...it is hard to not comment on ... several swipes at negative feedback:

use enough closed loop gain in a negative feedback amp with only smooth, low order nonlinearities and you get decreasing levels of harmonic distortion with harmonic order - not the 10-20 dB some "no-feedback" types may tepidly apply - it is easy to have >60 dB loop gain at all audio frequencies

there is no other practical way to obtain useful damping ratios - cathode, source, emitter followers are all 100% (local) negative feedback circuits - there are no "inherently low output impedance" amplifying devices

the "no-feedback" crowd's beloved Triode vacuum tube has loads of internal negative feedback - directly causing the observed plate resistance - a tube without internal feedback is a correctly biased Pentode{emphasis added}

negative feedback is a foundational principle and should be properly understood and used correctly - ignore audiophool mythology if you want to learn how to design sucessful circuits

another poster has gone off topic and is pursuing the throwaway comment about Triodes

so the title is the topic of This thread – is there “negative feedback” built in to triodes – is limited voltage gain, plate resistance a consequence?

I don’t remember where I 1st encountered the idea – is it just a feedback engineer’s “urban legend”?

Many circuits predating the wide-spread adoption of feedback analysis are still usefully analyzed with the more recent intellectual tools - followers, degeneration are taught today as feedback circuit techniques

Anyone interested enough to follow up with references? – if I cared enough I would look at 50’s electronic textbooks as Bode’s work, Blackman’s theorem only became widely distributed after WWII

maybe a start:

..for those still interested in tube theory just try some search: Langmuir-Childs law

Improved vacuum tube models for SPICE, Part 1

basically the Vgc and Vgp voltages control the current via the same nonlinear law