I've been prototyping a GSG variant with a SRPP (6SL7) driver. It's sounds ok but a bit harsh, and transistory in the upper mids. After conducting a THD test, I found that although the overall the 2nd order harmonic seems reasonable (-30dB), the THD is around 6.8% and it's dominated by the 3rd order harmonic (-25dB), which should be more like -60dB according to Tube CAD.
Other than the SRPP driver and a slightly different power supply design, the rest is pretty standard GSG with very similar voltages and biasing:
A NOS "Tests as New" Sylvania 6B4G
James JS-6113HS (3.5k tap)
I know grid current could cause distortion like that, but my test isn't driving it even close to 0 volts.
Any suggestions about what other things might be causing such high 3rd harmonic distortion?
Thanks!