Specifications

As power output from the driven channel increases, it draws more current from the power

supply. This drops the available B+ somewhat – to both channels of course. The idle channel

reacts to this by reducing it’s quiescent current draw, making more current available for the

driven channel to consume. In other words, the idle channel acts as a shunt regulator across the

output of the power supply, acting to maintain a rather consistent B+ voltage for the driven

channel to operate from. This has a significant roll in helping to produce the increased power

output and lower distortion readings obtained when each channel is measured individually. In

fact, all else being equal, if one channel of the SCA-35 was powered by it’s own power supply of

half the capability as that in the complete amplifier, it would not deliver the same performance

simply because the regulating effect of the B+ is gone!

So B+ regulation, and how each channel affects it is surely in play here: when only one channel

is driven, it effectively operates from a regulated supply, but when both channels are driven, not

only is the regulation effect gone, now each channel is acting to the detriment of each other.

However, in the SCA-35, there are even more devious gremlins at work.

That Common Cathode Connection

In the SCA-35, the cathodes of all four output tubes are connected together and

draw their collective current through one common cathode resistor, which is

bypassed by one common bypass capacitor (at right). It is likely the most dis-

cussed aspect of the SCA-35’s design. It was also surely an economic move,

and one which whether to stave off criticism, or herald its virtues, Dynaco felt

the need to comment on, stating it “improves the performance of the output

stage”. From the standpoint of stereo operation, it does somewhat due to the

unique signals being presented to each channel. But to understand why, again

goes back to the amplifier’s behavior on the test bench.

At issue here is the rather large disparity between the performance of one

channel when driven separately, versus that of both channels driven together.

Since the amplifiers employ cathode bias, its automatic adjustment capability

allows the bias to adjust itself to the prevailing conditions offered. That means

that as B+ sags under increased power output, maximum available power output

will drop, but a component of the cathode bias action will also react to help

maintain a low distortion operating point for the new conditions offered ––

except, that’s not what’s happening here. In the SCA-35, when both channels

are driven versus just one, available power output per channel drops about 20%

as expected, but distortion increases 3-5 times! What’s going on? The answer

lies in two important decisions Dynaco made: the use of a common cathode

connection between all four tubes, and the use of cathode bias to begin with.

To many, the effect of the common cathode connection is simply an economy that creates the

nuisance of having to use a matched quad of output tubes. It does, but that should be considered

a goal rather than a nuisance anyway if you are trying to achieve truly identical channels. But

there is an even bigger element at play with the common connection than simply requiring a

matched quad of tubes. Once again, a closer look at what is really happening here will give the

second clue as to how greater performance can be had.