Specifications

7
So existing efforts to regulate cathode bias are problematic at best or destructive at worst,
because the problems they solve are done so at the expense of introducing others. The problems
of the existing cathode bias system have already been established for the SCA-35, and conver-
sion to traditional fixed bias operation has been ruled out on the basis of complexity.
It appears that what is needed then, is a new type of bias regulator – one that apparently has not
been developed yet. In an effort to devise such a regulator, it is helpful to define the qualities
required of it:
1. From a performance and safety standpoint, this new regulator must fix the bias at a set level,
yet allow it to “float” around that level as necessary, in such a fashion as to always maintain a
proper relationship with other operating voltages in the output stage. This quality is highly desir-
able, as it creates a condition where both the B+ and bias sources are effectively regulated, since
they would remain relatively linked to one another. This requirement also means that the regulat-
ing element will need to be an active device, connected in series fashion to effect proper control.
2. The set point of operation for this new regulator must be adjustable, allowing ease in setting
the relationship between bias and B+ voltages (i.e., setting the bias) for maximum performance
as tubes age, or are changed out.
3. Since this new regulator will be series connected, it must exhibit essentially zero impedance to
all AC and DC currents flowing through it, allowing the bias to remain fixed regardless of
current flow. This quality establishes the basis of traditional fixed bias operation.
These qualities would result in an adjustable bias regulator that would hold the bias very steady
at the chosen operating point, regardless of power output, to maximize amplifier performance.
Yet it would also allow the bias to float as required, and automatically maintain a set relationship
with the available B+ under any conditions of use. This performance would represent the best
qualities of both regulated bias/B+ supplies, and fixed bias operation, and is exactly the kind of
bias regulator we need to fix the ills of the SCA-35’s existing cathode bias system. A bias regula-
tor with these qualities would create a brand new approach to bias control, unlike any other
known to exist. Can a regulator with this kind of performance be built in a simple way that main-
tains Dynaco’s design philosophy?
A Really New Approach:
An Enhanced Fixed Bias (EFB™) Regulator
The preceding paragraph describes the qualities of what I call Enhanced Fixed Bias (EFB™). It
can be applied in place of a traditional cathode bias resistor by way of an EFB cathode regulator,
or in a traditional fixed bias installation by way of an EFB supply regulator. In either case, EFB
can be achieved very easily from a common three-terminal voltage regulator. In the SCA-35, an
EFB cathode regulator does a very effective job of providing fixed bias operation at the cathode,
while also maintaining a set relationship with differing B+ voltages produced under various
forms of operation. EFB is so effective in fact, that with a sag in B+ due to the application of
power, bias is actually reduced for the output tubes accordingly! This is exactly opposite of what
happens with a cathode bias resistor, but is exactly and ideally what we want to have happen in