Charles just found some one who can explain why R-Cores make for very good output transformers, for triode SE amps here's what he said.
"Actually they appear to work and sound very well.
Maybe they are not so popular because you get lower inductance in comparison to Fe cores (of comparable size). This is more o less the same for other amorphous cores.
However they have a much smaller coercive field (near to zero) and tighter hysteresis loop that makes them more linear devices. In other words they have lower eddy currents losses and lower hysteresis loss. The permeability is higher and more costant (as function of the induction B) however when you place a gap for DC application it reduces to smaller values than those of a conventional core (i.e. same turns = less inductance).
They work very well with low impedance devices, like triodes, much less with pentodes.
One factor that definitely works against the R-core is the difficulty in winding. The coil former has to be placed on the core first, only then can the coils be wound. That's why you'll see a plastic gear on the formers."
Cheers George
"Actually they appear to work and sound very well.
Maybe they are not so popular because you get lower inductance in comparison to Fe cores (of comparable size). This is more o less the same for other amorphous cores.
However they have a much smaller coercive field (near to zero) and tighter hysteresis loop that makes them more linear devices. In other words they have lower eddy currents losses and lower hysteresis loss. The permeability is higher and more costant (as function of the induction B) however when you place a gap for DC application it reduces to smaller values than those of a conventional core (i.e. same turns = less inductance).
They work very well with low impedance devices, like triodes, much less with pentodes.
One factor that definitely works against the R-core is the difficulty in winding. The coil former has to be placed on the core first, only then can the coils be wound. That's why you'll see a plastic gear on the formers."
Cheers George