the Fairchild 670 signal output made by Chicago Transformer had some stability issues. why? well there is a feedback winding, and you know what can happen with those.
So Brian Sowter worked hard to get rid of the stability issues, resulting in an improvement over the original CTC xfmr.
i had no ringing problem with the Pultec innerstage from Sowter, but i used hand wound DIY inductors, using more than one to get better bandwidth.
there were a ton of different lam alloys available back in the golden age of audio, now your choices are very limited, especially with Mag Metals no longer stamping nickel, and other M6 companies going belly up, so your B-H curves are going to be different from the past on a lot of transformers. South Korea is making some good stuff now, you can get a few shapes on evilbay to experiment with, and you might be able to get them to stamp a custom lam if you sent them enough money. i sure would like to wind some WE 618 inputs on that weird lam,
i put up a couple of manly prints for the very mu, i believe they use the same geometry as the Peerless S-217-D (pultec output) if you want to wind and build an eqp or very mu, they are in the Magnetics section. i was surprised more people did not jump on those prints!
i believe the Triad output DCR readings are correct, back then i had access to one of those 1500 dollar fluke bench meters with all the decimal places,
Hammond makes so many transformers, i suspect some are in house, and the less expensive stuff sourced overseas. so it is hard to generalize about a company that has a substantial catalog.
Lundahl really likes the twin coil stuff, which is what UTC used a lot.
Jensen likes E-I lams and hi octane lams so they can use less turns,
their 4:1 output is probably the best xfmr i have measured and it is only $125. great for LA2a,
the best way to test a transformer is to install it into the circuit and listen to it, some are good for that rolled of sound of the 50's, some are good for the straight wire approach of the 21st century,
nobody uses the old wax used for impregnating coils anymore, and nobody uses foldback winding on their inputs like UTC anymore. but fortunately there is still stuff popping up on evilbay for all you retro freaks!
look at this beaut, ever seen one of these before?