Please see my response to your query on tube diy...
restated... the 26 has a r sub p of approx 7 to 8 kilo-ohms and should probably be used with a 15K or higher primary impedance.
the 45 tube has a r sub p of approx 1700 ohms and most often is used with a 5000 ohm primary.
also... secondary impedance of a small signal (preamp) output trans is typically a few hundred ohms... most often say 500 or 600 ohms.
45 tube is most often used in a power amplifier and will typcially have an output transformer with speaker impedances of 4, 8 or 16 ohms.
these two tubes are different enough in their characteristics as to reequire different transformers...
my suggestion might be to start out and decide to build either a preamp or a power amp first.... pick your preferred tube... we have many transformers for preamp and also for 45 tube that are not overly expensive to begin with and which will give you a good sounding end product.
For example check out our B7-15K preamp outputs.... or look at our EXO-45 output transformers for the 45 tube as an amplifier output. If your on a budget each of these transformers can be built with M6 core materials which would save you money over an all nickel core.
I hope I've been helpful.
Mike