After some further reading and experimentation, I'm pretty sure the effect I was seeing was "grid clipping". I actually think a little bit of this sounds good when the amp is driven into distortion, so I may lower the value of my (currently giant, 1M) grid stoppers somewhat.
I've also been playing with other values: 22nf coupling caps instead of 100nf, controlling gain with a cap-bypassed volume pot after the first stage rather than by varying the first stage plate resistor, etc. Fairly minor differences and it's hard to remember the sound well enough to be sure which changes I like.
One other thing I've been trying to do is bias things just right so that the amp has the maximum clean volume, but just beyond that point both the preamp and power tubes are simultaneously driven into distortion.
Here's a recent schematic. I may do another video once I get it tuned to my satisfaction.
I've also been playing with other values: 22nf coupling caps instead of 100nf, controlling gain with a cap-bypassed volume pot after the first stage rather than by varying the first stage plate resistor, etc. Fairly minor differences and it's hard to remember the sound well enough to be sure which changes I like.
One other thing I've been trying to do is bias things just right so that the amp has the maximum clean volume, but just beyond that point both the preamp and power tubes are simultaneously driven into distortion.
Here's a recent schematic. I may do another video once I get it tuned to my satisfaction.