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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!

Hi,

Different tube brands can have a small impact. Also the current through them can change things drastically.

Cheers!

Thanks for the suggestions, Geek.

I tried my other tube from my "matched" pair. As expected, it shows very similar results.

I lowered my bias resistor to increase the plate current by 5mA and it did improve the distortion. I also tried the 5k tap instead of 3.5k on the OT. Both changes together lowered the 2nd harmonic by 12db (-42dB) and the 3rd by 3db (-28db). Overall THD is down to 4%. It's sounds better too. Not stellar yet though probably due to the 3rd (-28db still seems pretty high). At least things are improving.