Kegger
R.I.P. 1/12/1966 - 6/1/2017
To touch upon what a few others have said and some of my own ramblings,, 
(I'll be repeating some of what has been posted here)
Optimum amounts of feedback, proper resistors value and all that is both an circuit
thing an the designer/rebuilder/listeners prerogative/preference, no 1 answer there.
----------
If doing it for yourself and no "tools" used you could use the pot like mentioned, and
can do the clip lead with resistor substitution, I'll run down a quick an dirty approach.
(even if you have tools the final say is quite often tweaked by ear)
(talking loop feedback here from output transformer secondary back to input/driver)
like mentioned 6db of feedback should drop the level in half, so with the amp running
and no feedback connected set the music playing level to decent amount but not to
loud, using the clip leads choose a resistor you think is in the area of "correct" clip it
in and judge the volume drop, was it about half? If not try other values till you find a
gain drop of roughly half, now that you found that don't go any higher value resistor,
that is the least amount of feedback you want to use, now decrease the fbk resistor
from there and listen, set it for what you think sounds best.
Some "pointers", amp that has no feedback and "needs" some will sound hairy, kinda
nasty, midrangy, the music isn't very clean, as you add feedback those things should
start to diminish, at 1 point you'll say hey ok all the bad stuff seems to be gone, now
increase the feedback(lower the resistor) until you here no more improvement, that is
the "max" you want to use, go between there and the point you felt that amp started
to sound pretty darn good, run the gambit on music you'd know on speakers you know
while listening pretty good and varying that resistor value to see what you'd like best.
(to low feedback resistor/to much feedback the amp can be unstable an motorboat)
Now a few other tips for listening and feedback, sound-stage and frequency response
besides distortions are some good keys to the feedback adjusting, as you increase the
feedback the frequency extremes will become extended, the amplifier will play "flatter"
or you could say the midrange starts to drop/less prominent, listening to solo vocals an
the sound-stage the singer will come from behind the speakers to right at the speakers
to in front of speakers as you adjust the feedback, an that may sway your preference.
(listen for detail, spaciousness, any harshness to vocals)
------------
(more of my own thoughts)
Some kind of generic guidelines, by no means is this correct in every case/for everyone.
Triode wired outputs do not necessarily need any feedback, if using it 6db's are often it.
UL wired outputs do not necessarily "need" any feedback, but 6-12db is generally sweet.
Pentode wired outputs "need" feedback, at least 6db, quite often 12db or more's needed.
Many, including myself air on the side of the "least" amount of feedback we can get away
with tends to sound better than larger doses of feedback, I don't want the amp to be one
that sounds "clinical" an stripped of sounding like a tube amp, but at the same time I don't
want an overly tuuby, midrangy non detailed amp either, just the right amount of feedback
is the key here, as others have mentioned if using feedback, general rule is no less than 6db.
(if running Triode or UL, sometimes feedback of less then 6db will still work though)
------------------
Now analyzing running circuits and gain/feedback..(in circuits needing/using feedback)
If your running a circuit that has an fair amount more gain then is "needed" to drive the amp
to full power an with feedback connected the amp isn't overly sensitive to yur active preamp
volume control your more then likely running a large amount of feedback, if the feedback was
on the lower side the volume generally is a little touchy at the low end of your volume control.
(a 12ax7's gain driving an el84 with "normal" volume control probably has high feedback)
If your running a circuit that has just a little more gain then the circuit needs an with feedback
connected the volume control seems "pretty good" your feedback is probably not to high nor to
low but somewhere in the middle. (this is the scenario I try an achieve most often when I can)
If your running a circuit that has just an little less gain then the circuit needs an with feedback
connected the volume control seems "pretty good" your feedback is probably on the low side.
These are just generalizations, but quite often can glean into the feedback being used in circuit.
(and again these are generalizations and opinions, nothings written in stone)
--------------------
This was meant to stay away from being technical really, but more subjective analyzing if yu will.
(I'll be repeating some of what has been posted here)
Optimum amounts of feedback, proper resistors value and all that is both an circuit
thing an the designer/rebuilder/listeners prerogative/preference, no 1 answer there.
----------
If doing it for yourself and no "tools" used you could use the pot like mentioned, and
can do the clip lead with resistor substitution, I'll run down a quick an dirty approach.
(even if you have tools the final say is quite often tweaked by ear)
(talking loop feedback here from output transformer secondary back to input/driver)
like mentioned 6db of feedback should drop the level in half, so with the amp running
and no feedback connected set the music playing level to decent amount but not to
loud, using the clip leads choose a resistor you think is in the area of "correct" clip it
in and judge the volume drop, was it about half? If not try other values till you find a
gain drop of roughly half, now that you found that don't go any higher value resistor,
that is the least amount of feedback you want to use, now decrease the fbk resistor
from there and listen, set it for what you think sounds best.
Some "pointers", amp that has no feedback and "needs" some will sound hairy, kinda
nasty, midrangy, the music isn't very clean, as you add feedback those things should
start to diminish, at 1 point you'll say hey ok all the bad stuff seems to be gone, now
increase the feedback(lower the resistor) until you here no more improvement, that is
the "max" you want to use, go between there and the point you felt that amp started
to sound pretty darn good, run the gambit on music you'd know on speakers you know
while listening pretty good and varying that resistor value to see what you'd like best.
(to low feedback resistor/to much feedback the amp can be unstable an motorboat)
Now a few other tips for listening and feedback, sound-stage and frequency response
besides distortions are some good keys to the feedback adjusting, as you increase the
feedback the frequency extremes will become extended, the amplifier will play "flatter"
or you could say the midrange starts to drop/less prominent, listening to solo vocals an
the sound-stage the singer will come from behind the speakers to right at the speakers
to in front of speakers as you adjust the feedback, an that may sway your preference.
(listen for detail, spaciousness, any harshness to vocals)
------------
(more of my own thoughts)
Some kind of generic guidelines, by no means is this correct in every case/for everyone.
Triode wired outputs do not necessarily need any feedback, if using it 6db's are often it.
UL wired outputs do not necessarily "need" any feedback, but 6-12db is generally sweet.
Pentode wired outputs "need" feedback, at least 6db, quite often 12db or more's needed.
Many, including myself air on the side of the "least" amount of feedback we can get away
with tends to sound better than larger doses of feedback, I don't want the amp to be one
that sounds "clinical" an stripped of sounding like a tube amp, but at the same time I don't
want an overly tuuby, midrangy non detailed amp either, just the right amount of feedback
is the key here, as others have mentioned if using feedback, general rule is no less than 6db.
(if running Triode or UL, sometimes feedback of less then 6db will still work though)
------------------
Now analyzing running circuits and gain/feedback..(in circuits needing/using feedback)
If your running a circuit that has an fair amount more gain then is "needed" to drive the amp
to full power an with feedback connected the amp isn't overly sensitive to yur active preamp
volume control your more then likely running a large amount of feedback, if the feedback was
on the lower side the volume generally is a little touchy at the low end of your volume control.
(a 12ax7's gain driving an el84 with "normal" volume control probably has high feedback)
If your running a circuit that has just a little more gain then the circuit needs an with feedback
connected the volume control seems "pretty good" your feedback is probably not to high nor to
low but somewhere in the middle. (this is the scenario I try an achieve most often when I can)
If your running a circuit that has just an little less gain then the circuit needs an with feedback
connected the volume control seems "pretty good" your feedback is probably on the low side.
These are just generalizations, but quite often can glean into the feedback being used in circuit.
(and again these are generalizations and opinions, nothings written in stone)
--------------------
This was meant to stay away from being technical really, but more subjective analyzing if yu will.
