Here are some further tests using automated measurements. Firstly I tried same configuration running the 4P1L at closely 38mA and 160V screen voltage. Second test, I dialled up the screen voltage to 171V and tweaked feedback to get current up to 40mA
Interesting to see that the performance is more triode-like and has a higher THD at lower output levels, however at higher levels the THD is lower. Mainly dominated by H3 and significant rise of H5 given grid current I suspect:
Here is the harmonic distribution of the first test (more pentode like)
Here is the harmonic distribution of the second test (more triode like):
From a frequency response perspective, it performs very good with -3dB from 10Hz to up to 35kHz at 1W tested level:
What surprised me was to find out the increasing distortion at HF. Will this be due to the 4P1L grid capacitance or the DN2540?
