a real challange?

[Kevin Kuehn]

Originally Posted by Yamamaya42

I just have to wonder WTH was RCA thinking when they came up with this bleeder power design...
By routing main ground (-100) trough the focus coil and the adjustment for it, won't that cause the -18 & -2 to change when you adjust focus? Not to mention reference gnd with respect to the positive bias voltages?
It's hurting my head trying to understand what they ment to do...

It's a pretty common scheme to obtain negative bias voltages. Putting the focus coil in there actually saves some power vs wiring in to it's own separate voltage dropper off of B+. Don't think of it as ground going through those resistances, because chassis ground is still at 0 volts. What's happening is part of the B+ is shifted to the negative side of the rectifiers because the HV winding center tap is not connected directly to ground. The 5U4 diodes block the positive cycles from going that direction, so it has no choice other than to produce a negative voltage. You are correct that the bias voltages will change with adjustment of focus, but it's a very small amount because those resistances closer to ground are much smaller in proportion. And vacuum tubes are very tolerant to loosely regulated B+