How to tell what is broken with my bass guitar

Question

I have a weird problem with my bass guitar and I wanted to have some sort of background knowledge before I bring it in so I understand what the repairman is saying and/or so I don't get taken.

I have two bass amps. A 350W "performance" amp and a 10W practice amp. When the bass is plugged into the 350W amp, the sound/notes it makes is very, very quiet, even when I turn things all the way up on both the amp and the bass. I should note, there is a passive and active port to plug the cable into and this is happening for both ports, although, most of my initial testing was with the passive port.

I tried 3 different batteries (new and tested to be strong) but the sound is still very low.

I borrowed 2 other bass guitars from friends and they sound nice and loud on the 350W amp so I think the amp is good.

I plug in my bass into the 10W amp and it's nice and loud (or as loud as can be on 10W). I also plugged my bass into my friend's 50W keyboard amp and it also sounds nice and loud (for a 50W amp).

With all this said, what might be wrong with my bass since it seems to work on some amps but not others? I haven't taken the bass apart yet (don't trust myself) so I don't know if I'll see something obvious (like a burned out component that's burned brown). What should I keep my ears out for when the tech is finished examining it and tells me what needs repair?

EDIT: My bass is a Ibanez SR405QM 5 string guitar.
The troublesome amp is a SWR Black Beauty 350W 15" amp.
The practice amp is a Crate BT15. My mistake, it's not 10W, it's 15W.

One friend's bass is a Yamaha RBX765A 5 string guitar. It also uses a battery. I forgot the other friend's bass b/c I had to return it =(

Answer 1

Oh, I've just read “I've tried 3 different batteries” – this suggests your bass is active, so the following may not be relevant for you. I'll leave it anyway, just in case:

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This is strange, but it almost certainly has something to do with impedance.

What I'd suspect is actually this: you bass is passive, but your friend's is active. If so, then the issue is probably that your 350W amp has, for some reason, a low impedance on both inputs. Normally the “passive” input should have high impedance (something between 100 kΩ and 2 MΩ), which is needed because passive guitar/bass pickups are pathetically unstable, electrically. The “active” input can have a much lower impedance (something between 5 kΩ and 50 kΩ, which is technically preferrable for a variety of reasons), because active basses give a properly stable signal.

There are two ways you can find out if that's really the case:

• Plug your bass in via some effects pedal. This should stabilise the input, regardless of the amp's impedance.
• Measure the amp's input impedance (i.e. resistivity) with a multimeter. As I said, an input labelled “passive” should have an impedance >100 kΩ (indeed for a good bass amp I'd expect >500 kΩ). If it less than that, something is wrong with the amp's input stage.

You could either have that repaired or make the bass signal stable, to circumvent the problem:

• Passive electronics are bogus, especially for bass. Even a simple decoupling circuit in the bass eliminates such problems, makes cable noise much less of an issue, prevents cables from changing the sound, and likely will make the bass a lot crisper and brighter. (If you don't like the treble you can always kill it later, much easier than doing the opposite.)
• Alternatively, especially for transistor amps, it can make sense to permanantly have a tube preamp before the amp's own preamp. E.g. the Presonus TubePre works great in this role. Of course there are also dedicated bass preamps, these are usually more aimed for really notable distortion.

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Now, going after that batteries comment, apparently your bass is active, so impedance shouldn't be an issue. ...Well, it shouldn't be, but unfortunately not all active circuitry is properly designed. Perhaps the circuitry has a too small capacitive output coupling; in this case it would be just as unstable as a passive bass, but sound much worse if the impedance is too low: a passive bass loses its treble if you operate it low-impedance, whereas a badly designed active one will lose it's bass, which is of course completely unacceptable for... well, bass. If some idiot used a 50 nF output capacitor then the bass will sound good with a 500 kΩ input, but horribly tinny even with a 100 kΩ input (which a passive bass could still cope with). If the 350 W amp has a 100 kΩ input, then this is perhaps it.

A completely different possible cause for the problem is if for some reason you're using a stereo (TRS) cable. Stereo cables are unsuitable for most active basses, because they use the ring as a “power switch”. If you use a TRS connector, the ring will just sit at -9 V and the active circuitry gets no power... unless the amp connects ring and shaft, which some input jacks do but others don't.
Probably this isn't the case for you either, but if you are using a TRS cable try replacing it with an ordinary TS one.