely need to be
moved from their GZ34 setting, and importantly, using these two expedients cools the power transformer notably. with a buck
transformer, the SS plug in rectifier and dropping resistor installed as described, and the unit biased to Dynaco
recommendations, the hottest point on the power transformer was just 130F in my lab after two hours of high power testing. This
of course was with the cover off, and the ambient temp in here is only 70F, but this is still quite cool operation for the transformer.
The amplifier was graceful under all conditions with this approach, showing no concern over the loss of the rectifier tube. All in all
then, they are improvements worth making to any stock ST-70.
The choke is also somewhat of a compromise, providing good filtering under even medium power conditions, but completely
saturating under conditions of high power output. Still, it does make for a quiet amplifier during normal use, with its high power
limitations covered by the sound level of elevated power output. In any event, it size was a good compromise, helping to achieve
quiet operation of the unit with typical use.
And finally, there's the selenium rectifier. Selenium rectifiers are well known to ultimately fail in high current applications (those
where significant heat is created within the rectifier) JUST AS they are well known to hardly if ever fail at all in very low current
applications, such as the bias supply represents. I would hardly suggest not to replace it from an ultimate durability standpoint,
but to replace it simply because it's a selenium rectifier -- in this application -- has little merit.
The Power Output Stage - Pt. 1
The output stage of the ST-70 harkens back to Hafler's earliest days at Dynaco. His first output transformer -- the A-430, as used
in the inaugural MK II amplifier -- was specifically designed for use with EL34 tubes. With a 4300 ohm primary, and screen taps
placed at 33% of the primary winding, the load the screen tap position was experimentally determined to produce the greatest
linearity from these tubes, coupled with a good degree of load regulation. The A-470 output transformer, as used in the ST-70,
has the same exact specifications, at a somewhat lower power rating. Unlike the original A-430 transformer, the A-470 also
includes a 4 ohm output tap as well -- this to accommodate the 4 ohm speakers (notably AR) that were starting to appear at the
time.
So the ST-70 operates the output tubes with the same loading conditions as Hafler's original amplifier does, but operates them
at a lower B+ level (~410 vdc vrs ~450 vdc) to achieve a lower power output level. Which brings us to the first myth about the ST-
70's output stage: Lowering the quiescent current level (i.e, Biaset voltage) to extend tube life. From a standpoint of absolutes,
lowering current levels will extend tube life. But in doing so in the ST-70, what is the practical return?
A common battle cry for this practice comes from those who categorically say something like: "Because tubes were so cheap
back then, manufacturers ran the tubes right to the ragged edge for the performance ratings wars, and didn't care about tube
life". No doubt in some cases that's true. But that line is said so often, that the statement has nearly become fact for all cases,
regardless of what the individual facts actually are. So what about in the ST-70?
The 15.6 ohm cathode resistor causes the output stage to operate at 100 ma total cathode current in each channel, when the
Biaset voltage is set for 1.56 volts as instructed. With a matched tube set, this is 50 ma per tube, with about 45 ma of it conducted
by the plate. With 410 volts across the plate and cathode then, this equates to 18.45 watts, which as a percentage of the very
conservative Design Center 25 watt plate dissipation rating for these tubes, equates to operating them at only 73.8% of their
rated dissipation level. Even when operating the amplifier from a 122 volt line and still using the recommended Biaset level, the
dissipation only climbs to 19.4 watts, for 77.6% of rated capability. These are both very conservative levels to operate these
tubes at -- particularly when you consider it is being judged against the Design Center Rating value. The more common Design
Maximum Rating System that was in place at the end of the tube production era rates the EL34 as a 30 watt tube, meaning the
tubes would only be operating at 61.5% of capability (64.7% at high line voltage) under that rating system. With such
conservative operation of tubes that are designed to conduct a continuous 165 ma each (Design Maximum rating) -- both from a
dissipation and current flow standpoint -- backing the quiescent current level down by 10 ma per tube to save on tube life will
gain you precious little of what you are trying to achieve, but it will give you a whole bunch more distortion!
If a THD test set is connected to the ST-70 and each channel is individually driven with a 1 kHz signal to 1 db below the rated
power of 35 watts (about 27 watts), it will be found that the lowest distortion operation point is achieved with a quiescent Biaset
setting of -- you guessed it -- 1.56 vdc, or very close to that setting. If you go below OR above this setting, distortion will climb,
and do so rather rapidly.
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