SM -- The modification for operating the phono preamp tube heaters from the DC current of the output stage would not be compatible with the EFB(tm) modification.
The EFB modification (in part) converts the output stage to operate with fixed bias, and as one byproduct of that, allows the output stage to operate at reduced quiescent current levels for cooler operation and longer tube life. EFB action then allows the tubes to idle at an even lower quiescent current level than straight fixed bias operation would require.
On the other hand, the modification to operate the phono preamp tube heaters off of the output stage current flow maintains the original cathode bias format of the original design, and necessarily means that the original current flow of the output stage must be maintained, since the 12AX7 preamp tube heaters require (optimally) 150 ma of current flow through them for proper heating (in 12 volt configuration), which is basically what the stock current flow level through the output stage is. By comparison, the EFB modification allows the output tubes to (collectively) draw only 104 ma, cutting the quiescent current flow by nearly 1/3 of that for the cathode bias arrangement, in addition to producing more power and lower distortion than the cathode bias arrangement is capable of as well.
Personally, I have found that the hum balance controls do a very good job of balancing out phono preamp hum in the SCA 35, since the wiring for these circuits is fixed on a circuit board. If you want your cake and eat it too, you might consider installing the EFB modification, and then powering the phono preamp heaters off of a little 12 volt 300 ma DC wall wart plugged into the back of the SCA-35. This would require you to convert the 12AX7s to 12 volt operation, but otherwise should be a relatively easy job to do. It's just a thought.
Good luck with your project!
Dave
