F30 making "fuzzy / crackly" noise

For the longest time I thought I was hearing my tubes rattling in my F30 combo when it is mic'd. But when I recently bought an extension cab and had it mic'd up instead of my combo the same "fuzzy" sound was still hanging around. I don't know how to describe this sound except that it is a sort of electronic sounding fuzz that happens when a chord or note rings out. The sound doesn't stand out too much when I'm in the same room with the amp but it is very obvious when the amp is mic'd coming through a PA.

Any thoughts???

You can try to isolate it (preamp vs. power amp) by running from the FX loop and/or recording out into the PA to see if you still hear it. And the reverse - run a separate source into the FX return and mic the cab (as you mentioned).

Could be a bad tube, bad tube socket or a bad solder joint. Does it get better or worse over time? Does it change when you change amp channels or contour? What happens if you swap tube positions?

Thanks for the quick response.

I've never used the fx loop before. I only run a fulldrive and a carbon copy delay so I've always just come out of those pedals into the input. Not really sure how the fx loop works. So if I was to test this what would I need to run the cable coming from the fx loop into?

I thought it could be a tube issue as well, so I replaced the power tubes and the noise is still there. I'm using groove tubes EL84 and Groove Tubes for the preamp section as well. Mesa puts 4 12ax7's in these from factory. I replaced tubes 1 and 2 with 12au7's and found that it had more head room in the gain. Didn't think this would cause any issues because from what I understand 12au7's are just lower gain options.

Think of your amp as a separate preamp and power amp.

The Preamp has the guitar input as its input, and FX send as its output.
The power amp has the FX return as its input and the speaker as its output.

The preamp out and power amp in are connected internally until you patch into the FX loop. Then it separates them (more or less - some signal always bleeds from preamp out to power amp in).

So use your amp as 2 pieces: Guitar to amp input, preamp out (either FX send or direct record out) to your mixer or another amp. Ignore the power amp part of your amp for now. In fact, switch on the power amp mute. Does it still crackle? If yes, the problem is in the preamp. If no, probably power amp.

Now turn the input volume to zero, and drive the FX return with another source - iPod, a different guitar preamp, whatever. Un-mute the power amp, and put it in the conditions where you tend to hear the crackle. Do you hear it? If yes, the problem is in the power amp.

If still no crackle, it could be in the FX loop. That circuit relies on a switching jack to disconnect the preamp -> power amp connection, and it may be bad. Stick a short patch cable into FX send and FX return, shorting them tigether. Does the crackle go away?

On a different note, you may want to try your delay in the FX loop. FX send -> delay in. Delay out -> FX return. Simple.

Thanks for all the info!

I did what you recommended and the buzz was still there...so I decided to replace the 12au7's with the original 12ax7's. And, whala! No buzz. Hopefully this will fix it for good, not just temporarily.

I also tried running my delay through the fx loop. Do you find you have more control over effects when run this way or is it simply to improve tone?

Thanks again for your help.

Kyle

Hi Kyle,
No problem - happy your amp's better.

The issue with the FX loop is that time-based FX (delay, flanger, chorus, etc) tend to get wierd artifacts when run through distortion circuits and overdriven preamps (same thing, really), due to phase problems. So the FX loop was invented to get around that problem.

Now certain people have made good use of those artifacts (like EVH) with phasers and flangers in front, so it's really a matter of taste. If you find that an FX unit sounds bad in front of the amp, stick it in the FX loop, and it will probably clean up.

Nice. I'll try it out asap.

Thanks again for all your help!