Some have asked for tips on using the Hammond 1650N output transformer with the Poseidon boards. The Poseidon boards are based on the earlier Eiclone project, itself based on the modified Mullard 5-20 design as used in the Eico HF-87 and HF-89. The stock Ike build did use 1650Ns, and I've always thought they were excellent iron.
Anyhow, I would build according to the Mark III schematic build but make the following changes:
Change R5 to 2k (Mouser PN# 71-RN60D-F-2.0K)
Change C2 to 220pF (Mouser PN# 5982-15-500V220)
Change R7 to 3.3k (Mouser PN# 71-RN60D-F-3.32K)
Change C3 to 470pF (Mouser PN# 5982-19-500V470)
Use the 8 ohm tap for the negative feedback.