newbie question about dual voltage switching

[Strat0kaster]

Hello all,
I'm new to the forum and am just getting into building amps.......I have a question....

Why is the series resistance needed in the low voltage option? To keep the current draw the same at the Rectifier? I'm in the process of building my first kit (JTM 50 BF, GZ34 Rect) and want to add a dual voltage option using a classictone 4018054 PT which has taps for 690 V and 625 V. How do I choose the resistor value?
FYI I'm using Georges wiki page as my main reference. http://metroamp.com/wiki/index.php/Dual ... _switching" onclick="window.open(this.href);return false;

Thanks a lot for any help!

-Paul

[toner]

Why is the series resistance needed in the low voltage option?

The Metro dual voltage tranny (and probably the Classic Tone) only changes the secondary HT voltage without changing the negative bias voltage. The resistor is there to reduce the negative bias voltage so the bias stays consistent relative to the new lowered plate voltages. Without the resistor, the bias would be too cold when in low power mode.