Driver Design and Fun with Distortion

My original *mono* prototypes had the 6B4G output tube biased using "Grounded Cathode biasing" or "Auto Biasing".  They included a 6SL7 driver tube configured in SRPP (Series Regulated Push-Pull).  The driver preformed pretty well: decent gain and distortion, but because SRPP has the cathode of one triode at half the HT voltage (125v in my case), I had to drive it's heater from a supply separate from the heaters of the output tubes or else I'd exceed the maximum heater-to-cathode voltage of the driver tube.  In the end, my power transformer had 2 heater windings, so I probably could have made it work by using one for the power tubes and one for the driver tubes, but I had already set off into the land of fixed bias for my output tubes with a simple 6SL7 common cathode driver (see my previous post).  This reduced my tube count to three (2 output tubes plus the 6SL7 with one triode to driver each channel). 

My first test with that configuration in October resulted in a much better overall sound.  The details were nice, gain was lower than expected but the distortion profile was generally more pleasing (~4.5% THD@1kHz, -27dB 2nd order, -35dB 3rd - see red spectrum graph below).

Still not outstanding, but better.  However, that was just one channel.  Unfortunately, duplicating the circuit to the second channel resulted in almost double the distortion, but more gain.  WTF!  Both channels were using identical components.  The only difference was the cathode bypass capacitor on the driver triode.  I had used a junk box generic 47uF cap on the original channel (the one with the good distortion) and a decent Nichicon VX series 47uF on the second channel.  Sure enough, swapping the caps pretty much swapped their distortion and gain profiles. So the crappy generic cap had some magic mojo for reducing distortion...

Well, kindof.  Turns out, it's value measured by my multimeter was about 0.02uF.  For a bypass cap, that's essentially not bypassing much other than very high frequencies.  It wasn't a magic cap, it was a blown cap which wasn't far from no cap at all.   Many designers believe that with a proper design, a bypass cap shouldn't really be necessary.  It increases gain, but also increases distortion.  It's used most everywhere because we generally don't want to "throw away" gain.  So to not include a bypass cap means that the lower gain has to be designed into the whole amp.  Fortunately, the 6SL7 driver I'm using has a high transconductance and yields about 25x gain even without a bypass cap.  Thus with an input signal of 1V RMS, I could expect about 25V RMS (~ 35V peak to peak) on the grid of the output tube which is just below the maximum for the operating point that I selected for my output tube.  Almost like the 6SL7 was born to drive the 6B4G!

Look Ma, no driver bypass caps!