Specifications

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increased, but flattened as well. In short, it has fully succeeded in achieving the original goal of
allowing both channels to perform at the same performance level as a single channel could of the
original design, and has done so while lowering the dissipation level of the output tubes by 20%.
The unit is now actually capable of developing 35 watts RMS on a continuous basis across
almost the entire 20 – 20 kHz power bandwidth.
10. Popular Modification: It appears that one of the most common modifications for the SCA-
35/ST-35, or DIY clone versions of these amplifiers, is to install separate cathode resistors and
bypass capacitors for each output tube. Since each resistor is adjustable, it then allows the bias
for each tube to be set individually. The effect this has when using Z-565 transformers is to not
only destroy Dynaco’s effort to provide an intermediate form of fixed bias operation under stereo
conditions, but also to forever condemn each channel to produce the sub-par performance exem-
plified by that of both channels operating in the original design, whether both channels are
operating or not!
When four independent resistors are used, a value of 380 ohms is often cited, which is four times
the original value used for four tubes with a common connection. That would mean that each
channel is effectively biased through a 190 ohm resistor, or double that of the original value.
With each channel pulling current through a dedicated cathode resistor of twice the original
value for four tubes, it will produce exactly the same shift in operating conditions – all the time
now – as the original design does when both channels are operating with the original cathode
resistor. This has clearly been shown to be a less than optimum way to operate 6BQ5 tubes with
a Z-565 transformer.
As independent evidence of this fact, Triode Electronics is a popular Illinois based business that
offers high quality clone copies of Dynaco transformers. On their website, they offer a perfor-
mance comparison of an original Z-565 transformer to three other competitor transformers in
common use. The test was performed using a clone version of the ST-35 amplifier, and tests were
made that included power output and distortion that started with ~ 1% THD at 1kHz, and then
extended at the same basic drive level either way from there down to 20 Hz, and then up to 40
kHz. Under these conditions, examination of their chart for the original Z-565 transformer shows
that power output and distortion levels achieved, mirror very closely the performance I achieved
when both channels are driven in a stock SCA-35.
While it does not specifically state so, it can safely be assumed that the tests were made with
only the tested transformer channel being driven, since the test was relating to the transformer
being tested, rather than that of the amplifier itself. It does state however, that each transformer
was simply dropped into the circuit, without any efforts to optimize performance. The driver cir-
cuits of the SCA-35 and ST-35 are different, but their output stages are not. This difference in
driver design is not so great as to cause the performance issues of concern here.
The take away point, is that the clone amplifier used employs four separate cathode resistors and
bypass capacitors to achieve the performance shown. By any measure of power or distortion, the
performance it produces is at best equal to a both channels driven condition of the original SCA-
35 at mid frequencies, and significantly inferior to it distortion wise at the frequency extremes.
Against the performance shown for EFB operation, this chart is further evidence of the lowered
performance that cathode bias produces with 6BQ5 tubes, and the Z-565 transformer.