I am trying to finalise a design of a two stage P-P el84 amp. For a brief description, there's an input transformer with a center tapped secondary that performs the phase splitting that is then connected to the grids of the 12AT7 diff pair, whose plates are RC coupled to the grids of the output tubes. As I have it now, B+ is around 310V going to around 300V at the plates of the output tubes and dropping to about 270V (through the use of a dropping resistor) right before the drivers' plate load resistors.
So this is my question: Instead of using the dropping resistor to get to 270V, could I order a power transformer with a set of additional taps to get to this voltage point? Are there any other downsides besides the obvious complexity and additional cost to this scheme? I want to have separate power supplies for the two stages, but how much of the (potential) benefit will be negated by the sharing of the same transformer?
So this is my question: Instead of using the dropping resistor to get to 270V, could I order a power transformer with a set of additional taps to get to this voltage point? Are there any other downsides besides the obvious complexity and additional cost to this scheme? I want to have separate power supplies for the two stages, but how much of the (potential) benefit will be negated by the sharing of the same transformer?
Not really a concern about the current arrangement, after all that's how most amps are set up when one supply feeds all stages. I am just thinking of ways to improve things, that's all. I am also not concerned about shift in the operating point with either arrangement, unless I should be concerned and am not seeing the whole picture - it's a possibility as well. Your idea about a regulator is not a bad one and I 'll perhaps revisit it in the future as another possible upgrade path.
I think the normal method where you use cascaded RC setions to drop the B+ voltage is actually the best design. The reason is that you end up with Good filtering after those multiple RC sections.
If you'd like to improve more replace the first resistor with a choke. Use a choke that has the same DCR as the resistor you are replacing
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