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

[Kenrod]

i was off track early on making some novice mistakes regarding Peak vs P-P vs RMS voltage.

i got back on track and readdress the same issues found at test points around U5 (IC5) see post #35 copied below

ok, I give it about 480mVpp (about 170 RMS correct?)

This time all the Test points are still on with the schematic values (after I took all p-p readings and multiplied X .3535, correct?) until you get to U5. TP12 (pin 1) reads 1.1V RMS (SB:800mV) TP10 (pin 7) reads really low and odd looking waveform with a spike. The base waveform is about .4V p-p with the spike up about another 1.0 V (scope on 0.5V/div). Not sure how you calc that, but that's not going make 2.0V RMS. I have 37V RMS at TP11, whether measured on my scope or meter.

I'm not sure what's happening with U5 or my higher than normal output voltage.[ATTACH=CONFIG]37943[/ATTACH]

Not so fast, what about TP3-8-4-5-6-7-12?

They tested fine so omitted them and only listed the discrepancies.

Now that I'm sure I'm putting in correct test signal, i don't at all mind to go back and recheck every test point and list them in order, listing spec and actual value

[Kenrod]

all test values in summary:

Test equipment: ExTech 430 True RMS Multimeter, Leader 1021 20 MhZ scope
Test tone: 1k hZ sinewave at 168mV from iphone 6 (Function generator app)

Test Point_____Spec_______Meter______Scope
1 ___________ 700mV_____704mV______742mV (2.1 Vp-p)
2_____________3.8V_______3.932V______4.03V_(11.4 Vp-p)
3_____________450mV_____468mV______495mV_(1.4 Vp-p)
4_____________450mV_____446mV______467mV_(1.32 Vp-p)
5_____________250mV_____283mV______297mV_(.84 Vp-p)
6_____________1.15V______1.347V______1.38V_(3.9 Vp-p)
7_____________1.15V______1.355V______1.39V_(3.93 Vp-p)
8_____________1.1V_______1.127V______1.18V_(3.35 Vp-p)
9__N/A____________________________________I cannot find this test point on the schematic
10____________2.0V_______106mV_______42mV_(120mV p-p) This is the small waveform with the spikes in it
11____________26.3V______37.0V
12____________800mV_____1.112V_______1.17V_(3.3 Vp-p)

**EDIT: nothing lined up as nicely as I typed it. Why? I tweaked to make it a little easier to read

[J M Fahey]

Thanks.
Most values are fine, within 10% of what's expected.
I had asked remeasuring TP3-8-4-5-6-7-12 because we didn't agree on Vpp to Vrms conversion, so by definition they were suspect.

The weird spikey signal on U5-7 does not surprise me at all, in fact I always see and expect funny waveforms there, because that Op Amp is *inside* the NFB loop, so it will desperately try to compensate for any non linearity at the output.

So even a slight dip at the output, for whatever reason, will produce there an exaggerated opposite signal to push the output into doing whatever is expected.

I bet the limiter does not allow it, but if output stage could clip, you'd definitely see a very weird signal there.

Only thing that slightly worries me is having a lower than expected value there but maybe it's a measuring error (just check it once again) or will take the proper value when you load the power amp.

As of TP11: "Houston, we do not have any problem" ........
37V RMS means very healthy 170W@8 ohms and 340W@4 ohms

Of course, such is an unloaded voltage.
By rule of thumb I'd expect some 120W@8 ohms and close to rated 220W@4 ohms , what Ampeg claims.

FWIW my own B300 Bass amps put out ~200W@8 ohms and ~305W@4 ohms with just +/- 63/65V rails so you know what can be achieved with those raw voltages.

Only difference is I use a very good very stiff low resistance power transformer, I maximize copper to the hilt (sometimes I have problems fitting EI laminations because of the very fat bobbin) so my losses are lower, but with your values 220W@4 ohms looks very doable.

Please connect an 8 ohms and later a 4 ohms resistor to the output (a speaker will be unbearable, as you already checked) and measure power output.

Amp *seems* to comply with schematic TP values.

Its input may be slightly hard to drive so giving the impression of less power, but if it's there ... it's there.

Personally I design for weak bass pickups, poorly adjusted Bass guitars, dead strings, so choose 50mV or at most 100mV input sensitivity, 170mV seems more in line with a very healthy Bass, but that's not a big point.

The real one is how does it compare, driven flat out and preferrably in a rehearsal studio or live, with a drummer playing nearby, to the 20W and the 80W amps.

When you play alone, you don't have a real sound reference, and knob numbers seem to mean more than what they actually do, but playing besides a drummer separates the men from the kids.

EDIT: misaligned columns?
I HATE that.
Forum software tries to "help" you by erasing empty spaces (so you have to use dots or lines to keep space "not empty" ) and to boot character spacing is not uniform (again for efficiency) so an I is assigned a narrower space around it than O , P , M , or W .

Old mechanical typewriters assigned same space for everybody , so probably a "monospaced" typeface such as Windows Courier which imitates it may do .

Let me try that theory:

Regular preassigned Forum fonts:
IIIIIIIIII
MMMMMMMMMM

Windows Courier New (imitates mechanical typewriters)
IIIIIIIIII
MMMMMMMMMM

Windows Fixedsys (imitates real OLD text only Green or Orange Monitors)
IIIIIIIIII
MMMMMMMMMM

SUCCESS!!!!!!

[Kenrod]

The real one is how does it compare, driven flat out and preferrably in a rehearsal studio or live, with a drummer playing nearby, to the 20W and the 80W amps.

This is why i resurrected this thread after almost a year. i took the amp to band practice a while back and after 10 seconds said "something's bad wrong here", unplugged it and started working on it again. It's still not better yet, but I'm sure we'll arrive at something.

I can keep talking about my opinion and experiences, but ANYONE would be severely disappointed at the output of this amp.

[g1]

What's strange is that the test point measurements look good up to the CD input jacks. This implies the front end is proper. And it sounds good when you use the CD input jacks. This implies everything after those jacks is proper. So the implication is is should sound proper .
Maybe some error on the test point values before the CD jacks? Maybe something funny with that mute switch?

Edit: can you hook it up to the crate again and measure what the signal voltage is at the CD jacks when you get loud volume out of the amp?

[Kenrod]

It really seems like an input sensitivity problem, although i suppose a parasitic problem could happen anywhere along the path.

I tested my Lakland 5 string (~100mV signal) into the 0dB input and it drove it pretty well, although technically I "should", and normally would, use the -15dB for an active bass.
When I put it in the -15dB input, it dropped off to unacceptable levels, just like a passive bass in the passive (0dB) input.

I believe I reported somewhere in a previous post that I can drive the amp to acceptable levels with a 170mV test tone from either the regular (0dB) guitar input or the CD input.

The problem being, most basses, especially passive, won't come near that level. Seemingly not a problem in most other amps.

[Kenrod]

What's strange is that the test point measurements look good up to the CD input jacks. This implies the front end is proper. And it sounds good when you use the CD input jacks. This implies everything after those jacks is proper. So the implication is is should sound proper .
Maybe some error on the test point values before the CD jacks? Maybe something funny with that mute switch?

Edit: can you hook it up to the crate again and measure what the signal voltage is at the CD jacks when you get loud volume out of the amp?

The CD input is on the Ampeg and I used a 170mV signal. You said "crate" did you mean Ampeg?

[g1]

No, I meant when you were using the line out from the Crate into the CD in of the Ampeg (post #89).
What level did it take at the CD input to be acceptably loud?

[Kenrod]

ok, i understand.

As I recall, I still had the Ampeg master at "10" and had the pre and master on the Crate at various settings and could tell that I had more than enough signal to drive the Ampeg power amp to seemingly full power (quite loud, moving a lot of air at the rear ports, etc)

So many variables at play there (Master of Ampeg, Pre and Master of Crate), i'm not sure what you might suggest to set a baseline, so please do.

[Jazz P Bass]

With that 'odd' input signal test point on the schematic, how about putting a boost pedal before the beast & calling it a day.

[Kenrod]

With that 'odd' input signal test point on the schematic, how about putting a boost pedal before the beast & calling it a day.

Although that might accomplish it, I hope you're kidding. i want it to work as designed.

[Jazz P Bass]

Actually, I am not kidding.

I have worked on these amps before and I have never been impressed.

All voltage measurements that you have made indicate, to me, that the amp is working properly.

You simply need a huge input signal.

Why the designers did this, I have not a clue.

[Kenrod]

Actually, I am not kidding.

I have worked on these amps before and I have never been impressed.

I agree in general.

All voltage measurements that you have made indicate, to me, that the amp is working properly.

I mostly agree, except it is far too low on output.

This is what has us all stumped, and why i keep having to give actual comparison after comparison to continue to seem to have a valid complaint.

As an example, a bass player would/should immediately return this amp for a refund, in this condition. I simply would not expect anyone to dish out a reasonable sum of money for an amp with these specs and be happy about it.

You simply need a huge input signal.

Why the designers did this, I have not a clue.

As a last ditch effort to anyone that believes i should leave well enough alone, that there is no real problem, I have ready access to my old Ampeg BA115 (100w) and will pick it up today for yet another side by side comparison.

Almost identical preamp section (no mute function, no tuner out, CD input is after master volume), uses only 40v rail voltage, has 2 less output transistors, other minor differences.

With what I assume should be a similar input sensitivity due to the almost identical preamps, I figure in lieu of an idential BA115HP(220w) at my disposal, this is as close as I'm going to get for comparisons. I would almost bet my paycheck that the 100w Ampeg is going to be much louder than the 220w Ampeg.

I will also swap the 15" speakers, assuming I still have my paycheck after the first comparison.

I still call myself inexperienced and mostly lucky in the limited amp repairs I have completed.

I have test equipment, willingness to learn, and perform any test you ask, and ears to continue to listen.

Above all, I appreciate the attention my thread is getting and your patience.

Thanks for hanging in there with me.

[Kenrod]

I'm also going to pull the chassis of the 100w amp and look at the waveform at pin 7 of U3B (same as pin 7 U5B of 220w amp)
I suppose I'll check all the same test points, even though not called out on the 100w schematic.

[g1]

On some specs for the BA115HP, I saw gain listed at 46db input to speaker (tones at 5).
I have no similar specs for other models, but perhaps some others here could say if that is comparable to other amps, or low gain.