Gibson Amp Filter Cap Polarity

[bobloblaws]

G'day, I'm uploading a schematic for the power section of a solid state Gibson GSS 100 from the Gibson Master Service Book.

If you look at the orientation of the 4 filter caps you see that the negative terminals of all 4 caps connect to ground. However, the way this amp came to me the polarity on C5 and C7 is reversed.

All 4 caps look too new to be original, so did someone install them incorrectly? The amp hums like crazy, but the amount of hum is steady even with volume at zero so I'm assuming that is a grounding problem or a bad component and not a filter cap issue.

The other curious thing is that the capacitance values for those 4 caps is shown as 4500uF each for the big pair and 1500uF each for the other two. In my amp the values have been changed to 9300uF and 3300uF, respectively.

I appreciate whatever insight you can offer.

[Enzo]

Yes, of course they are reversed. Look at the schematic. Those two caps are fed by the negative corner of the bridge rectifier. They have negative voltage on them with respect to ground. The schematic simply has a drawing error with the + signs there.

CAp values increased? At some point, someone decided larger caps would filter better. Works for me.

Hums a lot? Does it make just hum? Or can you still also hear the music along with the hum?

Is ther DC voltage on the speaker output post?

Is your hum 60Hz or 120Hz?

[bobloblaws]

Yes, of course they are reversed. Look at the schematic. Those two caps are fed by the negative corner of the bridge rectifier. They have negative voltage on them with respect to ground. The schematic simply has a drawing error with the + signs there.

Ok, that was not intuitively apparent to me. Negative terminal to ground seemed logical to me. Good to know it is the drawing that is in error and not the tech. I'll have to familiarize myself more with the bridge rectifier polarity.

Hums a lot? Does it make just hum? Or can you still also hear the music along with the hum?

Is ther DC voltage on the speaker output post?

Is your hum 60Hz or 120Hz?

I wasn't sure if it was 60Hz or 120 Hz so just now I turned it on intending to hook the scope up and see what it would tell me. And.......the hum was completely gone! So I grabbed my trusty chopstick and started poking around. Well, the hum is back, I suspect a bad connection in and around the mains fuse holder. Should be easy enough to solve. Thanks!

[Enzo]

Solder joint somewhere, sounds right.

If you have a simple single power supply voltage, then yes, the negative corner of a bridge would go to ground. But this is a dual supply - a plus and a minus voltage supply. In such cases there is no ground connection for the bridge.

See above the power cord plug symbol it says GROUND? Follow that over to the transformer where it connects between the two halves of the transformer secondary, and continues on to be the common to the power supplies and the audio circuits. I think there are better ways to draw it, but I can't say they drew it "wrong". We have to adapt to various drawing styles, especially on older circuits.

[bobloblaws]

Solder joint somewhere, sounds right.

If you have a simple single power supply voltage, then yes, the negative corner of a bridge would go to ground. But this is a dual supply - a plus and a minus voltage supply. In such cases there is no ground connection for the bridge.

See above the power cord plug symbol it says GROUND? Follow that over to the transformer where it connects between the two halves of the transformer secondary, and continues on to be the common to the power supplies and the audio circuits. I think there are better ways to draw it, but I can't say they drew it "wrong". We have to adapt to various drawing styles, especially on older circuits.

But the "+" markings are incorrect in any case, no?

[Enzo]

Yes, they are wrong, your amp is wired correctly as you described it.

[bobloblaws]

Well, I spoke too soon. The hum disappearing and reappearing when I did some poking around seems like it was a fluke. It might have been that it had not been powered on overnight and teased me with a few seconds or minutes of hum free bliss. I got some work to do now tracking down the source. The scope shows it is 60 cycle. Anyway, my question re: filter cap polarity has been more than adequately answered.

Edit: Indeed, I let it sit for a couple of hours and then turned it back on. The hum was gone for about 2 minutes, then there was a "pop", and the hum was back. what's the best strategy for tracking this down? Trace the signal with an audio probe to locate the point where the signal becomes noisy?

[Enzo]

How clean are the power supplies?

[bobloblaws]

How clean are the power supplies?

I don't know what you mean by "clean".

[Enzo]

The rectifier takes AC, and turns it into pulsing DC. The filter caps in the power supply smooth this out into DC. "Clean" means DC free of bumps and wiggles from the AC, which we call ripple. If you have a scope, clean means the DC supply voltage is a smooth straight line instead of one with lumps. This tells us how good a job the filter caps are doing.


In another usage, we might also refer to the output of an amp as clean if the signal is not distorted. As in the amp puts out 100 watts clean and 120 watts at 10% distortion.

[g1]

If you check for AC voltage on the filter caps, there should be very little. Note that some inexpensive meters can't do this where there is DC present, like on filter caps.

[bobloblaws]

The rectifier takes AC, and turns it into pulsing DC. The filter caps in the power supply smooth this out into DC. "Clean" means DC free of bumps and wiggles from the AC, which we call ripple. If you have a scope, clean means the DC supply voltage is a smooth straight line instead of one with lumps. This tells us how good a job the filter caps are doing.


In another usage, we might also refer to the output of an amp as clean if the signal is not distorted. As in the amp puts out 100 watts clean and 120 watts at 10% distortion.

Oh, I see, you're talking about the filter caps taking care of the AC ripple. As for this hum problem I had ruled out the filter caps since I have it written in my notes that AC ripple is 120Hz (and my scope is telling this noise is 60 Hz). My notes also tell me that hum from a filter cap issue will follow the volume control and that is not the case in this case.

Now ya'll go right ahead and burst my bubble

[The Dude]

......My notes also tell me that hum from a filter cap issue will follow the volume control and that is not the case in this case......

Hum from a filter cap issue will not "follow the volume control" because the power supply is poorly filtered regardless of volume. If the hum is signal related, then it will most often change with volume level.

[Enzo]

My general approach follows a path. Yes, power supply ripple will be 120Hz, unless one of the rectifier diodes is open. But nonetheless, we always ought to start with power supply.

Divide and conquer...another way of saying isolate the problem. Q20-21-22 are hte preamp, and they drive the power amp through the primary of the transformer through C3. I would unsolder and lift C3, OR short across that transformer primary, whichever is more convenient. Does that stop the hum or does it remain?

I forget, did we determine there is no DC offset at the speaker?

[bobloblaws]

I forget, did we determine there is no DC offset at the speaker?

My bad, you did ask about that but I lumped it in with the question of whether there was musical output. As for the latter, yes the guitar signal is amplified and reproduced through the speaker. As for DC on the output jack, my meter shows aproximately -1 VDC.