I built several 2A3 amplifier recently, this is my experimental set and I spend a lot of time to tweak it and finally completed by last week.
The idea was from Yamamoto A-08 SE amplifier, which using a WE-717A to drive vintage 45 tube. The first time I saw it on magazine, I keep searching for data for 717A. I can't get much detail from WWW . The 717A is a single penthode, I have really bad impression with penthode driven amplifier due to its low current and high output impedance. Several years ago, I had tried EF86 driving 2A3 with bad result, it sounds muddy and lack of control at lower frequency. I just can't get it right and give up. It doesn't means that EF86 always sounds bad but its quite hard to drive a 2A3 with best result.
By building a simple 717A driver stage and measure its characteristics, the result shows that it is quite a potential tube for driving 2A3. It operate at a few mA higher than 12AX7, The Rp is around 390K and I use 39K for plate resistor. I saw Mr. Yamamoto's schematic on WWW recently, his driver stage is similar but he used 33k of plate resistor. I add a 220ohm stopper resistor at grid 1 to reduce the chances of self oscillating. The grid 2 supply is regulated using 2 pcs of 56 volt 1 watts zener diode.
The power section, I used 5U4 as rectifier, I have several of them and similar type of rectifier tubes like GZ37 and 5AR4 also tested with good result. I choose 4 pcs of 20uF ASC oil cap for this set, it really improve the sound quality, more natural and huge sound stage. The big transformer in front of the 5U4 is a 10H choke. For filaments I use AC supply for 2A3 and hum balance pot. By carefully adjust the hum balance pot, I can get 2.2mV of noise level at the output, under 8 ohm of resistive load. That should be good enough for most of the fullrange speakers.
A pair of China ShuGuang 2A3C are used in this amplifier. It really out perform most of the 2A3 in market, its gold pin, gold grid and enhanced plate and looks macho in 300B glass envelope. I did adjust the plate current to push the plate dissipation to around 28 watts without any problem. I keep it working at around 16 watts for better sound quality.
The coupling capacitors are Russian made NOS military oil cap, and use Kiwami for all resistors in driver stage. I heard that carbon composition resistor will sound warmer, but I have no chance to try it, I will purchase a batch of carbon composition resistors soon, to try it out and also supply for DIY hobbyists. The bright color wires inside this amplifier are teflon-silver wire. I used silver contained soldering lead for this project too.
The 3K: 8ohm output transformer using Z11 EI 96 silicon steel stack to 40mm high, paper bobbin with 3 section of primary and 4 sections of secondary. More than 10cm of cross sectional core area seems over killed but I love the sub bass! I salvaged the bell covers from a pair of World Audio 5881 SE transformers.
Frankly speaking, I hate the sound quality which have penthode drivers, this set sounds lack of dynamic and a bit out control at lower frequency. After running for about 50 hours and I start to love this amplifier. It sounds natural , deep and wide ambience and sweet vocal, although it can't give huge dynamic bass compare to previous 6SN7 driven version.
This month is the month of Chinese ghost festival, I'm really busy repairing those broken pro-amplifiers, I got no time for the schematic drawing, I will post it by next month.
Updated :
Schematic available now, please click HERE for schematic.
