The amp was designed that way, the feedback was added last.
The first several variants of the Princeton were single ended amps & all featured global negative feedback.
While a few of the early smaller Fender amps did not feature global negative feedback, I'm unaware of the Princeton being among them.
We have to remember, Leo & his team (along with many amp producers of the day) did not so much "design" these amps, rather, they lifted the designs straight from the pages of published tube applications manuals. Tube manufacturers wanted people to use their tubes, so they published circiut design examples of all sorts, along with necessary info on adapting them to ones needs. Look through some of these tube application manuals & you'll see what I mean when I say they "lifted" these amps circuit block designs from these manuals.
Negative feedback has several effects.
Among these are:
It attempts to correct distortions, imbalances etc generated in all of the stage's encompassed within the global network.
These include the driver tube (single ended designs) or phase inverter (push pull designs), output stage & the OT.
It reduces "peaks" & extends frequency response.
It reduces the effective impedance of the output, thereby increasing the damping factor.
The "amount" of (global) negative feedback taken is governed by 3 things;
The voltage supplied by the OT secondary tap. The higher the impedance secondary tap, the higher the voltage. Alternately, the lower the impedance secondary tap, the lower the voltage (though the voltages appearing on the different taps isn't proportionally linear).
From there a voltage divider is used to reduce the NFB voltage to the proportion we want. The voltage divider is made up of the "series" resistor & the "shunt" resistor (to ground).
The shunt resistor is part of the PI circuitry (or in the case of the cathodyne, the gain stage directly preceding it), so it can't be changed to a different value to alter the ratio of feedback.
This leaves us with the series resistor, which value we can change to alter the ratio of feedback. Larger value resistor = a lower feedback voltage. Lower value resistor = higher feedback voltage. So amps whose OT have multiple secondary taps have got two places they can use to alter the ratio of NFB, the OT secondary tap & the series resistor value. If your amp only has one secondary tap you can only change the series resistor value.
Regarding the amount, or "ratio" of feedback to utilise &/or disconnecting the NFB loop, global negative feedback is generally considered to be a good thing, but as with most things, too much can become a bad thing in that it can cause power amp instabilities & oscillation due to phase shifts in the various stages encompassed within the feedback loop. Even when not taken this far, too much negative feedback can make the power amp as a whole "lifeless".
Depending on several design factors, no negative feedback can make the power amp feel somewhat out of control. A good example is that mainstay of rock, the 100 watt superlead. Early examples featured the 0.68uf cathode bypass cap on V2a, the feedback series resistor was (usually) 47k & the feedback (also usually) was taken off the 8 ohm tap.
When the 0.68uf V2a cathode bypass was done away with, gain (from about 720Hz up) was reduced, so the amount of NFB taken was reduced by increasing the value of the series resistor to 100k & the feedback being now taken from the 4 ohm tap (less voltage off the OT tap & more attenuation by the voltage divider), resulting in a gainier power stage that feels looser & sounds dirtier.
A common mod on these (100 watt superleads) is to increase the value of the NFB series resistor even more, often to about 200k (some use 220k or even 250k), but this seems to be about the limit at where the power amp starts to become too loose & out of control. Try disconnecting the NFB loop on your 100 watt superlead, crank the volume up around 8 & tell me you like the effect. So yes, some amps do well without NFB, others not so much.
The amount of negative feedback used can be a "personal taste" thing.
This brings us to the cathodyne (or split load) type phase inverter, which can be a decent phase inverter. It only utilises one triode which can be economical, however it produces "less than unity" gain, so usually the other half, or triode of a twin triode is used a a gain stage directly preceding the cathodyne (as in the Princeton).
Because of its less than unity gain the cathodyne is used very successfully in cleaner or mildly crunchy output stages, most notably Orange amps, where the distortion is predominately generated the pre-amp.
Like any tube stage, it has inherent characteristics that must be taken into account. A couple of these characteristics are "grid clamping" (on the non inverting side) when the cathodyne overdrives the power tubes (which we don't hear as that power tube is in cut-off when this occurs), & a frequency doubling effect on the inverted side when the cathodyne itself is overdriven (that is audible).
Normal audio amps don't look to push the various stages into overdrive so these characteristics aren't a problem, however in guitar amps we do, so require a fix for these undesirable characteristics.
The (inaudible) grid clamping effect when overdriving the power tubes can be taken care of with large value grid stopper resistors on the power tubes control grids (100k).
The frequency doubling effect when overdriving the cathodyne can similarly be taken care of with an even larger value grid stopper on the control grid of the cathodyne (500k to 1 meg). This won't cause high frequency roll-off due to the very low input capacitance of the cathodyne.
So while in its basic design the cathodyne does not like to be overdriven (or heavily overdrive power tubes), for the cost of a few resistors (less than a buck) we can change that & enjoy the benefits of its wide frequency response.
You can read all about the cathodyne (& fixes) here.
The Valve Wizard
As the author (Paul C) of the article that Sysco linked noted, that the cathodyne "fix" he showed wasn't of his design, so the "fixes" shown on the Valvewizard site aren't Valvewizard's design & are documented in tube manuals. Cheers
