Specifications

the SCA-35: With reduced B+, the EFB regulator reacts accordingly to maintain it’s set relation-

ship, so the lowest distortion operating point is automatically maintained. And, because the

regulator automatically reduces the voltage drop across itself in this condition, power output

increases as well.

Finally, from a distortion standpoint, there is now little need to actually regulate the B+ supply

anymore. Once set, the EFB regulator always maintains the minimum distortion operating point

under all conditions of use, whether one or both channels are operating. With EFB, the only

improvement that B+ regulation would offer, is to eliminate any power output loss due to sag

from an unregulated supply. For the purposes of this article then, I will concentrate on EFB

achieved with a cathode regulator, since it is appropriate for the SCA-35.

Circuit Description And Operation

In its most basic form, the EFB cathode regu-

lator consists of no more than two resistors,

one small capacitor, an adjustment control,

and an LM337 adjustable negative voltage

regulator. The basic concept of the design is

shown in Figure-1. As can be seen, the 337

acts to hold the output stage cathode bias

steady, at a value above ground that is slightly

greater than the 337’s own bias voltage applied to its adjust ter-

minal. By making the 337’s own bias voltage adjustable, it

adjusts the set point of this device, which ultimately adjusts the

bias on the output tubes themselves. Therefore, this control

becomes a standard bias control for the output tubes, just like

any other fixed bias amplifier would employ.

But notice what voltage source is used to supply the bias voltage for the 337.

It originates from the B+ for the output tubes. This is the connection that

turns normal fixed bias into EFB. Notice what happens if the B+ to the

output stage drops, for whatever reason. The bias to the 337 then also drops,

which then drops the bias to the output stage – all in direct proportion to the

new B+ level – so that the original bias/B+ relationship for the tubes is maintained. If the B+

increases, the exact opposite happens. Traditional cathode bias reacts in a somewhat similar way,

except it is reacting to current flowing through the tubes. Unlike traditional cathode bias, the

floating feature of EFB reacts only to changes in B+ voltage, and not to current through the

tubes. This means that EFB maintains the bias/B+ relationship for the tubes under all operating

conditions, in a manner that only regulated bias and B+ supplies could achieve in a traditional

fixed bias application. This is why further efforts to regulate the B+ supply of an SCA-35 operat-

ing with EFB only serve to restore lost power from B+ sag, and not reduce distortion in any way.

Finally, capacitor C is added to help ensure the stability of the 337, with a value small enough to

still allow for quick response to changing power supply conditions. The complete schematic of

the modification as installed is shown in Figure-2 (on the next page), while before and after per-

formance measurements for the stock design, and that with EFB are shown in Figure-3 on p.10.

Figure-1. Basic concept of EFB operation. 337 operates

as a unity gain buffer, with output set by B+. By varying

the voltage divider tap, the bias/B+ ratio is adjustable.