Hammond 124A 5W Push-Pull Interstage Transformer, 10K:90K CT

The 124A is a tube driver interstage transformer from the 124 Series, designed for general purpose or replacement use in push-pull or phase inverter tube driver circuits. It is intended for tube (valve) audio circuits, both new builds and replacement work.

It has a 10K ohm primary and a 90K ohm center-tapped secondary (turns ratio 1:3) and is rated at 5 watts of audio power. The series notes that these transformers should not be used for single ended applications, as they have no gap for the DC bias current present in SE mode.

The core uses grain oriented silicon steel laminations (listed as Grain Oriented Steel in the series table); the 124B is the same design with 49% Nickel laminations. Typical measurements from the drawing: DC resistance 405 Ω primary and 1525 Ω secondary (the series table lists 403 and 1524 ohms), inductance 6.3 H primary and 50.5 H secondary, 0.11 H leakage inductance, impedance at 1.0 kHz of 40 kΩ primary and 320 kΩ secondary, and 500 Vrms dielectric strength. The series gives a minimum frequency response of 150 Hz - 15 kHz (+1/-3 dB) at 1 kHz.

The 124A is open style with a 2 point mount and minimum 5" long leads: primary RED–BLU (10,000 Ω) and secondary GRN–BLK–GRN (90,000 Ω CT). Dimensions are A 2.38 in (60 mm), B 1.35 in (34 mm), C 1.43 in (36 mm) and D 2.00 in (51 mm), with 0.19 in (5 mm) mounting holes. It is marked Made in Canada, ships at 1.2 lbs (0.54 kg) and is RoHS and REACH compliant.