Output Section Design

The output section includes the output tubes, the output transformers which drive the speakers, and the biasing circuitry.  The job of the output transformer is to take the high-voltage/low-current/high-impedance output of the output tube and convert it to a low-voltage/high-current/low-impedance output for driving the low impedance (4/8/16 ohms) speaker.  Early on, I settled on a pair of James output transformers, partly because they look awesome in their potted cans, but also because budget conscious Single Ended amp builders seemed to like them, they pair well to the 6B4G output tubes I chose and gave me an option of 3.5kOhm windings and 5k.

In my first mono prototypes, I biased the output tube the good old fashioned way, Grounded Cathode (aka Auto bias).   Because I didn't have enough 6.3v heater windings on my power transformer to heat two output tubes plus two driver tubes in a SRPP configuration, I decided I needed to be able to power both output tube heaters from one 6.3v winding.  That suggested that I should consider using Fixed biasing of the output tubes so that both cathodes could be tied to the same potential (ground).  Fixed biasing adds a negative DC potential to the grid of the output tubes.  That allows a positive AC voltage (the signal) to applied to the grid and still be at a lower potential than the cathode.  The main reason fixed biasing isn't often used is that it requires an extra power supply.  The supply draws almost nothing though (a small handful of microamps in my tests) so the only real design negative is the extra parts count and associated real-estate they will occupy.

I built a quick and dirty (yet remarkably clean) negative supply, reconfigured the output stage, and was amazed by my first listen.  The upper-mid distortion that had bugged me for weeks was almost gone.  When I looked at how other designs (like my Cary SE-1) implemented fixed bias, I was surprised that they usually use the same negative supply to bias both channels.  That seemed odd to me since that basically means they are tied together.  Granted there's 400k ohms or so of resistance between the two grids, but still the signal will take that path with a resulting crosstalk.  I tried shorting the Right channel of my SE-1 to ground and sure enough the Left channel bled to an audible level to the Right speaker.  Granted, it was maybe only 2 to 5% the volume of the Left channel, but still, that's substantial for "Hi-Fi".

I decided I could do better, and I did.  I searched online for hours for a small power transformer with dual primaries.   It's common on large trannies, but not small (5 to 12 volt) trannies.  My plan was to not connect the transformer to the main voltage, but to connect it backwards to a heater winding of my main power transformer keeping it as THE isolation unit and not having to worry about fusing two (or three) power trannies, etc.  So the voltage gets stepped down by the main power tranny from 110v to 5v, then back up to about 90v by the small biasing tranny.  Since it has dual primaries (which I'm using as secondaries), I get two -90v adjustable supplies, one for each channel - zero crosstalk introduced by the biasing.  This was the one idea in my whole design which was completely my own.  I'm sure someone has done it before, but still, it felt great to have that spark of a novel idea and avoid the need for a second transformer.  Below is what the final dual -90v supply looks like:

I needed about -90v so that I could slap a high resistance potentiometer on the end and have the resulting voltage divider produce my target biasing voltage (roughly -40v) somewhere in the middle of the potentiometer's range.  A fixed bias configuration really simplifies the output stage circuitry.  Below is my final output stage schematic with the bias supply applied where you see "-39 v".