Roger's Output Transformers
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This special cored universal purpose KE57613-E tube output transformer, which is a C+ derivate and excists only on paper, has the following design parameters:

Primary windings (50W):
- 3X230V@45Hz with 40% taps (Imax=0.22A)
Secondary windings:
- U0=6.5VX4, Imax=5A
Miscellaneous data:
Dimensions/Weight: 52X100mm/1kg
Total primary inductance: 80H@5V/50Hz (95H@10V/50Hz)
Leakage inductance: -
Total primary DC resistance: 200Ohm
Maximum power input: 50W@230V/15Hz
Inductance vs DC unbalance: 67H@+/-5mA (33H@+/-20mA)

KE57613-E, Roger's Ultimate Output Transformer

Which makes the transformer useful as for instance:
- Standard Williamson OPT using push-pull KT66/807/6L6GC/EL34 drive with 8Ohm speaker load only (15Wsin@807).
- Boosted Williamson OPT using push-pull KT88/6550/PX25/300B drive with 4-8Ohm load (Psqr@6550>30W, +/-30% Class AB if biased@6R).
- Universal OPT using true Ultra Linear push-pull KT88/6550 drive with 4-8Ohm load (Psqr@6550>50W).
- May also even be used as mains isolation transformer for f.i tube preamps (<2X230V@0.2A/60Hz, 4X6.5V@<5A)

More info:
KE57613-E, The ultimate Knoppson OPT taps and additional features