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

[Kenrod]

Is the tweeter making noises with the alternate signal sources?

Yes, whether pugged into the guitar input or the CD input.
50mV signal (sine wave)
It starts getting noisy around 65hz and quits around 225+hz. In particular, it made noticeably more noise at 105, 155 & 205 hz

[Tom Phillips]

Kenrod,
All your data is based on number settings on gain and master volume controls. None of those knob numbers are calibrated. The absolute number setting is not really all that meaningful on a guitar amp. Especially when comparing different model amps. A good approach is to ignore the knob numbers and just set the controls with your eyes closed. If you can achieve the sound and tone you like at any setting then the amp is most likely OK.
Tom

[g1]

And at 105db I doubt you are full power yet (depending on spkrs), so the higher power amp may still have more on tap.
But like Tom said, the numbers really don't mean much and vary greatly between amps of different models.

[Kenrod]

And at 105db I doubt you are full power yet (depending on spkrs), so the higher power amp may still have more on tap.
But like Tom said, the numbers really don't mean much and vary greatly between amps of different models.

I tested it basically like you said. it seems like a relative & reasonable test to me.

And Tom, at least twice you questioned the fact that I didn't have the master maxed, and when I do, and compare the gain settings, you don't give that any credibility either? now I'm confused

I could make the comparison 2 ways and kinda arrive at the same conclusion. "same" gain & master settings on a low power and high power amp and get the same output level, or max the both out and find the lower powered (by almost a factor of 3) much louder than the higher powered amp.
Admittedly, that's not terribly scientific, but I think a reasonable explanation of the differences I'm hearing.

[Kenrod]

Kenrod,
All your data is based on number settings on gain and master volume controls. None of those knob numbers are calibrated. The absolute number setting is not really all that meaningful on a guitar amp. Especially when comparing different model amps. A good approach is to ignore the knob numbers and just set the controls with your eyes closed. If you can achieve the sound and tone you like at any setting then the amp is most likely OK.
Tom

Let's back up to a previous comparison between amps.

My 20 watt Peavey Microbass (8") speaker puts out almost as much perceived volume, using the same bass, as the 220 watt Ampeg.
With the 20 watt gain all the way up (no master), I have to turn the Ampeg master to 10, and the gain to 6.5 - 7.0 (almost out of adjustment) to match the 20 watt amp

Anybody else see anything wrong with the Ampeg performance?

[Kenrod]

It could just be the nature of the master volume control in that amp. Master volume circuits are simple and un-calibrated. Your description implies that you still have the master turned down. Are you saying that even if you turn the master up (even to 10) that the amp does not produce the loudness you expect?

Yes, exactly what I'm saying.

[Tom Phillips]

...And Tom, at least twice you questioned the fact that I didn't have the master maxed, and when I do, and compare the gain settings, you don't give that any credibility either? now I'm confused...

I admit that it's confusing and it is hard to evaluate without making quantitative measurements. I was just suggesting that the front end gains of each amp were not being compared apples to apples. The actual knob number settings can vary by quite a bit.

...I could make the comparison 2 ways and kinda arrive at the same conclusion. "same" gain & master settings on a low power and high power amp and get the same output level, or max the both out and find the lower powered (by almost a factor of 3) much louder than the higher powered amp...

OK. I agree that if you "max the both out and find the lower powered" (IF you mean both the volume and the master volume) and the lower powered amp is very much louder then their are significant gain differences between the two amps. However, if you mean that the master is maxed but the volumes (or front end gain is set to a different value) then it could still be differences in the pre-amp sections of each amp. The differences could be a malfunction or just different volume pot tapers. It's really hard to make an assessment without playing the amp in person or making absolute measurements on the test bench.

[J M Fahey]

My 20 watt Peavey Microbass (8") speaker puts out almost as much perceived volume, using the same bass, as the 220 watt Ampeg.

You must be kidding.

1) set EQ flat on both, i.e. on "5" on a 0-10 scale.

2) set Master Volume to 10 if available , what basically puts it out of the circuit.

3) set gain to 10 on both.
Amps will not clip, since both have limiters preventing so.

4) set your Bass with volume on 10, any pickup you like, tone fully to the brightest setting.
By the way, what Bass is it?

5) play alternatively plugging into one or the other, same picking strength of course.
Does the 20W amp still overpower the 220W one?

Please answer only yes or no, no qualifications or explaining why you didn't follow instructions 1 to 5 to the letter.

More questions and tests will follow, but only after this basic test is answered, otherwise we are all wasting time.

Before you ask, I'll answer: pot taper can vary a lot and explain a large part of what you perceive, but "all pot tapers are the same on 10" so we are testing under that condition only.

And EQ can vary perceived loudness a lot, neither speakers nor ears are flat (at all) , so we also standardize that too by running all on 5 which roughly is flat response.

Thanks.

[Kenrod]

You must be kidding.

Nope. this is why I gave the example, to continue to assert that my ears aren't lying to me.

I'll perform the test exactly as you state this evening. Thanks for jumping back in.

[Kenrod]

You must be kidding.

1) set EQ flat on both, i.e. on "5" on a 0-10 scale.

2) set Master Volume to 10 if available , what basically puts it out of the circuit.

3) set gain to 10 on both.
Amps will not clip, since both have limiters preventing so.

4) set your Bass with volume on 10, any pickup you like, tone fully to the brightest setting.
By the way, what Bass is it?

5) play alternatively plugging into one or the other, same picking strength of course.
Does the 20W amp still overpower the 220W one?

Please answer only yes or no, no qualifications or explaining why you didn't follow instructions 1 to 5 to the letter.

More questions and tests will follow, but only after this basic test is answered, otherwise we are all wasting time.

Before you ask, I'll answer: pot taper can vary a lot and explain a large part of what you perceive, but "all pot tapers are the same on 10" so we are testing under that condition only.

And EQ can vary perceived loudness a lot, neither speakers nor ears are flat (at all) , so we also standardize that too by running all on 5 which roughly is flat response.

Thanks.

here are my findings, tested to the letter.

I must start with this disclaimer: I didn't say the 20 watt amp was louder, I stated that it was almost as loud as the 220 watt amp.

Now, the findings.

The 220 watt amp is only slightly louder than the 20 watt amp

if i back the gain down to 8, I have practically matched the the output of the 20 watt amp.

The tweeter is still "crackling" during the testing.


I also did the test with the 80 watt Crate B80XL. It it noticeably/obviously louder than the 220 watt amp when maxed.
If I back the gain down to half on the 80 watt amp, it will match what the 220 watt amp is doing at wide open.

I'm using a passive jazz bass (SX brand from Rondo Music), all controls wide open (how I play anyway).

[Kenrod]

Well, the amp is either not amplifying the small signal enough to fully drive the power amp or the signal is being lost somewhere along the way to the power amp.

I know that the schematic has ac signal voltages marked at different points in the circuit, some of which seem to be wrong. When you sent in the test tone or music at levels high enough to get the loudness where it should be, did you monitor the input signal strength or the signal strength along the signal path?

Is the tweeter making noises with the alternate signal sources?

I did not monitor the signal in the amp because i couldn't reach anything with it plugged in to the speaker (speaker lead length and molex plug design)
I'll have to get creative to make that possible.

[g1]

Thanks for hanging in Kenrod.
I think a lot of the doubt we showed here was because of your initial misunderstanding of the master volume which I was getting at in post #47. It's important to run masters full when testing levels.
The 220W amp should sound roughly twice as loud as the 20watt amp to the ear. It does not sound like this is the case, and your post #80 also seems that the BA115 has a problem.
The best way forward here I think is to try to determine whether this is a preamp or power amp issue.
Does the Crate B80 have preamp out and power amp in?
Can you try the crate pre-out into the ampeg power amp in, then the ampeg pre-out into the crate power amp-in?
(pre-out is same as effects send, power amp in is same as effects return).

[Kenrod]

Thanks for hanging in Kenrod.
I think a lot of the doubt we showed here was because of your initial misunderstanding of the master volume which I was getting at in post #47. It's important to run masters full when testing levels.
The 220W amp should sound roughly twice as loud as the 20watt amp to the ear. It does not sound like this is the case, and your post #80 also seems that the BA115 has a problem.
The best way forward here I think is to try to determine whether this is a preamp or power amp issue.
Does the Crate B80 have preamp out and power amp in?
Can you try the crate pre-out into the ampeg power amp in, then the ampeg pre-out into the crate power amp-in?
(pre-out is same as effects send, power amp in is same as effects return).

I'm hanging in there. I'm in it for the long haul if you guys can hang in here with me.


I think I've got the necessary connections on the Crate. The Crate pre to the Ampeg pwr in should be easy as I have the CD input on the Ampeg I can easily use. I may have to make a new patch cable to get out of the XLR of the Ampeg and back into the Crate. Stand by, but don't hold your breath.

[Kenrod]

I sent the "line out" from the Crate to the CD input on the Ampeg. I could turn up the gain and/or master on the Crate and drive the Ampeg (master on 10) to goofy loud levels.
I only have my hands on a very short XLR to 1/4" patch cable. I'll make up a longer one tomorrow so that I can test the preamp properly.

[J M Fahey]

Then you seem to have a preamp problem.
Ampeg power amp and limiter (which usually is speaker out driven optical Led/Ldr) seem to be working fine.
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Stopped and opened the schematic supplied by kenrod on post #20 so will refer to that exclusively.
Although we have a discrepancy from the very beginning: +/-50V rails in no way are enough for 220W RMS ... unless we are talking 2 ohms loads which is not the case.
26.5V output as shown was correctly calculated above as 85W@8 ohms, 170W@4 ohms ... fine for a 150W amp, not a 220W one.
That said, kenrod measured +/-60V rails which is a whole new game, expected power amp voltages should be ~20% higher , meaning power ~40% higher ... now we are talking.

So although the posted schematic does not exactly match the amp model (at least the 220W version), it should be close enough.
In fact I expect exact same electronics, with larger transformer and maybe higher voltage caps, and improved heatsinking (or addition of a fan) and not much else.

a) limiter is optical as I expected (it's an Ampeg/Crate staple by the way) .
Not a type I like, it's bland sounding and robs amp of a few Watts (can't set it too close to actul clipping and must be set at least a couple volts below for reliable operation) but it does its job .
So as I mentioned earlier, do not expect clipping on the scope screen because you will never reach it.
At worst you will clip the last preamp stage trying to overdrive the power amp.

b) power amp sheet asks for the exact same setting as I suggested earlier: gain and volume on 10 , as crazy as it looks, and all tone contols on 5.

c) there is a typo on TP1 .
Injected voltage is 170mV, TP1 as drawn must show exactly the same.
so either TP1 is 170mV and somebody typed 700mV by mistake (sh*t happens) or more probably TP1 arrow should point to the right, towards pin1, U1A *output*, where Math says you should have 720mV if fed 170mV to pin 3 , the input.

Let's see kenrod's voltages:

I ran a 1kHz signal in at 170mV per the schematic. cranked gain and master to 10, EQ flat.

Good.

Rails are +/-60V, IC supply is at +/- 17V

Good, this makes 220W@4 ohms at least possible.

As far as test point voltages, I have a few discrepancies on TP 1, 10, 11 & 12, the worst being TP 10 & 12 on IC5A/B

Ok, let's see.
---------------------------------------------------

All read on my scope peak-peak (making TP11 ok I think)

RMS is ~PP* 0.36 or for quick mental calculation, ~ 1/3 of what you see.

TP1 reads the same 170mV input I'm injecting, schematic shows it should be 700mV.

Yes, it's a typo or drawing error.

TP2 reads correctly though, just downstream of TP1, making me wonder if it's actually ok? I dunno

If you read 3.8V RMS or 10.8 VPP fine. Do you?

TP 10 (pin 7 of IC5) reads 1.3V, should be 2.0V

Not so fast, what about TP3-8-4-5-6-7-12?
And TP10 again, but remembering I think you mean VPP or VPP*0.707 while proper RMS value is VPP*0.3535 (or 0.354) .

TP 12 (pin 1 of IC5) reads 120mV, should be 800mV

In any case, way too low, please confirm reading anyway.
We not only need 800mV RMS there, BUT also 1/5 of it between C11/R25 or some 170mV (funny, huh?) .
If less, there's a preamp or connectivity problem; if present, I suspect the turn on mute Q9 .

TP 11 (Drain of Q7) reads 42-43 Vpp (30-ish RMS? 43 X .707)

Well, that's actually some 14.8V RMS, some 27W@8 ohms or 55W@4 ohms.

Ouch.

But we need rechecking along the chain to see where the pipe leaks all that petrol and later we'll check whether with approppriate drive that ampcan put out some 28 to 30 V RMS into a load.

Sorry for being such a Measurements Nazi but it's the only way to "remote control" solve anything.
Maybe I should buy one of these fashionable Hugo Boss designer clothes?