2A3 Single-Ended Direct-Coupled Class A2 Power Amplifier Part 2 【DIY Amp Lab】

The output tube load is set relatively high, prioritizing damping factor and distortion characteristics over sheer output power, Designed and Built by Soya Susumu

Parts Used

The main components of this amplifier are listed in Table 2. For the vacuum tubes, I simply made use of those I had on hand; any readily available brand will work fine. The 2A3 (Photo 1) does not need to be a matched pair. Since a relatively high cathode bias resistor of 3kΩ is used, the plate currents will balance well enough even with unmatched tubes.

[Table 2] Major Parts List

[Photo 1] Vacuum tubes used in this amplifier. From left: 2A3 (RCA), 6J5GT (Hitachi), 6SL7GT (Toshiba), 6G-K17 (Raytheon, Japan-made). Magnifications differ.

Tube sockets are critical parts, so even with new ones, care is required. Always insert the tube and check the fit to ensure proper contact.

For transformers, I used products from General Trans Hanbai. Since these are all catalog items, they should be readily obtainable. The output transformer (Figure 5) allows switching between 2.5kΩ and 3.5kΩ secondary impedances by changing the primary connections. Its frequency response is excellent, which made it an ideal choice. However, since this amplifier operates without negative feedback, if you have an output transformer on hand with a 3.5kΩ primary impedance and capable of handling around 20W, that would also be suitable.