[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Steve Thompson]

A few thoughts covering several recent posts:

Stability with tetrode amps should not be a problem if you use ceramic 
tubes with the proper bases. More than anything, I think it's the lead 
length in glass tubes that brings problems.

As opposed to rf -ve feedback, you might look at envelope -ve feedback. 
Here's my take on it: if you build your basic amp, it will probably 40+ 
dB IMD for starters (and be one of the cleanest signals around anyhow). 
At these IMD levels, the distortion is primarily amplitude 
non-linearity. Once you head into gain compression, you start to get 
AM/PM conversion, so there's phase errors as well. While it's mainly 
amplitude distortion, you can apply envelope feedback to get improvement.

If you haven't already, have a read of:
http://www.highfrequencyelectronics.com/Archives/Nov03/HFE1103_RaabPart4.pdf
for an overview. Amplitude detectors with lots of linearity are easy, 
two diodes and an opamp. Somewhere I think I still have a pile of stuff 
from Gosling and Petrovich from the 80s.

Steve