Tuesday, February 12, 2013

New Project: a SE 6CB5A amplifier

After the completion of my 1625 "Command" amplifier, I began to cast my eye around for another project.  This time I wanted to yet again tackle a triode amplifier, perhaps doing a push-pull 2A3/6B4G type.  However, the pesky cheapskate that lives inside my head just couldn't spend that sort of coin on a set of output tubes.  While searching around, I read a thread on diyaudio that lead me to this blog post on Vinylsavor.  Check it out for the signal schematic.

The 6CB5A is a 1950s television sweep tube that wasn't designed specifically for audio, but the curve traces in triode mode are quite good.  This cheap tube can also produce 6-8Ws in single-ended, which is pretty close to the mojo of a 300B.  I also liked the idea of using the 6N7 driver, thought I opted to not implement the expensive interstage transformer, and instead went with RC-coupling.  Hey, this is a budget build after all.

Output and power iron will be from Edcor - I really liked the quality of sound I got from them in the 1625 amplifier.  Chokes will be from Triad and the rest of the parts will be standard bog stuff you can get from Mouser, Allied, and Digikey.  I've also opted for some current meters to monitor the health of the output tubes.

Since a single 6CB5A takes a mighty 2.5A @ 6.3V to light up, I had to purchase a separate filament transformer to power the 6N7 tubes.  Other than that, this is a pretty standard power supply with a fake dual-mono separation for the two channels.

Output iron has been ordered.  Everything but the top plate metalwork has already arrived, so I will provide another post when all the parts have come in and everything has been wired together.  I'm quite curious to hear what this amplifier sounds like since the build/design approach is much different than the 1625 amplifier.  The 6CB5A amplifier will have no negative feedback, a pure triode amplification chain, and minimal parts.  In my experience this should lead to a big sound with a slightly underdamped bass.  Stay tuned to find out!