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

[Kenrod]

I don't understand what you are asking here. U5A is part of the limiter circuit. Signal from the output is sent to the network of diodes that feed the led in the optoisolator OC1. The resistor side of OC1 is part of the feedback loop of U5A, which controls the gain of the stage.

*EDIT*: now I see what I did. I meant to specify output of IC5"B" side

The input and output at U5A, looks good. Where does U5B get it's input? I was thinking from U5A, and since it looks ok, I thought maybe I should be looking at stuff around the B portion, even the output side. Just wondering. I don't know what I'm talking about, just looking at where the symptoms are, just from a basic troubleshooting stand-point.

I'm still listening closely and checking anything you tell me to. I really appreciate the descriptions of what each component is doing in the circuit as we test and troubleshoot. That's as valuable to me as the eventual repair itself.

Have you checked all of the diodes and resistors in that part of the circuit? What are the two 50 volt supplies measuring at. I would guess that if they are grossly mismatched, the balance of the circuit would be thrown off.

I have not checked the various diodes and resistors there, but I will do so. The diode arrangement is converting the AC signal back to DC to properly trigger the LED inside OC1, correct?

The 50 volt supplies are measuring 60 volts +/-. I've checked this numerous times at various places in the circuit. My IC supplies are at 17v, rather than the 16V called out on the schematic.

[Kenrod]

all diodes in that circuit check good. The resistors: something I've seen before, not sure if it's normal depending on how they are arranged in the circuit, but the 2.2k (R6 & 7) resistors start low and slowly ramp up toward the spec value. All the other resistors in that circuit immediately measured correctly.

I thinks I noticed some imbalanced values across R72-R75 when verifying the bias voltage, in particular R72 (higher than expected). I'll report those actual values later.

[52 Bill]

The resistors: something I've seen before, not sure if it's normal depending on how they are arranged in the circuit, but the 2.2k (R6 & 7) resistors start low and slowly ramp up toward the spec value.

That is caused by a capacitor in the circuit. You are charging or discharging the cap with the voltage of your ohmmeter.

[Kenrod]

I thought it was something like that.

I did just check the voltage across the big resistor, per the bias adjustment instruction, just to see what they show.

I now understand how ridiculous my original thread title was after schooling myself on transistor biasing.

I'm not checking bias because I think it's contributing to any of the current issues, but just want to see what the measurements might show. It's easy to get to.

Here they are:

initial check
R72: 10.2mV
R73: 7.8mV
R74: 7.8 mV
R75: 8.3mV

adjusted AP1 to 18.0mV while taking a reading on R75:
R72: 20.0mV
R73: 14.5mV
R74: 17.7mV
R75: 17.9mV

Seems like both "sides" should be in balance with each other to operate optimally?
Are these values of any concern at this point?

[52 Bill]

adjusted AP1 to 18.0mV while taking a reading on R75:
R72: 20.0mV
R73: 14.5mV
R74: 17.7mV
R75: 17.9mV

Seems like both "sides" should be in balance with each other to operate optimally?
Are these values of any concern at this point?

Yes, in an ideal world you would get the exact same reading across all four resistors and when driven all four outputs would respond in perfect unison push/pull.

But, this is the real world where the four transistors are not perfectly matched and the four ballast resistors are not perfectly matched and the four gate resistors are not perfectly matched and the two power supplies are not perfectly... You can see what I am saying, this is not a laboratory experiment, it's a guitar amp made on a production line with production grade parts. There will never be an ideal amp, unless the heavens smile upon the random choice of parts and all of the stars align just perfectly. You probably have better odds of winning the Power Ball lottery.

The schematic states that the bias adjust should average between 15-20mV across the four resistors. Your numbers come close to that exactly, the highest is 20mV the lowest is 14.5mV. If I was rebuilding the output stage after a meltdown, I might spend a little time trying different Mosfets, but that would be my personal want for perfection in the rebuild. But that would have nothing to do with the power loss problem that you are looking into.

[Kenrod]

Thanks 52 Bill

Anything come to mind yet that I should check next?

[52 Bill]

I'm kinda late coming to this party, so when I get a chance, I'll read through the entire thread and see what you have done so far, but have you checked the speaker? Have you taken a signal out of the line out and checked to see that the preamp is putting out a good strong signal?

There is no easy way to insert a signal directly into the power amp, or does this one have jacks for a cd/mp3 player? If it does can the external player drive the power amp to full output?

[Kenrod]

I actually have not checked the speaker, but I did use the CD input and drove it pretty darn loud with some music. I know that's not a number, but I'll use the function generator app and put in a test tone of known voltage and report back.

Have not tried the DI signal, but the voltages before and after that stage looked good. In fact, every test point has looked good except the output of IC5"B".
I'll send that signal to another amp just for verification.

In case it's related I'll mention, again in case you popped in after first mention, along with the low output, the tweeter is "crackling" that intensifies with signal peaks (me playing bass through it). I thought there was a tweeter problem before I experience the low output so I replace the tweeter diaphragm.

[gbono]

Sorry for the highjack - are the output FETs isolated from the heatsink? I don't think this is the case from the schematic but I have a BA115 in pieces and don't see any silpads on the heatsink. THX

[Kenrod]

So here's a quick update and some learning:

I put both a test tone and music into the CD input. either one will drive the amp to the loudness I would expect from an amp this size. Same effect at the "0bD" (passive) 1/4" guitar input.

I also tried the DI, but it was a bit noisy. I'm not convinced it wasn't the patch cable. I put a TRS 1/4" plug on an XLR plug that already had some cable attached. I need to double check that.

The test tone is still the 170mV RMS as specified on the schematic. I got to wondering why such a difference when I plug in a bass. The bass is passive jazz bass. I plucked it as consistently as possible, not too light, not real harsh, and only recorded about 50-60mV RMS signal. I tested another of my passive basses and found the signal average around 60mV RMS.
I have another passive bass and 2 actives at the house I can test for output voltage, but haven't had the chance yet.

While the difference in input voltage (phone test tone or music vs actual bass guitar) can explain the large difference in volume I'm experiencing on this amp, I still don't understand why the same basses drive a different lower powered Crate BX80 much louder at a much lower master volume setting.

Real world experience says there is still a serious volume problem.

[52 Bill]

While the difference in input voltage (phone test tone or music vs actual bass guitar) can explain the large difference in volume I'm experiencing on this amp, I still don't understand why the same basses drive a different lower powered Crate BX80 much louder at a much lower master volume setting.

Real world experience says there is still a serious volume problem.

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?

[g1]

Set the masters on both amps to full.
Control the level with the preamp volume or gain, whichever it is called.
How much higher do you have to turn up the gain on the BA to match the loudness of the other amp?

[Tom Phillips]

...I still don't understand why the same basses drive a different lower powered Crate BX80 much louder at a much lower master volume setting...

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?

[Kenrod]

Set the masters on both amps to full.
Control the level with the preamp volume or gain, whichever it is called.
How much higher do you have to turn up the gain on the BA to match the loudness of the other amp?

I did it like this (and tell me if it's a completely invalid test)

Set master on BA115HP to "10", put in a 50mV RMS sinewave at 500hz. I couldn't stand to listen to 1k hZ long enough to test.
I set my SPL met in front of the amp (at 1 meter) and turned the gain until I hit 105 db (slow response, C weighted).
I set the SPL meter in front of the Crate B80XL and turned the master to "10" then eased the gain up to about 2.5 to reach 105 db.

So, almost the same gain settings with the masters on "10". Seems like the 80 watt amp and the 220 watt amp are on the same playing field at roughly the same setting. Doesn't seem like it should be so. Arguably both quite loud under these test conditions. I'm still confused.

Makes me want to recheck the output on the basses again since I purposely lowered the test signal voltage to something similar to the plucked bass output.

[Kenrod]

It could just be the nature of the master volume control in that am. 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?

I did in that example, gain probably about 4-ish, master about 3 and it was way louder than the Ampeg ever tried to get.
See my test on the latest post.