All transformers are made by Tamura of Japan. The custom-made STU-OOI is expected to become a standard Tamura item. The other custom transformers were made to my order and are not likely to become catalog items.
This amorphous core interstage transformer (DC150mA) was a collaborative effort based on two years of listening tests. Tamura's good quality ITs enable me to use the same type of tube for driver and output.
Although Tamura ceased production of the Sk:20k 1 50 mA driver transformer because of high manufacturing cost, you can substitute a STU-5K (DC80mA), which is sold only by Sun Audio, the Tamura distributor in Japan.
Let me explain about amorphous cores. There are three kinds of audio transformers: Si-Fe type, Ni-Fe (permalloy), and amorphous. Amorphous is composed of Fe-Ni or Fe-Co, but it is not an alloy. It has no regular crystals of metal. Physically, it is like glass and quite brittle but Tamura got around this with special manufacturing technology. The other challenge that Tamura had to overcome was that amorphous material saturates even more easily than permalloy or pure nickel, so here again special techinques were employed.
I find the amorphous core to be excellent throughout the frequency range, however, its extremely clear sound requires color and power elsewhere in the amplifier. Some builders who tried amorphous cores say the low frequencies are weak. My answer is simple- their grief comes not from the amorphous core but the poor quality of the parts used elsewhere in the ampIifier.
The output transformer's core is also made of amorphous strip material. It has two secondary windings for separate outputs for drivers and woofer, to bring out the powerful and clear sound of the Altec speaker system.
Capacitors are Nikkemi or Nichicon make. Resistors are cheap metal oxide-type.
I always use wire meeting Japanese AC code for the speaker cable and general-purpose hookup wire inside the amplifier. I use Belden wire for power supply, filament, and signal line.