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

[Tom Phillips]

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

Your calculation is off by a factor of 2.
Vrms = Vpp *0.3535
Therefore, 42Vpp is 14.8Vrms

[Kenrod]

awe crap! i see what i was doing now....Vp X .707 get to RMS right, but half that factor (.3535) for both halves of the waveform (Vpp). That's why i showed my data and calculation.

Because i didn't know what i was doing, I didn't believe my meter when i got approx 15V at TP11. All other voltages were jiving on the scope; except maybe there are all off.

I'll go back and start over with the correct input voltage, unless 170mV RMS is not what i'm supposed to do.

[Jazz P Bass]

170 mvs
700 mvs.

Either way I don't know of many inactive bass guitar pickups that are going to output that much voltage.
Well, maybe if you slap the strings with a brick.

I have always thought that these amps where not very loud.

Maybe that is why they came out with the HP version.

[Kenrod]

well, looks like i was only giving it a 60mVRMS input signal. I'll go give it the right amount and see if i get more than half my voltage out this time, If not, it still got a problem.
I'll report back in an hour or so.

[Kenrod]

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.

[Jazz P Bass]

The amp. How does it sound?

[g1]

Does the waveform look proper at the output, or like TP10 ?

[Kenrod]

Does the waveform look proper at the output, or like TP10 ?

Looks fine

[Kenrod]

The amp. How does it sound?

Gain at "6", master at 7 1/2, just to get to "bedroom practice level" loud enough but you wouldn't likely annoy anyone else in the house. Sounds good except for a little crackle in the tweeter when digging in just a little harder on notes on the D and G strings.

As a comparison, because I keep insisting this thing isnt nearly loud enough, I plug into a Crate BX-80 (with a 15" speaker) only rated at 80 watts and I set the gain at "5" and the master at "3" and get the same perceived volume with the same bass

[g1]

I don't like the look of that waveform at IC5, but it must be getting cleaned up if it looks good at the output. Someone more familiar with that limiter circuit around IC5 might have a better idea, but I've seen that kind of waveform come out of bad IC's. If it were me, I would just replace IC5 and see if it makes any difference.

[Kenrod]

I don't like the look of that waveform at IC5, but it must be getting cleaned up if it looks good at the output. Someone more familiar with that limiter circuit around IC5 might have a better idea, but I've seen that kind of waveform come out of bad IC's. If it were me, I would just replace IC5 and see if it makes any difference.

Neither do I, shape or level.

Enzo mentioned the limiter circuit in an early post.

I lieu of anyone really addressing the IC5 anomolies, and perhaps suggesting I swap it, I ordered some last night. I figured no harm in swapping "known good" parts for parts "producing" bad results. I hate blind part swapping, but I had a similar issue on another amp and swapped a IC where I was getting wacky readings and fixed it.

[Tom Phillips]

Why would you turn the master down on a bass amp?
Have you tried it with the master full up?

[Kenrod]

IME, I shouldn't be able to tolerate the volume level of the master turned all the way up on an amp of this size.
I suppose one could turn the master full up and then barely crack the gain, but that's typically not how I have run one.

[Tom Phillips]

It just seems strange to me that your concern is that the amp isn't as loud as you think it should be. However, you are playing it with the master turned down and you now say it is too loud with the master turned up.

[Kenrod]

It just seems strange to me that your concern is that the amp isn't as loud as you think it should be. However, you are playing it with the master turned down and you now say it is too loud with the master turned up.

It's not. You saw my side-by-side comparison to the other amp? Not exactly scientific, but close enough: about 1/3 rated power, 1/2 as much master volume = same perceived volume.

I never said this one was too loud turned all the way up, I said it should be, yet it's not.

I could turn the master volume all the way up, but on amps with pre and power volumes i've never had to. It takes at least half the gain and most of the master volume to begin to wake up and start to have a usuable volume level. That's just not right on an amp of this size, in my experience (see above)

I suppose imagine if these amps did not have a preamp gain level, but only a master volume (like a lot of amps), I still wouldn't have to turn the volume up to "10", especially not on an amp of this size, in my small shop, for it too get too loud.