A very interesting Russian directly-heated pentode related to 4P1L is the 2P29L. It has a similar mu (μ=9), much higher anode resistance 2.8-3KΩ and transconductance of 3mA/V when triode-strapped. The filament requirements are much lower at 120mA. I picked one valve from my collection to submit it to the mercy of the curve tracer:
2P29L test point (pentode)
The triode curves are really nice:
2P29L triode curves and model
This valve is as linear as the 4P1L (hooray). As a preamp it can be easily implemented like the 4P1L Gen2 preamp using a gyrator PCB which simplifies the building process:
2P29L preamp
Running it at 15mA and slightly above the recommended 160V achieve its lowest distortion.
We could also use this valve as a driver for a 4P1L preamp, which comes very handy for filament bias:
