power supply question

[ernie_jr]

I am working on at SUNN type build. The power rail on the 200s is choke, 15k and 33k. If i want to add a second preamp, would I reduce the 33K resistor to a 22K and leave the 15k as is, or should I also decrease the 15K to a 10K as well?
Thanks,
Ernie

[PRR]

Whatever. Simplest would be to hookup one preamp, measure B+ points, hookup your 2nd preamp, see if B+ drops a lot. If so, shunt the dropping resistors with equal values (tack-solder) and see if it comes back to nominal. If so, replace with resistors half the value and double the watts.

OR... build a second B+ string off the choke for the second preamp. This also improves isolation between the two preamps. In most cases this won't matter, but sometimes it might. Anyway more caps is more-better, right?

[eleventeen]

"Anyway more caps is more-better, right?"

I have always thought that, but some have opined that too many ufds on the preamp tubes produces a "stiffness" that isn't that desirable. I don't understand why. The early tubes use very small current; I thought the function of filter caps on the preamp tubes was to remove as much p/s noise as possible.

[jjasilli]

"Anyway more caps is more-better, right?"

I have always thought that, but some have opined that too many ufds on the preamp tubes produces a "stiffness" that isn't that desirable. I don't understand why. The early tubes use very small current; I thought the function of filter caps on the preamp tubes was to remove as much p/s noise as possible.

Apples & oranges.  The question here is where to branch-off the existing B+ rail for the 2nd preamp:  a) downstream after 2 dropping resistors; or b) upstream at the choke output?  If "b)" the 2nd rail would have better signal isolation (if needed).  It could be built to duplicate the filtering and B+ drops of the original rail.  Then it would have its own, isolated filter stages, but not more filtering than rail 1.

[stratele52]

Whatever. Simplest would be to hookup one preamp, measure B+ points, hookup your 2nd preamp, see if B+ drops a lot. If so, shunt the dropping resistors with equal values (tack-solder) and see if it comes back to nominal. If so, replace with resistors half the value and double the watts.

OR... build a second B+ string off the choke for the second preamp. This also improves isolation between the two preamps. In most cases this won't matter, but sometimes it might. Anyway more caps is more-better, right?

+1

[sluckey]

This is how I would add the extra node. Start with 47K for Rx, then adjust up/down for desired voltage at Node D after you have the new preamp working.

[jjasilli]

+1.  Adding the 2nd rail after the PI filter stage is good -- see schematics of 2 channel fender amps like Deluxe Reverb, e.g.  We got a bit sidetracked into the quantity of filtering, instead of placement of the 2nd node