Variable Feedback - By the numbers...

[Bob M.]

I installed the popular variable feedback control on my Fender Princeton amp (sometimes known as the 'Soul Control'). Now, I usually adjust this control by ear for what I think sounds best to me but frequently it's handy to know how much or how little negative feedback one is using and what exactly is one's preference. The matter is complicated by the fact that a true resistance reading is not possible in circuit so I've made this handy little chart. I cannot say if this will be true for all amps or, at least, all amps using an 8 ohm output transformer; further research is required but this might be of help to those Princeton owners with inquiring minds. By nature, this chart is pretty approximate:

Feedback resistance used: 5.6K resistor and a 50K pot; variation = 5.6K to 56K
Resistance reading in circuit = 1.228K to 1.538K

Resistance -------- Measured in circuit

5.6K -----------------1.228K
6.8K -----------------1.277K
7.5K -----------------1.30K
10.0K ----------------1.364K
12.5K ----------------1.40K
15.0K ----------------1.43K
20K ------------------1.467K
25K ------------------1.487K
27K ------------------1.49K
35K ------------------1.51K
45K ------------------1.528K
56K ------------------1.538K


Does a Princeton amp have a 'magic number' for the perfect amount of negative feedback? I don't know but lately 6.8K ohms has been sounding very good to my ears in BF and SF Princeton/Princeton Reverb amps. The above research was conducted on my brown 6G2 Princeton amp. Time will tell how much resistance I end up selecting.

Bob M.

[ThermionicScott]

I'm confused -- what are you measuring for your "resistance in circuit"? Different Fender amps used different resistors to ground in the divider, so it's not possible to generalize what's "best" by the feedback resistor alone.

- Scott

[JoeM]

I'm confused too. The pot is in series with a 5.6K?

I don't know if there could be a 'magic number'. If you like the way it sounds with a particular value, then that would be the correct number - for you.

[Jazz P Bass]

Here is a nice take on "why" we use negative feedback.
It may help in your quest to understand what you are changing.
Link:Amplifier Output Impedance

[Bob M.]

Hi Scott,

I don't think there is any 'best', only what sounds good to each of us. There are just too many variables to come to any one best number, resistor, circuit, etc. When using a variable control, once it is dialed in to the user's preference, sometimes it's handy to know how much variable resistance one has chosen. As I said, this chart will give an approximate indication in the Princeton 6G2 amp by reading across the resistive elements, in this case a single resistor and a pot wired in series. You can measure this out of circuit but most folks don't and won't take the time. Measuring across the feedback resistance elements in circuit will skew the numbers because of the influence of the output transformer. I am merely shining a light on this bit of amp arcania; more information almost always being preferable to less. Next time I take apart a BF or SF Princeton, I'll repeat this experiment and see what correlates.

I usually view a variable feedback control as something to be used temporarily until one discovers how much variable feedback sounds good to him/her in that particular amp. Once that is decided, I usually remove the pot and resistor, measure across both components and hardwire in a single resistor equal to that value in the feedback circuit. This is a pretty common practice. Hope that clarifies things.

Bob M.

[R ski]

Basically the term soul control is drivel, the more feed back you enter into the phase splitter antenuates the final power out put, so maybe a single coil guitar sounds nice to your preference but a hotter humbucker could make that set up sound farty when dimed

[ThermionicScott]

I'm totally on board with using a large-value pot to determine the amount of NFB that sounds best. I'm just not sure about the applicability of measuring it in circuit. The results wouldn't be useable with a different cathode resistor or OT.

[J M Fahey]

Please post a schematic of your mod, otherwise we can't answer anything useful.