2A3 SE amp
I built a 2A3 SE (Single Ended) amp a long time ago. I built it because I wanted to see what all the fuss was about, so I built it using the parts I had. I had received a pair of Chinese 2A3 in return for some help with a phono preamp. I also had some 6SN7s and a Hammond power transformer. But what about the output transformer? I looked through my junk, and found a Beovision 3400 output transformer. The Beovision used a PL84 output tube (EL86 with different heater), so the transformer was the standard 2k4 often used with that tube. It looked small, but the PL84 is usually run with the same current as a 2A3 (60mA), so it had to be big enough for that. It had interlieved windings (and a dual C-core), so it couldn't be all bad...

So I sat down with a calculator and the tube datasheets, and came up with some circuit values. But I forgot about the loading of the output stage, so the driver stage didn't work correctly the first time I tested it. The driver stage clipped before the output stage... But I recalculated the driver stage, and the circuit then worked. I built my "prototype" on a piece of wood. The lethal result can be seen below.

2A3 amp photo

The Hammond transformer didn't have a heater winding for the 2A3, but it did have a center-tapped 5V rectifier winding, so I used half of that for the 2A3 heater. But then I didn't have a heater winding for the rectifier... So I found a nice "junk" tube solution - the 6BY5GA! A dual rectifier or damper tube with a 6.3V heater. The 6BY5GA has a very low voltage drop, but it is not meant for use with large input capacitors. So I used a 5H choke for a choke input filter. The resulting B+ voltage is about 285-290V from a 600Vct winding!

The amp sounds great, even with the small output transformer. The dual 6SN7 input/driver stage makes the amp very sensitive - a bit too sensitive probably... I have replaced the Chinese 2A3 with a Sovtek. Both tubes sound good, but the Sovtek is a bit more HiFi'ish. I have bought some better output transformers, but I haven't installed them yet. When something sounds good I don't mess too much with it. I also have a Hammond chassis waiting to make the amp a bit more cat safe...

The schematic can be seen here:


You can get a higher resolution version here: Schematic


Comments? Questions? Just send me an e-mail!

MailMail: mcs@post5.tele.dk