Crackling/buzzing with higher volumes in a Carvin X100B combo.

[bclemons]

I have a Carvin X100B Series III combo that makes a crackling/buzzing noise when played at higher volumes. Distortion masks the noise. It's more prevalent on lower notes. It doesn't sound as harsh as cold solder joints I've heard before. It's almost like a bad speaker sound, but I tested the EV12 with another amp at at higher volumes than created the noise, and it was fine. Here's what I've tried.

1. Chopsticked the tubes while running. No unusual noises.

2. Cleaned the sockets and pins with contact cleaner/lubricant. Still makes the noise.

3. Chopsticked the board and all pot/jack connections. No unusual noises.

4. Ran the preamp of another amp into the power amp of the Carvin while in the combo cab. Still makes the noise.


It's definitely related to the vibration of the speaker. Could a tube still do this even though it passed being tapped? The only things I couldn't get to were the connections between the sockets and the board. The board was also discolored around one of the power tube connections.

[guitician]

Tapping a tube isn't like having it subjected to 100hz rumble. The amplifiers current and voltage is going to set up other electrical anomalies too. Can you look at it on a scope? Discoloration of a circuit board is ok until it starts to get really burnt looking. Can you run a sweep tone into the amp?

[g1]

Assuming there are several power tubes of the same type, if one has discoloration where the other ones do not, it could be a sign of some arcing going on. You said you couldn't get a the connections between the sockets and the board, perhaps there is some iffy solder causing a hot spot. Also, check the sockets for any sign of arcing.
And, as guitician noted, it could still be a tube issue that the speaker triggers but not the chopstick test. Is there a way you can separate the speaker and run it away from the chassis?

[bclemons]

It's fine when running a separate speaker cabinet. I don't have any way of checking it on a scope.

[Mick Bailey]

Speaker vibration can be fairly difficult to track down because the dynamics are different to what you get with chopsticking and can be far more invasive. I've just had a real problem with an Eden SS bass amp that crackled on the low string of a double-bass. I've chopsticked the hell out of, banged it on the bench all-ways and pushed and pulled every wire, component and connector. No problem showed. Stick a 30Hz signal through it and whack up the volume and it plays up, but only on top of the speaker cab.

So I ended up getting it where it was noisy all the time, wearing ear-defenders and pressing with a chopstick to stop the vibration, which was a defective solderless IDC joint.

A tube may act-up under vibration, though my gut-feeling is that you have a bad connection somewhere else. Even a failure within a cap or resistor. A tube-swap is an easy way to eliminate tubes. Inspecting solder joints under magnification often shows up those annular cracks around component legs. If a joint is breaking down but not yet failed completely that's when vibration can affect it more so than chopsticking. You need to closely inspect the solder-side of the board for signs of cracking. Also inspect every connection to the board.

[guitician]

Then it's either a tube that has vibration issues, an electrical component that is vibration sensitive, or a speaker.

[bclemons]

Ok, I'll pull the board as the next step. I don't have any spare tubes on hand. Would the problem still persisting while using another preamp into the Carvin's power amp in jack eliminate the preamp section of the amp as the problem?

[guitician]

Yes, then the pre-amp is disconnected from the power section, or the normal effect loop operation.

[bclemons]

Ok, so I remembered that you can pull the inner 2 or outer 2 power tubes on this amp if you give it double the expected load, so I did, to see if I could eliminate a bad power tube. When I pulled the outer 2 tubes, the noise was still there. When I pulled the inner 2 tubes the amp was dramatically quieter. I had the volume maxed out and it did not equal the volume on 2 or 3 the other way. The noise was still there, however, once I got the volume up enough.

[guitician]

So you rotated through all the tubes and they are all fine. A sweep tone would help. That's what I use to find problems with speakers. Can you pull the chassis and run wires over to the speakers in the combo cab?

[Chuck H]

I remember (foggy, but still there) reading a thread here that revealed the plate resistors Carvin used as inferior for some reason. This problem has been fixed before by replacing the plate resistors with 1/2 watt metal film resistors. Considering the age of the amp it could certainly be any number of things. But this has come up before as a common problem and new plate resistors was the solution.

[drewl]

I get this alot.
Check the PC board for bad connections.
One of the main problems is the lack of tube retainers, especially on the upside down combos.
I've replaced the phase inverter socket on a couple.

Replacing the low voltage filter caps quiets these down alot.

[bclemons]

Now that I've had time to really get into the amp, it seems there are maybe some parts missing from the board. It also looks like it has gotten a little burnt at some point. Here are some pics.

[52 Bill]

Yes, that board looks pretty sooty.

It looks like there is an added terminal strip that probably holds the resistors that used to be on the board. Try and clean off the soot and inspect the fiberglass itself for carbonizing.

[bclemons]

The black discoloration would not come off. I suppose where the terminal strip is tied in would be a place to start looking for a bad connection. Would this produce the result of having very low output with the inner pair of tubes removed, but normal output with the outer pair removed?