Disclaimer: someone asked me how difficult it would be to build an amplifier similar to the Zana 2. My first reaction was that it is just plain crazy to use 6C33C for headphones. Still, the concept has been bugging me and I had to try and see how this thing could even work in theory. This will be all sims, I really have no interest in pouring hundreds of € into something with high chances of failure.
Still, playing with theoretical amps is definitely part of the hobby for me ;-)
What I gathered from reviews is that the 6C33C in the Zana are run at 8W, with a 90r plate load resistor. The amp is dc-coupled in-between the stages and has no input cap. Driver is a 6sl7. Now, there is a lot in that which doesn't make sense to me.
- 8W ? No matter how I run the sims, lowish B+/high current or the other way around, 8W is too low to get the best performances of the 6C33C. It seems ok at 17W though, still a long, long way from its max power.
- dc-coupled ? Why ? The amp has by definition a massive output cap. Would a small interstage cap of the highest quality make such a big difference on top of that one ? And if the amp is dc-coupled, it means that the 6C33C's cathode has to be way above ground (to give breathing room to the input stage). Which means higher B+ than necessary and plenty of heat dissipated in the cathode resistor.
- 6SL7 driver ? Ugh, everyone on the web seems to be pushing for a decently low impedance driver for the 6C33C, to get an adequate hf response. So that means the driver has to get some help. µ-follower here we are.
Starting from there, I had my parameters :
- input stage must be a high-µ tube, to get decent gain to apply feedback and linearize the not very linear 6C33C.
- if the input stage is also the driver, we can use a solid state ccs to also get a low impedance output. Most high-µ tubes won't like to be run with too much current, so let's divert part of the current through another ccs, from tube anode to gnd.
- interstage ac-coupling. We can now use supplies adapted to each stage.
- 6C33C in fixed-bias. No need to worry anymore about cathode resistor decoupling nor the dissipation of the cathode resistor. A small negative supply is a small price to pay for that.
It all leads naturally to the following schematic. Sims are pretty good, even at 32R (sims promise 0.2% thd at 1Vrms into 32r). Distortion spectrum is nicely decaying. I really have a hard time to understand why someone would want to build such a monster though.
As it is an intellectual exercise, I'd be very interested in reading your comments and suggestions on this.
Btw, I found a prior attempt at this game: http://www.head-fi.org/t/340309/6c33c-b-se-otl