High frequency transformers were used in the anode circuit of the HF amplifier stage and coupled the HF signal to the grid of the detector stage. Usually, the primary coil would be tuned by a variable condenser to the frequency required. The one shown in the photograph covers the 300-600 metre range when a 300pF variable capacitor is used.

The McMichael barrel transformer was also used as a neutralising coil in the Anglo-American Six construction project (see Wireless Constructor, February 1925). Scans of the leaflets that came with the box are shown right (click to enlarge).

Two adverts are shown, right. The first is a scan of an advert taken from Wireless Weekly dated April 29th 1929 (click to enlarge). The advert mentions that matched transformers could be purchased. Matched transformers were required if more than one HF stage was employed because a dual capacitor was often used to tune both HF transformers and if they weren't matched then maximum efficiency would not be achieved.

The second advert is taken from Wireless World dated January 19th 1927.