Specifications

conditions in both channels. We have seen what that compromise produced: under-biased tubes

at quiescent conditions which shortened their life, and over-biased tubes at full power conditions

in both channels, leading to excessive distortion and power loss. This situation required the

output tubes to draw about 35 mA each under quiescent conditions, and resulted in nearly 12

watts of dissipation for each tube. This also gave the SCA-35 a well deserved reputation for

eating up output tubes.

Converting the output stage to EFB now eliminates those compromises. It allows maximum

power and minimum distortion under all conditions of use. As an added bonus, with the Z-565

transformer operated under conditions of fixed bias, the 6BQ5 output tubes now produce the

lowest distortion with a quiescent current of only 27 mA per tube! Now, not only are the output

stages operating with much lower distortion and greater power output when both channels are

driven, they are also doing so with at least 20% less dissipation under quiescent conditions. The

effect this has on tube and component life is enormous. All things considered then, the original

cathode bias system in the SCA-35 is simply no match for the performance enhancements that

EFB brings to this amplifier.

Installation

Installation of the EFB cathode regulator is a

relatively easy operation in the SCA-35, as the

circuit associated with it is so simple. In my

unit, all of the basic components for the regula-

tor itself have been wired point-to-point inside

the back panel — in the area between the output

terminal strips and RCA jacks, as seen in Fig-

ure-4 and Figure-5. This required the

installation of one dual lug terminal strip,

mounted to one of the screws for the RCA

jacks, while the 337 is mounted to a screw

securing the top output terminal strip.

The 337’s mount tab is internally connected to

it’s input terminal, which in this application is

connected to chassis ground. However, an iso-

lation mounting kit should still be used to

prevent the possibility of ground loops being

formed. The heat sinking so provided is more

than adequate, as the 337 dissipates less than

2.5 watts under worst case conditions. For

ease of construction, before installation, I

snipped the leads of the 337 back close to it’s

Figure-4. No drilling needed: The bias pot goes in an

existing hole, originally intended for an optional head-

phone jack.

Figure-5. The LM337 is mounted to the rear panel using

an existing screw of the speaker terminal board and an

isolation mounting kit.