Specifications

to each power amplifier module as well to

implement this change. The original bias pot

would then be used to bring the bias voltage in

range, and then the individual controls would

be used to tweak each tube or channel as

needed for equal current draw. Once they were

set, the original bias control could then be used

to move the bias uniformly up or down as

needed.

13. In the listening room, EFB makes itself

known rather immediately in the SCA-35.

More power is available for previously mar-

ginal applications, and overload characteristics

are greatly improved. The new power is deliv-

ered with greater dynamics and clarity,

particularly in the bass region, where more

power is required. Overall, the sound is effort-

less, with excellent presentation of detail. This

is one of those wonderful times when the test

bench and listening room agree: The modifica-

tion to EFB takes the SCA-35 to a whole new

level of performance.

14. Please! As one last little piece of support for the SCA-35, be sure the bass and treble knobs

are installed correctly on this unit! There’s no telling how many “clean ‘em up” folks took them

off not knowing the correct installation procedure, and then reinstalled them merely centered on

the control shaft rotation. This is absolutely wrong, and will destroy the flat performance of the

otherwise good performing tone control circuits. Correct installation has the bass control point-

ing full down at 6 o’clock when turned to minimum, and the treble control pointing full down at

6 o’clock when turned to maximum. Then, when the controls are both turned straight up to 12

noon, the response will be flat.

Modifying the SCA-35 with an EFB™ regulator is relatively simple, and will produce a signifi-

cant improvement in its performance. I will be happy to address questions regarding it, in the

Reader Comments section for this article on the Tronola.com website.

Finally, I would like to thank Steve Lafferty for offering me this dedicated space on his website,

Tronola.com and for his invaluable assistance in producing and posting this article on it. Look

for more of my articles to be published here in the future. Happy Listening!

Figure-10. For those who want to provide individual bias

controls for each channel, this circuit can be used. The

Bias Range pot sets the common cathode voltage, while

individual controls fine-tweak the grid voltages.