According to Fender literature, this allows you 'to roll off treble, without boosting bass'. I think that this is erroneous, since the passive tone knob in this circuit is cut only.
I've looked at the circuits. They are out on the Fender site:
[Invalid or Expired Link Removed]
[Invalid or Expired Link Removed]
The [Invalid or Expired Link Removed] shows a .100uf 50V cap, a .02uf 50V cap, and a 4.7k ohm 1/4 watt resister used in the 'grease bucket' tone circuit. A standard P bass uses a single .47uf cap or a .100uf, the former is used in most modern circuits, wile the latter was used in many of the vintage Fender basses.
A P Bass circuit (excluding the S1 switching) standard circuit simply allows the signal from the pickup to go to the output jack, but by turning the pot, the shunting of signal to ground through the .47uf or.100uf cap is increased. This is aurally perceived as rolling off the treble. In this arrangement, the high frequency band that is rolled off is fairly wide and the decrease in treble also causes rolloff of other frequencies and overtones, consequently increasing the tonal quality that we perceive as 'thump'.
I think this circuit works the same way, but more precisely trims the high frequencies (Anyone with more electronics background, PLEASE correct me if I've got this wrong. I'm an amateur electronics buff, and could easily have this wrong!). It looks to me that when the knob is fully in one direction, the signal flows fully through the .100uf cap and the signal is also minimally shunted to ground through the .02uf cap and 4.7K ohm resister. As the knob is turned in the opposite direction, less signal is sent through the .100uf cap, but more signal is being shunted through the small cap and resistor. Fully rotated in the opposite direction, the .100uf cap has no effect on the signal, but we see full shunting through the small cap and resistor.
From a sound perspective this means that the .02uf cap w/ 4.7K ohm resistor is filtering out a certain frequency, full time when the tone knob is set for no treble cut. (I have no idea what this frequency is, so if anyone who knows more about this stuff than me can provide the frequencies that are being filtered by the .02uf cap & 4.7K ohm resistor, by a .100uf cap, and by a standard .47uf cap, I would be really appreciative and interested!) As the knob is rolled in the opposite direction, a specific frequency begins to be filtered by the .100uf cap (the same freq as a vintage bass with a .1uf cap) and the filtering by the .02uf cap/4.7k resistor is decreased. At the extreme opposite end of the knob range full treble attenuation through the .100uf cap, while still shunting a hair of the signal through the small cap/resistor combination.
In the end, I did not like the audio effect of the 'grease bucket' tone circuit. As I roll off the tone knob on a P bass, I expect to have some of the highs muted, but I also expect for the tone to get darker and more thumpy. The 'grease bucket' does not do that. As you roll off the treble, there is very little change in the low mid and bass quality of the tone.
As a tone circuit, the 'grease bucket' is more advanced and does the job of reducing the highs without affecting the rest of the tonal spectrum, exactly as intended. It does a better job of cutting only high frequencies, due to it's design.
However, the more 'primitive' single cap tone circuit is the sound most of us associate with a P Bass. I want to hear the other frequencies and overtones attenuated along with the highs...that's what I'm used to from a P Bass.
It's kind of like the story I was told about butter and margarine. Apparently, during WWII butter was rationed and the use of oil based margarine as a substitute became very common. A whole generation of kids grew up eating margarine, many never tasting real butter until they were much older. They found that they preferred the taste of margarine to that of real butter! What we are used to has a determined effect on what we prefer later in life.
I wanted more of a traditional tone, so I replaced my electronics entirely, thereby removing the 'grease bucket'. I put a Fender Original '62 pickup in and loved it for a purely vintage tone. It sounded a little too polite for me. I wanted a little more clarity and bite, so I went with a Lindy Fralin and a single .47uf cap for the tone. I'm perfectly happy with my sound. The only 'improvement' I might ever try would be to have the pickup overwound about 5% for a little more output and low/low-mid fullness, but I'm not in a hurry, if I ever even do it. I would have to already have the bass out of commission or the pickup out for another reason to even consider it. Getting a pickup tweaked just right is an art...I've got it there and don't necessarily want to f**k with it.
Scott
(which is of course a terrible thing). It definitely pushed the mids, reduced the thump factor and made the bass sing more than growl. 
