troubleshooting amp/cab buzz

[Luthierwnc]

Hi Guys,

I just replaced the board in a homebrew combo and am having some trouble with a low-end buzz/crackle. The amp is basically a single-channel 6L6 blackface with no trem. Tubes are known good. The speaker is a WGS 12 smooth cone. It is installed flush with a good seal around the baffle. The baffle seems secure in the cab.

When the chassis is in the cab there is a nasty buzz more on the low strings than the high ones but it isn't pitch-specific. When the chassis is in the cradle, no buzz. When I ran another amp into the speaker, no buzz. When the chassis is in the cab and I'm driving another speaker, there is some buzz but not as much as the combo.

Haven't run across this noise before when it didn't happen both in and out of the cab.

I haven't run the signal through the scope. That's next. But I keep thinking it must be something shaking in the chassis when it's bolted down.

Any ideas would be appreciated. Thanks, sh

[xtian]

I have no sympathy for sympathetic vibration.

Best way to approach is to use a signal generator, so you have both hands free to find the rattling. Plug in a sine wave at the input, and sweep the frequency and volume until you are hearing the bad noise. Now you can use your hands to press on the tubes (power tubes are my first guess), and various points on the chassis until you have identified the trouble spot.

[Phil_S]

I'm thinking same as you. Have you got something that is thin but a little squishy that you can use to insulate the chassis from the cab? Thin rubber washers, thin sheet rubber, thin sheet of that foamy packing material you sometimes get with computer parts. You see where I'm going...a bit of damping may cure the problem and in the process will also prove the problem. This is something you can do quickly and cheaply.

You've determined the speaker is fine, as is the cab. So, that's not it. It's got to be the chassis in relation to the cab.

I didn't ask how the chassis mounts in the cab. It could be the way you are bolting it in -- maybe something is loose?

[xtian]

thin but a little squishy that you can use to insulate the chassis from the cab

1/4" weather stripping.

[Phil_S]

thin but a little squishy that you can use to insulate the chassis from the cab

1/4" weather stripping.

Yeah! 3M makes foamy double sided tape. They call it mounting tape. Easy to use. Stays put so you don't need 3 hands. You can leave the protective paper on the chassis side so you won't have any trouble positioning it.

[Stevem]

It would not be the first time that I have had that issue from a build due to oscillations !
It makes you think you have a blown OT or a bad speaker!
One easy check is to see if reversing the mounting of the coupling caps end for end makes any change .

That being said the first thing to check really is that cap wise you have not fractured the connection of one of the leads loose in the cap body , I have even had this issue on new caps that I just layed my hands on !

[Luthierwnc]

Thanks guys. It might be a while before I can crack it open again but I'll let you know how things go. sh

[Luthierwnc]

It could have been any of several things but I gutted the board, shielded some of the wires and isolated the input jack. The latter made a big difference in ground hum. Works well. Sometimes, I just have to blow it up and start over for niggling problems.

thanks, sh