I have been looking at the schematic for the Torpedo (quick link : http://www.diyforums.org/Torpedo/schem/TORPEDOschem.gif) and there are a few things I can't figure out. First is the bias current : is it only 2 or 3 mA ? Second is the biasing method : does it serve a purpose to have the same current flowing through D12, D13 and D10 (other than saving a few resistors) ? Actually, why the use of a transistor + led under the cathode ? Wouldn't a string of leds or diodes do the same ?
A data I can't find anywhere as well is the B+ voltage. I guess it is in the 200V (and it doesn't matter because of the ccs) but since the bias current seems different than in the other l'espressivo designs ...
By bias current, you mean the current through the tube, or the current through D12/D13/D10? The former is ~16mA and is set by R4, the latter is 2-3mA and is set more or less by R6 and R7.
We use the same current as a method of recycling -- ~250V * 3mA is 0.75W. X2 it is the difference between throwing away 1.5W or 3W, which is enough to put significant extra stress on the power transformer, which is heat we don't want to worry about and noise we want to avoid.
The difference between this biasing method and a string of LEDs is that this method is lower impedance and considerably more linear. Keeping this Z low keeps rp lower which helps drive the transformers. And while an LED is an improvement over an RC circuit, this is much better then either.
Yes, B+ is not critical - IIRC it is around 250V, but Tomb probably knows better.