25-FEB-2005
Here's a 2A3 SE amplifier that I had just finished yesterday. I used a pair of EH's 2A3 gold grid and a pair of RCA's 6SN7 for driver stage. The photo shown Chinese 6N8PJ( Chinese Military 6SN7) for testing. I use solid state rectifier and use 2 pcs of 100 ohm paralleled to replace the choke. There's no hum at all without choke because I'm using 820uF 450V for the supply. The output transformer is 2.5K-8ohm rated 15W, I use paper bobbin and Japanese Z11,EI96 silicon material.
I seldom design SE amp more than 2 stages, but my client request for higher gain. 6SL7 can give enough gain with only 2 stages but less current, I prefer higher current, and the sound quality of 6SN7 but less gain, that's why I'm using 3 stages in this design. The 2A3 working at plate current of 60mA with cathode-plate voltage of 300V. The EH's 2A3 can handle this 18W of dissipation. I had tried to change it to chinese 2A3 with better sound quality but it can't take that high dissipation, need to reduce to 14W. Since my client love the EH's sound, I leave the EH tubes on. I use DC filament supply from 5V ac, using a bridge rectifier and a 0.25 ohm resistor to get the 2.5V DC. If I build 2A3 for myself, I will use AC as filament supply, and use tube rectifier and choke to supply B+, that will sound even better, I think.
Nothing much to talk about the sound quality. As expected, 2A3 always put a grin on my face every time I I had build one and listen to it for the first time. So far I haven't hear anyone says that he or she don't like the sound of 2A3 SE amp. Tell me which 2A3 SE amp sound bad if you could find one..........
I'm designing another 2A3 SE amplifier (for my own use) that using 717A door knob tube, and will put it on here when finished. Now I'm still trying to find out the characteristic of 717A tube, and optimized it to drive 2A3. I can't get complete data sheet for 717A, if anyone of you found the data sheet, I would like to have a copy!
The schematic of this 2A3 will be on soon, I'm quite lazy to draw it using any design software, I will draw it manually and scan as jpg file. Stay tuned for the schematics and the new 717A driven version.
The simple but messy layout, I build it
without a schematic in hand, The 2 big rectifier is DC for filaments. I don't
like using DC for 2A3's filaments. Again, customer's request that I must follow.
I really NOT enjoy building this 2A3! can't
follow what I like ,such as rectifier tubes, AC 2.5V filaments, oil caps etc
. :(