Tone knob kills volume. Why?

[Pet Sounds]

Here's a weird problem: When I turn down the tone knob on my 1975 P-Bass, the volume drops off radically (but not completely), even though the volume knob is all the way up. Is this a wiring problem? Is the bass-side pickup dead? What's going on here?

[abemo]

Can you post pics? It sounds like a miswire, with the tone almost acting as a second volume. There may be a partial ground somewhere.

If the pickup was going out it would affect the entire frequency range, not just the lows. That said, what size cap are you using? If its too big, it could be bleeding off too much.

[Pet Sounds]

I just bought the bass at a pawn shop, and they only had a crappy little practice amp to play it through, so the problem wasn't apparent until I got it home. Haven't taken it apart yet.

[FranF]

Wrong value cap would do this too. P Bass should have a .050 cap between the center lug of the tone pot and the pot body. I just took a .100 cap out of a Mustang Bass circuit that cut the volume by 80 percent. Replaced it with the .050 and it's perfect again .

[tbplayer59]

Bad tone pot? Possibly shorting the signal directly to ground instead of through the capacitor.

[Meddle]

In the worst scenarios this is indicative of a broken wire in a pickup coil.

[joelb79]

Originally Posted by FranF

Wrong value cap would do this too. P Bass should have a .050 cap between the center lug of the tone pot and the pot body. I just took a .100 cap out of a Mustang Bass circuit that cut the volume by 80 percent. Replaced it with the .050 and it's perfect again .

This. I bet you its a .1 uf Cap. Use a .05 uf and watch the volume return.

[thehangingmist]

Happened to me when i wired one of my pickups to the tone pot mistakenly.

[Pet Sounds]

Thanks for the input, guys. I'll let you know what it is when I get it figured out.

[Pet Sounds]

Chapter Two: Armed with your suggestions, I took the bass in to my local Fender dealer. The guitar guru there, who is very experienced, had a look under the hood and said I have a mixture of ALL the problems you guys mentioned.

He said the tone pot was badly corroded and replaced it. Then he put in a .022 cap. I came in and played the bass, and it may have been a LITTLE better. But the perceived volume still drops off when you roll back the tone knob. And the sound is extremely harsh when the tone knob is on full.

The output of the bass is terribly weak when compared to another P-Bass in the shop (an old Hot Rodded P with the J pickup turned down) through the same amp.

Based on the low output and harsh tone, I elected to have him order and install a Fender '60s RI P-Bass pickup set. Depending on how this works out, I may have him reinstall the original cap, too. He said the .022 may give me a fuller range of tones. Does this make sense to you experts?

[tabdog]

Use a 0.047uf cap with 250k pots. (not 500k pots)

Tabdog

[Cadfael]

Maybe the strings are totally wrong for "the heart of YOUR bass"???

I have 7 basses. Some like brand/type A, others hate it!

You have an original 1975 P-Bass?
So, the body is not made of alder like the 60s, but made of ash?
A friend of mine has a 1974-76 P-Bass with ash body and he has d'Addario HalfRounds on it. They give you the punch of flatwounds but sound a bit more modern.

There should have been a 250k pot in the bass - and it should have been replaced by a 250k pot. The "size" of the pot can influence the harshness. That's why tabdog wrote "not 500k".

Did you take the old pot home???
If not, ask if he still has the pot and take it!!!
This pot may prove when your bass was built! There is a stamp on the pot which dates the week of production - and often says more than serial numbers! Keep it in a plastic bag! If you might sell the bass somewhen, this pot "tells a story" to every Fender expert.
If the volume pot has to be changed, keep it as well! Same with the capacitor and everything you remove (especially the PUs)!

Try the original cap when the new pickup is built in.

[SGD Lutherie]

Originally Posted by Pet Sounds

The output of the bass is terribly weak when compared to another P-Bass in the shop (an old Hot Rodded P with the J pickup turned down) through the same amp.

Is it also kind of thin sounding? I'd say you have an open coil on the pickup. You still get sound because the windings are capacitively coupled. As Meddle said, that makes the tone control act like a volume control.

[Meddle]

Thin sound could also be because of 500k pots (not actually the result of anything missing, but a brighter sound that is mistaken for lacking in bass) or... a degauzed pickup?

I had a telecaster partscaster that took a knock at some point. The magnet actually fell off the back of the pickup. That sounded really thin!!!

[SGD Lutherie]

Originally Posted by Meddle

Thin sound could also be because of 500k pots (not actually the result of anything missing, but a brighter sound that is mistaken for lacking in bass) or... a degauzed pickup?

I mean "thin" as in lacking low end. Otherwise it's "bright". Capacitively coupled coils often lack low end because the low end is being filtered out due to the capacitive action.

Degaussed pickups are weak and dull sounding, as in lacking top end.

[Pet Sounds]

Yeah, it's very thin sounding as well as weak in output.
Cadfael, I'm keeping all the parts. The pot was made on a Thursday in October 1973 and the neck is early '74, so the bass is a '74, not a '75 as I first thought.
As a side note, the bass appears to be all original, but it sports lollipop Fender tuners. (Yeah, I know they're typically found on '60s Jazz basses.)

[SGD Lutherie]

Originally Posted by Pet Sounds

Yeah, it's very thin sounding as well as weak in output.
Cadfael, I'm keeping all the parts. The pot was made on a Thursday in October 1973 and the neck is early '74, so the bass is a '74, not a '75 as I first thought.
As a side note, the bass appears to be all original, but it sports lollipop Fender tuners. (Yeah, I know they're typically found on '60s Jazz basses.)

It's very common for old Fender pickups to die like this. The coil corrodes where it touches the magnets. I recently rewound a '76 Jazz set that had the same issue.

You can have it rewound to factory specs. That's better than a replacement pickup.

The pickup will have a number stamped on it in ink. The last two digits are the year, i.e., "74".

[Pet Sounds]

Thanks. I may indeed have it rewound. Send it to Lindy Fralin?

[SGD Lutherie]

You do need to have it rewound. It has a break in the coil. If you try and get a reading off of it with an ohm meter it will show an open circuit.

Lindy does a good job, as do a bunch of other winders.

[Pet Sounds]

Just checked out your site, David. It looks great. I may be sending a couple of pickups to you. Thanks for the advice!

Sorry for being such a noob about SGD Lutherie. I'm not that familiar with pickup experts, obviously, so I just threw out the one name I knew!