Moderators: pompeiisneaks, Colossal
They said there would be no math.Phil_S wrote:It seems to me, all this can be figured using Ohm's Law. If you give me a schematic (as built) with a complete set of actual voltages, and provide the desired preamp voltages, I think I can get really close on the resistor values for the B+ ladder.
Not arguing with you here...quite the contrary. The OP said he's fiddling with preamps. If we use Ohm's Law, that will give us the wattage (V*I) for each tube section. Using that wattage figure as if it is a constant will work to back-solve the problem. In reality, it is not a constant. It is the limited mA range on a 12A?7 that allows this to work. Besides, resistors come in standard values, so there is a forced choice when all is said and done.matt h wrote:The only issue with that is that current draw is also determined by plate voltage. If you're adjusting voltages within a narrow range, it may not matter a ton. It is conceivable within a narrow range of adjustment, you can rely on the current not to change enough where you can predict desired resistor values. If, however, you're trying to figure things out over a much broader sweep, resistance will start creeping in the opposite direction as current for the same voltage drop.Phil_S wrote:It seems to me, all this can be figured using Ohm's Law. If you give me a schematic (as built) with a complete set of actual voltages, and provide the desired preamp voltages, I think I can get really close on the resistor values for the B+ ladder.
I am willing to show my work if the OP will provide the data. How about it?I like the decade box idea. Right now I just use alligator clips and a box of resistors. A bit reckless, but it does the trick. BTW, who is limited by standard values? What about parallel resistor combinations -- works great.Structo wrote:I have a resistance decade box, it comes in handy to tune the preamp.
For more power I would probably go with a high wattage rheostat to dial it in.
Once you are satisfied, measure the resistance and replace with a fixed resistor of the proper wattage.
Phil, tomorrow I will re measure voltages and let you know.Phil_S wrote:Here I am wondering about all this. I'd still like to see the OP's schematic, voltage chart, and list of desired plate voltages. Maybe I can still salvage something?
