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

[Kenrod]

Actually, I can't think offhand of any schematic that warned we MUST use a load. Solid state amps don't ccare, but tube amps MUST have a load. That is something any tech knows, but I don't see it on schematics.

Yeah, the fender SS schematics I just looked at mention a resistive load, but I believe that's just for testing output power.

[Kenrod]

Haven't verified, but suppose it's a 4 ohm internal speaker and Ampeg advertised peak output, that would get us in the ballpark correct?

[Jazz P Bass]

26.3 x 26.3 / 4 ohms= 173 watts output.

As I said there are number of these BA115s.
The latest one states 100 Watts at 4 ohms.

[Kenrod]

I had the BA115, it was 100 watts.
This is the BA115HP. Supposed to be 220 watts IIRC.

[Kenrod]

To clarify, we're talking about this one.Ampeg BA115HP 220 Watt 1x15_ Bass Series Combo Amp _ Musician's Friend.pdfAmpeg BA115HP.pdf

[Enzo]

The BA115HP is the ...High Power... version, and says 220 watts in the ads.

Just measure the main voltage rails, and assume the output can come within a few volts of that. So subtract maybe 3-4 volts. That will be peak volts. Time 0.707 for the "RMS" voltage.

RUn a 100Hz signal or something your meter can read, and crank it up. Does the output come up to that level? 26v sounds cool. 220 watts into 4 ohms is about 29-30v, so there is your target.

[Kenrod]

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

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

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

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

TP1 reads the same 170mV input I'm injecting, schematic shows it should be 700mV. TP2 reads correctly though, just downstream of TP1, making me wonder if it's actually ok? I dunno

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

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

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

[Kenrod]

Also note the waveform from pin 1. Not sure what to call that.

[Kenrod]

To me it looks like the waveform is bouncing on top of itself slightly.
I would expect distortion to be a clipped waveform (based on my limited knowledge) but when googling images I ran across a picture related to op amp distortion and it had a "smeared" waveform kinda like mine.

I will add that I had "clean/normal" waveforms at all the other test points

[Jazz P Bass]

First off, the schematic indicates TP 1 as the input signal amplitude.
700 mvs. (which to me is crazy high)

It is not clear to me what we are trying to fix.

[Kenrod]

First off, the schematic indicates TP 1 as the input signal amplitude.
700 mvs. (which to me is crazy high)
(

I agree, TP1 measured the input signal, which the schematic specifies as 170mV in the test parameters.

The next downstream TP2 measured correctly, making me question the 700mV at TP1(with no apparent amplification stage to get from 170 to 700 at TP1) I begin to wonder if it's a typo or my test value is a problem.

It is not clear to me what we are trying to fix.

At this point, with it operating "correctly", I still question if it's loud enough, so I'm going through all the test points and finding some anomalies, particularly on IC5, that I'd like help understanding.

[g1]

Run 170mVAC (not peak to peak) at the input jack. Volume & level at 10, tones at 5.
You should get 26VAC at the output into a load. Unloaded should be a bit more.
What do you get at the output?

[Kenrod]

Run 170mVAC (not peak to peak) at the input jack.

I ran a 1kHz sine wave in at 170mV per the test parameters on the schematic. cranked gain and master to 10, EQ flat.


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


Are you suggesting I take the test parameter as RMS, then calc the sine wave at 240mV and use that for the input signal?

Seems counterintuitive to me, but I claim ignorant from the beginning

[g1]

As far as I know, the test parameters are AC volts, which means RMS. Any time the voltage is peak to peak, it will be specified as such. If it's just "AC", then it's RMS.

My point was to look at the big picture first, with specified input do you get specified output. Once you establish that, you can decide whether various points along the line are problems, or just typo's.
It sounds like you are getting the right numbers at the output, it's just in disagreement with your idea of what sounds "loud enough".
The numbers don't lie. But between the output wattage of the amp and your ears is the speaker/cab efficiency.

Edit: I see that Tom P caught a math error that means the output is low and you are not getting the "right numbers".

[Kenrod]

Any idea about the discrepancy on IC5?