Bias on Ampeg BA115 too low cause occasional random-ish noise?

[J M Fahey]

As a last ditch effort to anyone that believes i should leave well enough alone

Nobody said that, but that we find it performing to Factory specs, so it is not "broken" nor needs to be "repaired" , there are no "bad parts" there and I'm quite certain that if you send it to Crate/Ampeg for repair, they will return it with a "no fault found" report.

But you don't like its sensitivity , as Factory set.

Cool, then let's not repair but *MOD* it.

a) replace R12 from 22k to 47k

b) replace 50k lin P1 by 25k lin

c) replace 10k R67 by 22k or 47k, depending on desired chili heat

d) add a 4700pF cap in parallel with R65 to make that input worth using by an active Bass. Ceramic is fine.

e) replace R36 by 220k and set rotary selector switch to that position.

Apply these one by one, all at once may turn this Ampeg into a Bass amp as hot as a Fahey, not kidding, so do it only until satisfied with your Bass sound and sensitivity.

Too much and your Bass may distort or hiss may become objectionable.

[Kenrod]

Nobody said that, but that we find it performing to Factory specs, so it is not "broken" nor needs to be "repaired" , there are no "bad parts" there and I'm quite certain that if you send it to Crate/Ampeg for repair, they will return it with a "no fault found" report.

But you don't like its sensitivity , as Factory set.

Cool, then let's not repair but *MOD* it.

Do you agree?
If so, let me have a look at the schematic (I closed that Acrobat window) and suggest some change.

Agreed on all points, it just took seemingly several pages of posts to convince anyone that its' severely under performing, even for an Ampeg.

If it's not "broken" as I suggest (after all, even I can drive the thing to very loud volumes with an input signal that most basses, even active, cannot produce), I'm not at all opposed to mod it to gain it's full potential.

[Kenrod]

Nobody said that, but that we find it performing to Factory specs, so it is not "broken" nor needs to be "repaired" , there are no "bad parts" there and I'm quite certain that if you send it to Crate/Ampeg for repair, they will return it with a "no fault found" report.

But you don't like its sensitivity , as Factory set.

Cool, then let's not repair but *MOD* it.

a) replace R12 from 22k to 47k

b) replace 50k lin P1 by 25k lin

c) replace 10k R67 by 22k or 47k, depending on desired chili heat

d) add a 4700pF cap in parallel with R65 to make that input worth using by an active Bass. Ceramic is fine.

e) replace R36 by 220k and set rotary selector switch to that position.

Apply these one by one, all at once may turn this Ampeg into a Bass amp as hot as a Fahey, not kidding, so do it only until satisfied with your Bass sound and sensitivity.

Too much and your Bass may distort or hiss may become objectionable.

Alright, now to get started. Thanks

[g1]

And please let us know if your other BA115 has the same issue.

[Kenrod]

And please let us know if your other BA115 has the same issue.

It seemingly does, but I blame it on Ampeg at this point. This BA115a also suffered an obvious loss of treble recently. I'll want to dig into that as well. I suppose I'll start with the piezo tweeter. Are they supposed to be very reliable, or somewhat sketchy or what ?

The 80w Crate still kicks the crap out of both these amps. :-/

[Kenrod]

i finally caught a little time and made the first change, swapping R12 from 22k to 47k. It got a little louder, but it's still not quite acceptable so i'll order a 25k linear pot to swap in.

I still need to check out that bad crackle from the tweeter. I think i'll look at (scope) a test signal coming into, and through the crossover and see where it gets funky.

[Jazz P Bass]

A 'crackling' tweeter usually indicates that it is "toast".

[52 Bill]

I think that the tweeter was replaced earlier. It seems to me that the spike that was shown in the one oscilloscope photo may be the cause of the crackling noise.

[Kenrod]

True. I replaced the tweeter diaphragm last year for the same reason, although I don't remember if it was this bad.
I still have the old one as well.