> snag is gm gets really small
We want that.
The snag is that maximum output must happen at *minimum* current. If you go down to 0.1mA to get enough GR, you won't have enough output to drive a transformer; even resistance coupling gets lame. Nevermind the Fairchild approach of driving the output straight off the VCA tubes.
I use a '2mA' low-limit for fast estimations into ~~10K load. This keeps the THD small. After picking likely tubes, then a detailed analysis would show just how much output can be supported for what range of GR.
We want that.
The snag is that maximum output must happen at *minimum* current. If you go down to 0.1mA to get enough GR, you won't have enough output to drive a transformer; even resistance coupling gets lame. Nevermind the Fairchild approach of driving the output straight off the VCA tubes.
I use a '2mA' low-limit for fast estimations into ~~10K load. This keeps the THD small. After picking likely tubes, then a detailed analysis would show just how much output can be supported for what range of GR.