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Originally I wasn't very interested in adding a digital circuit to my Ella but Brian convinced me that he had made sure no digital noise would affect the sound so I changed my mind.
The first job was to change a resistor on each ABS board. It comes with a 3K3 which sets the bias to 40mA but with KT88's I wanted 50mA. A table of values included in the instructions says that means I'd need a 4K3. The problem is that it would then be fixed, but if I ever changed back to EL34 (or 6550) I'd have to swap the resistor again. So I decided to use one of the existing bias pots in parallel with the resistor and wanted a range of 0mA to 60mA, which the table said would need a resistance range from 0R to 5K6. With a 22K pot in parallel that meant using a 7K5 resistor, because (1/22) + (1/7.5) = (1/5.6)
Then I had to make some space for the boards, which meant moving the bias bridge rectifier, removing the additional bias capacitors I'd added (which probably aren't needed now due to the extra filtering in the ABS) and rerouting the main A/C supply wires.
After that it was easy to mount the boards on L-shaped brackets off the tube sockets. I would have preferred to have points to solder the wires to on the boards instead of the screw fittings but I suppose if you use a fixed resistor instead of a parallel pot then you want the board as easy to remove as possible so you can get to the resistor to change it. And it would be nice if the LEDs on the boards could be moved so they were chassis mounted but I suppose that would make the manufacture of the boards much harder.
In use the bias goes to the value set and stays there regardless of the amp temperature and it does keep the pairs of tubes nicely balanced. Because I had already changed the bias diode to a schottky rectifier the improvement in sound wasn't as big as it would have been for a standard Ella, but there was still an improvement in details and bass. On a standard Ella the improvement would be huge.
Photos and a layout diagram on my web page.
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