Looking for Help with Tobias - Bartolini TCT Wiring

[bucephylus]

I have a Hollywood era Tobias Signature 5. It is a very nice bass; but, I have struggled with the tone that the preamp produces. There are two issues. First, the gain in active mode is VERY low. Second, the active treble boost seems decades less than the Mids and Bass. It almost seems like Mike wired this particular preamp for a studio cat. I'm trying to figure out if I can move the wiring around to get it back to a more standard behavior; and would hugely appreciate any insight the techies here might be able to lend.

Here is the standard Bartolini TCT w/MCT wiring diagram for reference:

Here is my as-built sketch for the circuit in my particular bass:

The stacked blend pot(s) is labeled 500kohms, which I am showing. The Vol, Treble, and Bass pots are all 440 coded, indicating that they are all 500kohm pots. The mid pot seems to have the MCT module soldered to it, and is not labeled; I am presuming that was a stock MCT part from Bartolini at the time.

Here are the things that I see being different.
1) There appear to be grounding networks on the active circuit switch. In particular, the RC network is a question for me.
2) The Vol pot is a 500kohm pot vs the 25kohm pot Bartolini shows. This alone may explain why the output gain is low; just guessing.
3) The MCT is wired to the TCT via the Orange (variable) wire rather than the brown wire, and the brown wire goes to the Mid pot tap rather than orange (variable).
4) The Mid pot has no resistors between the taps.
5) The Blend pot is 500kohm vs 250kohm.
6) The blue wire running from the Treble pot to the Bass pot connects to only the center tap of the Bass pot; there being no jumper from the center tap to the outer tap on the Bass pot.

I think that is pretty much it; the rest being pretty much as Bartolini shows. Any feedback to explain the low output gain and nearly non-existent treble boost would be greatly appreciated.

BTW, I intend to send this same information to Mike and see what he says.

 

[JustForSport]

Couple things:
looks like a treble-bleed circuit of the a/p switch,
another resister to keep the highs,
the vol and blend, at 500k helps keep the highs avail.

Maybe with the 500k vol and bal pots plus treble bleed circuit, there's not much left for the treble pot?

Some Barts (pickups) are known for low vol output, and I don't see a gain pot on your preamp, so maybe that's why the overall output is relatively low.

 

[bucephylus]

Couple things:
looks like a treble-bleed circuit of the a/p switch,
another resister to keep the highs,
the vol and blend, at 500k helps keep the highs avail.

Maybe with the 500k vol and bal pots plus treble bleed circuit, there's not much left for the treble pot?

Some Barts (pickups) are known for low vol output, and I don't see a gain pot on your preamp, so maybe that's why the overall output is relatively low.

Thanks. Pretty much along the lines I was thinking. I suppose I could just rip all that stuff out and wire it the way the Bartolini diagram shows.

OTOH, I can wait until I hear back from Mike. No hurry and I'd rather do the right thing here.

I still find it curious that the MCT seems to be wired backwards. But, what do I know?

Anyhow, thanks for reading and considering the questions.

 

[JustForSport]

OTOH, I can wait until I hear back from Mike. No hurry and I'd rather do the right thing here.

There could very well be a reason for the choices made, due to components such as the particular pickup model with that body/neck wood combo.

I still find it curious that the MCT seems to be wired backwards.

Thought I had a diagram for it somewhere, but didn't find it. Depending on the components inside, it may not matter which way it is.

 

[bucephylus]

There could very well be a reason for the choices made, due to components such as the particular pickup model with that body/neck wood combo.

Thought I had a diagram for it somewhere, but didn't find it. Depending on the components inside, it may not matter which way it is.

Exactly.

I'll post up when the Maker gets back to me.

 

[JustForSport]

A couple Bart PDFs you may find useful (or not):



Would a TC Booster maybe help with the signal strength? Ask the maker?

 

[bucephylus]

A couple Bart PDFs you may find useful (or not):

Would a TC Booster maybe help with the signal strength? Ask the maker?

Thanks. I do have those.

Output strength is something I can work around, so, not a deal breaker. I'm more concerned about the voicing. As it stands, it produces a tone that is very close to what an engineer would EQ for on a recording. However, that tonality is also not particularly useful in live mixes. Kind of a curious thing. I do more live gigs than recording dates. Ergo my concern about the voicing design.

 

[bucephylus]

Bit of an update on this one.

I've had several email exchanges with Mike Tobias, who BTW has to be one of the finest gentlemen working in this field today. Mike couldn't comment on this particular case, too old of an instrument, but suggested I try to contact TB'er BassLand (Bob Lee) who installed many of these circuits; and Mike contacted Bartolini, the result of which was the following response:

"The wiring looks okay. It just has an active/passive switch and some anti-pop passives. I’d verify that the Volume is a 250K or higher. If the AP switch was an add-on, and it still has a 20-25K volume, that will squash the treble from the pickups.
I suspect the capacitor on the active/passive switch is not what he thinks it is, but if it is even close, the treble would be shot. I would start with removing that capacitor. The difference will either be nothing or very dramatic. Probably nothing – a 100uF X5E dielectric would be quite large. It is probably a 10pF.
Assuming the difference is nothing, I would replace the MCT. That one has almost no treble boost. An MCT-250 would be a good choice. If that doesn’t do it, I would replace the TCT."

So, they pretty much see the same things we have discussed.

One other thing I noticed last night, and I was a bit surprised Bartolini did not comment on, is that the volume pot (and low pass filter network) are upstream of the TCT circuit. The Bartolini diagram clearly shows the volume pot being downstream of the TCT module. So, what this circuit is doing is both cutting signal and only passing lows to the TCT. That definitely doesn't seem right to me.

As to Bartolini suggesting I didn't read the ink on the capacitor correctly, I'll just take that as indication of incredulity that someone would put that into the circuit.

OTOH, while I don't prefer having the highs attenuated ALL the time, I will say this circuit produces a VERYstudio friendly tone, which is by far the woodiest tone I have ever heard. So, I am actually giving some thought to using the AP switch to the switch that RC network in and out of the circuit vs having a passive state that I never use. Food for thought. I probably won't get the soldering iron warmed up until this weekend; but lots to work with here.

 

[bucephylus]

Here is the wiring modification I intend to start with this weekend. It allows me to switch that RC network in and out of the circuit, and puts the Vol pot after the TCT. Haven't decided about that 100kohm output grounding resistance; probably will try without it. All I lose from the original circuit is the option to go passive; which I never use. Small steps and see what happens.

 

[bucephylus]

After looking at this change, it seemed so minor that I went ahead and got out the soldering iron and went after it.

Look Out!

Totally fixed the bass. Almost like lifting a blanket off of the amplifier. The interesting thing to me is that even with the more full spectrum tonal controls, the bass still retains the woody cajones that is Mike's trademark. Big success!

 

[JustForSport]

Interesting replies- thanks for sharing.

I didn't know there was an MCT that wasn't an MCT 250/375/500.

Edit- missed posting this earlier.

 

[JustForSport]

What value vol pot do you have after the pre?

 

[bucephylus]

The Vol pot is 500k.

 

[JustForSport]

Usually a 50k pot is used after the preamp, but I guess a 500k is needed with the a/p switch. Hows the sweep when in active mode?

Wouldn't it be better to have the vol between the bal pot and a/p switch, which puts it ahead of the preamp in active mode?

 

[bucephylus]

Usually a 50k pot is used after the preamp, but I guess a 500k is needed with the a/p switch. Hows the sweep when in active mode?

The Bartolini diagram (that I did not show) for the AP option shows a 250kohm Vol pot with the switch, instead of the 25kohm pot. So, you are correct that they want the higher impedance pot with the AP switch. For whatever reason, Mike's shop just used the same pot that they were using for the Treble and Bass pots - a 440 series 500kohm pot - and called it good enough. What do you mean by "sweep when in active mode?"

Wouldn't it be better to have the vol between the bal pot and a/p switch, which puts it ahead of the preamp in active mode?

Right. That is clearly the way Mike's shop had it figured, because that is the way it was done. But, the Bartolini diagrams all show the Vol pot following the TCT. And when you think about it, all the Vol pot does is attenuate signal. If you put it in front of the TCT, then the TCT has less signal to work with. So, it does make sense to me that the Vol pot ought to follow the TCT, thus allowing the EQ network in the TCT to operate with full signal. And, that's the way Bartolini has it.

 

[bucephylus]

Interesting replies- thanks for sharing.

I didn't know there was an MCT that wasn't an MCT 250/375/500.

Edit- missed posting this earlier.

In reality, I have no idea what this MCT even uses as a center frequency. The module has no markings at all. Must have been a very early version. There are also no trim resistors (27kohms and 47kohms), which Bartolini calls for to obtain correct biasing on the Mid circuit. This does concern me a bit; and I may yet experiment with these.

 

[JustForSport]

What do you mean by "sweep when in active mode?"

Sometimes a high yalue pot will have all the 'action' at one end of the sweep, like from 1 to 3, instead of spread out over the full rotation avail.

 

[bucephylus]

Sometimes a high yalue pot will have all the 'action' at one end of the sweep, like from 1 to 3, instead of spread out over the full rotation avail.

Gotcha. I must confess that I generally only use the last 35% of the taper. I looked up this particular pot. It is a very common type, and has an audio taper. So, that would explain the way I am using it. A linear taper would probably make more sense for a Vol pot; but, that isn't what I have. No big deal.

 

[JustForSport]

I was curious-
some say that a linear vol pot is the way to go,
but I have basses with OEM audio vol pots and they work just fine.

 

[bucephylus]

I was curious-
some say that a linear vol pot is the way to go,
but I have basses with OEM audio vol pots and they work just fine.

I think that is correct. In a perfect world, the Vol pots would be linear. That provides a much more even and intuitive gain structure for the user. Audio tapers go between the same endpoints; they just are more progressive, kind of like a logarithmic gain structure. So, they are still usable. Just one of those things.