Which output transformer is best used for the 300B?

Most applications with the 300B use one tube in single-ended Class-A operation at an anode voltage between 350 volts and 400 volts. The suitable primary impedance is then around 2.5 kΩ to 3.5 kΩ. On the secondary side, 4 Ω or 8 Ω are usually used.

This makes the following output transformers from the BTB product range suitable:

Lundahl LL1623 with 90 mA – ideally made for this purpose, best audio characteristics

Hammond 1630SEA – sounds excellent, fair price, proven design

Lundahl LL1620 with 60 mA – ideal if the focus is not on maximum output power, but on extended bandwidth

For affordable entry-level projects, the Hammond 125ESE is also suitable

Price tip: Lundahl LL1664 / 100 mA – it offers excellent transformer characteristics and therefore very good audio properties, with up to 100 mA idle current for high output power at low distortion.