Not really sure but - a few of the Bijou prototypes used the same R-core that was sourced by Jeff Rossel.
From what I know about transformers:
1) Toroids - low noise (less magnetic radiation) & efficient and smaller VA for VA, however core saturates easily if there is DC imposed upon the AC wave resulting in transmitted noise.
2) EI transformers - less efficient, more magnetic noise but with air-gap allows it to tolerate DC in the line
3) R-core = similar to toroids ?more linear? less noise
Actually looking at the R-core specs, they are really specifically targeted towards the tube amps with filament secondaries and high voltage secondaries.
her shann
