Specifications

4
Earlier, it was shown that when a single channel was driven and measured, the idle channel actu-
ally helped contribute to the excellent performance produced by the driven channel, because it
was actively working to help regulate the B+ supply. The common cathode connection has a
huge part in the effectiveness of this action.
Again, consider what happens on the test bench when a single channel is driven to full power.
The average current draw of the stage has increased, meaning that the bias across the cathode
resistor will increase also. However, because of the common cathode connection, the increased
bias created by the driven channel also works to reduce current flow in the idle channel, which
now amplifies its effectiveness to function as a B+ regulator. But most importantly, it can also be
seen that this process is ultimately working to effectively regulate the bias itself, since increased
current draw in one channel causes reduced current draw in the other. The end result is that now
the driven channel is actually operating under conditions of fixed bias!
On the other hand, when both channels are driven, they both pull increased current across the
cathode resistor. In that condition, the bias cancellation effects previously noted vanish, and con-
ditions revert to that of traditional cathode bias operation, with elevated bias voltage across the
common cathode resistor.
Tying It All Together
Now it can be seen why there is such a significant performance discrepancy between a single,
and both channels driven condition in the SCA-35. Between these two conditions, the unit auto-
matically transforms itself from optimum conditions of fixed bias operation from a regulated
power supply when a single channel is driven, to much less than optimum conditions of cathode
bias operation from a non-regulated power supply of much smaller capacity when both channels
are driven.
Obviously, some of the power loss in each channel is due to power supply sag when both chan-
nels are producing power. But much of it is also due to the increased drop across the cathode
resistor as well. However, the notable rise in distortion between the two conditions is entirely due
to the huge shift in output stage operating conditions created by the increased bias generated
when both channels are operating.
From the preceding then, the following facts can be drawn:
1. The Z-565 transformer is optimally matched to 6BQ5 tubes when they are operated
with fixed bias.
2. The Z-565 represents too low of a load (numerically) for 6BQ5s operated with
cathode bias. Use of cathode bias with this transformer results in excessive shifting of
the operating point, resulting in reduced absolute power output and increased distortion.
In designing the SCA-35, Dynaco was clearly aware of these facts. But in using a common
cathode connection, resistor, and bypass capacitor for both channels, they cleverly achieved
economy, simplicity, and for stereo operation, a dynamic form of operation that lies somewhere
in between cathode and fixed bias operation – all at the same time. It was assumed that with
Russian EL84M