Locating Buzz in Distortion

[Tim Hall]

Hi all,

I'm hoping someone can help me identify the source of buzzing I'm getting on my Guytronix Gilmore Jr amp. I'm having a hard time determining if it's the speaker (Weber 12A125-O) or the power section of the amp. The buzzing is worse on the low end and only occurs at a distortion level (power tube saturation).

I have used several different tubes in the power section (6N1P, 6CG7, 12AT7, 12AU7 and ECC99) and the buzz is present with each of them, but at slightly different volume/distortion levels.

I'm a bit limited as to the speakers I have on tap to test out. I tested the amp on two different Hi-Fi speakers in addition to the Weber, and they seem to buzz-out (and flub/fart-out) sooner and worse than the Weber. I have heard that this Weber model might flub out with bass-heavy guitars (my guitar is somewhat bass-heavy), but the speaker is rated at 20W and the amp is only putting out 2W max.

Can someone tell me how I might go about eliminating potential culprits to narrow down the problem?

Thanks for any help,

Tim

[Tim Hall]

Just played on it some more....

Just played on it some more, and had a thought. This speaker is designed to break up early, but even so I didn't think I would be able to drive it into break-up with my puny 1-2 watt amp.

Is it possible that the speaker is right on the verge of break-up, and just sounds harsh and buzzy because it hasn't been broken in?

[capnjuan]

Hi Tim: can't answer as to speaker break-in period but would apply to output tubes. No, I don't think 1-2 watts would break up a +/- 20w speaker. My experience w/ hi-fi speakers is the same as yours; they break up as soon as the power is turned on.

Non-technical suggestions. Have you gone through the entire chassis with a chop-stick poking at things to see if they move, flex, or budge? Same for baffleboard; loose amp badge, ply gaps, weak edges, or grillcloth spots not fully adhered at the edge? Any unsecured chassis edges close enough to the cabinet to rub when vibrated?

Did you try the guitar in another amp to rule it out? What about dialing the vol control on the p/u back?

Don't ask how I know about these 'did ya trys'....good luck...not sure which is worse; buzz or hum but nothing worse than beating the hum only the fight the buzz...

[chipaudette]

Hi,

My amp also makes a noise. Mine is more of a rattle than a buzz, though. It turned out that my rattle was from my tube rectifier.

My listening to the front of my amp vs the back of my amp, I heard the noise coming more from the back. Then, I started pulling out tubes and shaking them near my ear. When I got to the rectifier, it rattled like a blown light bulb.

When I swapped in a different rectifier, the problem went away.

So, don't forget about these basic mechanical noises from your tubes. They may be the cause.

Chip

[chipaudette]

Oh, also, did you try detaching the speaker from your cabinet/baffle. Still keep it WIRED to the amp, just mechanically detach by unscrewing it and letting it lie on your work bench (you can either unscrew the speaker from the baffle, or you could unscrew the whole speaker+baffle from the amp...for me the latter was a lot easier to do). Then, once detached, play through the amp.

If the noise goes away, then you know that your buzz is not due to the mechanical coupling of your speaker to your amp. On my amp, my rattling went away because the speaker could no longer shake my rectifier tube. That was another important clue for me.

Chip

[Tim Hall]

Thanks for all the replies. Already ruled out the guitar...sounds fine through an amp simulator (or as good as digital can sound, at least.) As far as mechanical vibration in the amp/chassis goes: I don't think this is the issue since it's a seperate head, well distanced from a not-so-loud speaker.

I also pulled the speaker...voice coil doesn't seem to be rubbing...speaker is probably fine. But the buzz seems the be more prominent when the speaker is not in an enclosure (but it's hard for me to guage since the sound is really tinny and harsh).

It may be my bad soldering job on my first build, but I would think this would translate more into bad hum or failure (at some point) of the circuit. I'm only getting the buzz at distorted levels. Not necessarily loud levels...using a lower-headroom 1/2W tube the buzz occurs in the distortion just like a louder 2W tube. Otherwise the amp seems to be very quiet...there is faintly perceptible hum at idle.

I'm beginning to think it has something more to do with saturating the power tube.