Specifications

10
New Output Stage Operating Conditions
The development of the EFB circuit provides new artillery in the arsenal of weapons to achieve
really great amplifier performance economically. The cathode regulator provides exacting, fixed
bias operation at the cathodes, and solves the problems attributed to the original cathode bias
installation in the SCA-35. But not to be over-shadowed is one other very important change that
results from the SCA-35’s conversion to fixed bias.
The original cathode bias resistor’s value was chosen as a huge compromise: one that would not
let the operating point fall completely off a cliff with the application of full power (to both
channels in this case), but one that would also keep output tube dissipation within limits under
quiescent conditions. In the end, a value was chosen that kept the tubes safe (but pushed them
right to their limits), and then delivered whatever performance it could under full power
Figure-3. The table readily shows the drop in performance of the original design when both channels are driven,
versus performance when EFB is used. In most cases, EFB performance with both channels driven is better than
single channel performance of the original design.
Notes
Ÿ Original specs are: THD 1.0% or less, from 20 - 20 kHz, within 1 dB of 17.5 watts, (each channel).
IM 1.0% or less at 17.5 watts (each channel). 0.2% or less on any input at average listening levels.
Ÿ These represent average, but realistic figures – some tubes favored power or distortion at one frequency, other tubes at
others. All power is RMS.
Ÿ (1) Max Power is measured at the onset of visual clipping.
Ÿ (2) Total harmonic distortion is measured at 13.65W, which is 1dB below rated power.
Ÿ (3) Intermodulation distortion is measured with 60 Hz and 7 kHz, mixed 4:1, at 17.5W equivalent for each channel.
Ÿ (4) Unable to produce 13.65W for measurement. Distortion is at max power.
Ÿ (5) Unable to produce 17.5W equivalent for measurement. Distortion is at max equivalent power of 14.65W.
Ÿ (6) Note that this IM test is at the specified 17.5W equivalent power – an increase of 19% over the test before the mod.
Cathode Voltage Change at Max Power (From zero to maximum power output at 1kHz.)
Before After
Single Channel 14.05 to 15.27 14.65 to 14.03
Both Channels 14.05 to 17.85 14.65 to 13.39
1 2
3
4
5
Single Channel Driven
Before After
Freq Max Power (W) Distortion (%) Max Power (W) Change (%) Distortion (%) Change (%)
20Hz 17.0 1.60 18.9 +11.2 1.60 0
25 17.0 0.90 18.9 +11.2 0.75 -17
1K 18.0 0.17 18.9 +5.0 0.12 -29
20K 17.2 0.65 18.5 +7.6 0.52 -20
IM Distortion 0.80 0.40 -50
Both Channels Driven
Before After
Freq Max Power (W) Distortion (%) Max Power (W) Change (%) Distortion (%) Change (%)
20Hz 13.6 X2 2.00 17.6 X2 +29.4 1.70 -15
25 13.6 X2 1.40 17.6 X2 +29.4 0.80 -43
1K 14.6 X2 0.95 17.6 X2 +20.5 0.18 -81
20K 13.3 X2 2.6 17.2 X2 +29.3 0.57 -78
IM Distortion 2.00 0.75 -62
3
1 2
1 2
1 2
6