My issue is at the input, but does this necessarily mean it's the transformer itself? I can't imagine Sowter making a transformer for the redd that wasn't actually suitable. I'll swap it out for the a-10 that I have and see if there's any improvement, although it seems more likely to be impedance rather than the transformer itself right?If your scope is showing the frequency response is dropping off and you are measuring right on the transformer with no load on the transformer, your problem is the transformer.
Change it for something better.
I do have a question regarding the testing method I'm using. My LA-2A has utc a-10/a-24 (500 ohm in and 500 ohm out), same as the redd. When using the same method, I get a flat response (slight rise in the HF). Is this a characteristic of using a 12ax7 vs the ef86, or am I misunderstanding something here? I would've expected to see a similar response if impedances are the same between both units, and if it was an impedance issue with the redd.
Thanks again everyone