I just checked some websites for info, and found that C-Core transformers are ideally used with negative source impedance when using mixed-feedback circuit designs. If you are using a no feedback design, this may not be the best choice for your application. Tamura makes some really great iron. They have a Model F5004, that uses an "Amorphous" core. Don't ask me what that is. It is advertised as the "ultimate" transformer for 2A3 and 300B. Costs $1600/pair. Magnequest makes some great iron too. They recommend their air-gapped DS-025 model for the 2A3. Price is $360/pair. You can get'em at Bottlehead. If you can find an old Citation amp, those trannies were recycled by some high end companies in some pretty expensive gear during the 80s and 90s. Notably, MFA. Good luck.
