Here is another 2A3 amplifier for one of my customer from Hong Kong. This design is actually different with the previous one, the gain is lower than the first version so that a pre amp can be use with this 2A3 power amplifier.
The rectifier is 5U4, GZ37 or 5AR4 type, I had tried all types of tubes with different characteristics. 5AR4 gives more dynamic and measured output power is slightly higher than others. Sovtek 5U4G sounds good at mid range, and good imaging. GZ37 is most musical in this design. The first capacitor just after the rectifier tube is SCR 15uF, and another 15uF Matsushita oil cap just after the choke, and 3 pcs of Matsushita 15uF oil cap in parallel after a 56ohm resistor. I am thinking of adding a 100uF in parallel with the 15uF oil cap that is just after the choke to reduce hum, but I can't hear any hum 1 foot from my Tannoy Stirling so I didn't add it ,( wanted to keep the power supply section electrolytic-free)
The output transformer is paper bobbin hand wound Z11 type 3K : 4, 8 ohm, Rated 15 Watts and the stack is EI96 X 40 mm. This oversize output transformer gives tight and firm bass and it can go really low. I use 3 K primary in this design, it will sounds better but gives slightly less output power. The B+ is around 311V ( when 5U4 is used, 5AR4 and GZ37 will be higher) and 2A3 is biased to 57mA.
Driver stage is SRPP,and using SILMIC cap for the cathode bypass. I had tried normal plate follower but it sounds a bit dim, less high and also less control. It doesn't means plate follower can't use as driver stage for 2A3, SRPP is matched in this design especially most of caps are oil cap. The cathode resistor for 6SN7 is fixed at 820ohm, it runs at around 4-5mA, this will reduce the output swing ( around +- 50V before clipping,) but will sound better than lower current. Coupling cap is 0.5uF military grade oil cap, this will improve the sound quality a step further.....
I use schottky diodes for DC filaments, low voltage drop between the schottky diodes produce less heat and higher efficiency..
Schematic can be download HERE
