Bartolini preamp in Tobias Growler

[Aryeh Barson]

I've got one of the older Tobias Growlers, with the single volume, balance, two stacked bass/treble controls and two toggle switches. My understanding is that the pickup and EQ are from Bartolini, and it's a model that wasn't used on any other production bass.

My problem is that this bass no longer has the same power to it, no "oomph". The controls all operate as they are supposed to, but the whole thing just lacks the powerful output it had when I bought it. I've replaced the batteries and changed strings (obvious and simple), but it didn't do much. Anything else I should be checking?

Does anyone have a wiring diagram for this rats nest? It might give me a clue what to check, even though the controls are working.

If that fails, does anyone have a good replacement recommendation? The less modification to the bass, the better.

[Greg Robinson]

Does the preamp have a model number printed on the circuit board or something? That would help us track it down more easily.

[David King]

I believe what you have is probably a Bartolini DNTBT-18, it has three black epoxy encased modules?
The growlers came in all sorts of configurations with different controls.
If the volume is down you might check the internal gain settings (small blue trimpots inside the cavity). Some of these came with 4 trimmers, two for gain and two for midrange 1 ea for each pickup. Plug the bass in and start twisting to see what happens. It might be smart to mark the current locations first
You may have a short between the shielding paint and one of the pots or the jack. If your cavity paint is carbon black it can have just enough resistance to suck your power without killing off your signal completely.

[Aryeh Barson]

Full name: Aryeh Barson
Thought I'd gone through this before, but no biggie!

[Jim McConkey]

Fixed now. Your full name was not entered yet.

[David Schwab]

How does the battery clip look? Is it oxidized? Same with the output jack.

[Aryeh Barson]

Yes, it is a DNTBT-18. Three epoxy modules, two with dual trim pots. Any idea what each one does?
Shielding is copper tape, not paint. Multiple wires are soldered to the shielding in numerous locations. Shouldn't they all go to one point?
Rechecked the batteries and found both of them to be below 7 vdc. No oxidation anywhere. They're only a couple of months old, and I've been careful not to leave my cord in the jack when I'm not playing, so I'm surprised that they drained so quickly. I poked around looking for a short to ground that might cause this and didn't find anything. Any other ideas?

[Greg Robinson]

7V is considered end-of-life for 9V batteries. Have you tried replacing them?

[Aryeh Barson]

Ha! Low battery voltage is much too obvious!
But seriously, I did install brand new Duracell's and it didn't make any difference. That's why I continued to look for a voltage drain (short) somewhere in the circuitry.
On an alternate forum, someone mentioned pickup height. Anyone have any guidelines for this?

[David King]

Barts can be mounted quite close to the strings as they generally have very low Gauss levels on top. I'd go for 1/8" (3mm) for the bridge PU and 5/32" for the neck or even less if you don't play hard.

The cavity shield is considered a single point since it surrounds the guts and has very low resistance so unlikely to induce any V offset between any two points.

You should definitely check the power consumption with an ammeter. It should be between .8 and 4mA probably around 3.2mA -depending on the components Bill chose for the job. That's high for a bass preamp but it doubles because you are running 18V and again because you are running two parallel preamps. Seven Volts is nothing, that's a dead battery.

A typical alkaline 9V has a capacity of 560mAh so you should get about 150-200 hours (a NiCad 9 V has only 180 mAh -50 hours at best)

So test for current draw at the battery clip between + and + with your red meter lead on the battery side and black on the clip side. With the bass unplugged you should get 0mA, with it plugged in you should get something measurable.

Read my earlier post about the blue trip pots.

[Aryeh Barson]

Resurrecting this thread. Here's where it stands.

1. Pickup height adjusted as recommended.
2. Batteries replaced - negligible difference.
3. Ammeter used to make sure that
a) no current is being drawn when the cord is not plugged into the output jack, and
b) no more than 4 mA are drawn when it is plugged into the jack.

How do I further troubleshoot whether the problem is with the pickup (low output from the pickup into the preamp) or that the preamp is kaput, even after I've checked the current draw?

Neither Bartolini or Gibson/Tobias are any help. I'm on the verge of giving up on this bass, but right now it's the only one I own.

[Joshua Levin-Epstein]

How about bypassing the preamp? Do the pick ups sound decent "dry"? If the pick ups are good, run the thing passive or replace the preamp with something new.

[Greg Yancey]

Keep in mind this is a weird preamp and uses a bipolar(+-9V) power supply. If I recall the switch for the battery that runs to the ring connection of the stereo jack is not directly hooked to the batteries. It probably is the gate connection for some internal FET's to disconnect the batteries from the circuits, but even then it would just be a very high impedance connection, not a true disconnect. When measuring current I'd do it for each battery (but with both in circuit though) in the system to see if the current draw is the same. Between system + and battery 1+, then between system GND and battery 2+.

There's probably a reason Bartolini doesn't make +-9V preamps anymore...