For those who wonder ... the black wire is a tap on the choke winding; using it instead of the blue wire for the plate gives 30% fewer turns of wire, and hence more current can go through those turns. So that way the choke can handle 50mA, as used in Stereomour for the 2A3. Using the blue wire limits you to 35mA.
Of course inductance goes as turns squared so the inductance is half with the black wire. That's fine in Stereomour, because the transformer is used at 4K instead of 8K, so the ratio of inductance to load impedance is the same.
Not as obvious is the fact that, in Stereomour with 45s, the inductance is full value but the transformer operating impedance is 4K so the ratio is twice as good. This is why I want to use this combination for the SR45 - an extra octave of bass goodness.
