Possible sources of buzz

[osing]

Hi folks,

While playing guitar, I am hearing a buzz on certain notes that ideally I'd like to eliminate. I thought at first it was speaker related, but I am able to hear it when plugging directly into an instrument input on my mixer (I recently got a Radial CabLink which allows me to combine two speaker cabs and also send a signal to a mixer).
 
I subsequently assumed the source of the problem was my Super Reverb (AB763 DIY build), but I also get a similar buzz using my DIY 5F6A Bassman (currently running 6L6GC). I then assumed it might be guitar specific, but am hearing the same buzz using two different strats. Although I do often play my Super Reverb with the bright switch on and both treble and mid controls set high (~8), it seems like the sound is still present if I turn these down (albeit perhaps less pronounced).

I tried sending a ~140Hz signal (close to a Db note on my guitar where I easily hear this sound) through the amp and viewing it at the preamp, PI, and output tubes on an oscilloscope, but I did not see anything that looked odd.

I don't think this is necessarily a new noise, but it is making me curious what might be causing it.

I have included links to two short sound clips of two different strats into the Super Reverb. I'd be curious if anyone could venture a guess as to what might be causing this?

https://soundcloud.com/geh321/amp-buzz-direct-out-black-strat-sr

https://soundcloud.com/geh321/amp-buzz-direct-out-sunburst-strat-sr

[EL34]

posted in wrong board..... moving

[osing]

Sorry - I was about to fix it but damn you were quick!

[acheld]

I've had this occur several times over the years, and have found it tough to diagnose.   

I am assuming that the buzzing occurs in tones around the same octave, and that if you play quiet it does not happen, but louder it might.  Does the temperature of the amp have an effect?   ie, happens when cold, or well warmed up after a period of time?

This could be a cold solder connection, or could be related to poor contact in tube socket, and there are probably other issues possible.  Once had an EL84 fail as the culprit (had the slight buzz, kept playing, it got worse, and only then saw some arcing in the tube).   

There are some really talented folks on this forum who may provide a systematic approach to the problem.

[ac427v]

Interesting that it happens when you plug directly into your mixer. That sounds like a guitar problem--on both strats. I have had that happen with my favorite guitar. Lots of amp troubleshooting later and I discovered that it was due to grooves worn in the guitar frets and a slightly over tightened truss rod causing string buzz.
I know, it's a long shot. but desperate times...

[acheld]

I may have misunderstood.   

So you have two Strats that are associated with the noise, and two different amps that each send the same signal to your cab and, through an additional output to a cab sim that sends to your mixer?

My head is spinning . . .

[osing]

Thanks guys,


I am indeed able to produce the same buzz using two different strats, and I can produce the sound using either of the two amps as well. I might need to try another guitar or two in case this is indeed a guitar issue rather than an amp issue. The solder joints and tube socket contacts are also interesting. It might be worth re-tensioning the sockets and maybe carefully clean the contacts. I am also considering just replacing one tube at a time to see if anything changes.


I am able to play one guitar through one amp but with two speaker cabs connected at once using a Radial CabLink. When I do so, I also run a signal directly to a mixer as the CabLink has a line out. I wanted to rule out a speaker issue with this approach.

[PRR]

Loose lamp?

I've heard a metal case fluorescent lighting fixture resonate with a buzz if excited at the right pitch. That at least covers why it happens on multiple axes and amps.

[proaudioguy]

More pronounced on low notes or does it matter?

[osing]

I don’t think it is an external rattle as I am able to record it directly using the line out feature of the CabLink mentioned above (that is how the sound clips were recorded). I definitely have had issues like that before though where a metal panel on a desk rattled only at certain frequencies. I had to play a clip into a looper pedal and walk around the room until I could find the source.


It does seem more pronounced on certain notes. For example, it is easy to produce if I strike an open D or G string (in my case the guitars are  tuned down so the notes are actually Db and Gb. However, in the sound clip I also struck a few 12th string harmonics and got the sound. I am not sure I can say it is only happening on lower notes, but it does seem to be happening at some frequencies more than others . I will do some more testing with more guitars, try to see if there are any patterns, and report back. I am going to be away a few days but will definitely get to it next week.


In the meantime, thanks again for your thoughts!

[HotBluePlates]

I don’t think it is an external rattle as I am able to record it directly using the line out feature ...

Are any speakers being powered when you're recording this rattle?  It's starting to unclear whether there is volume-in-the-room when it happens, or not.

If yes, then microphonic output & rectifier tubes will often manifest as a "rattle" noise.  Old & new GZ34s have been known to have the issue.  I'd try swapping in a known-quiet rectifier, even if it's a 5U4 or 5Y3 (for a short test).

How about the 6L6/5881 you're using in Super Reverb & Bassman?  Are they 2 different sets of output tubes (not moved from one amp to the other)?  Do you have any other 6L6-based amps not making rattling noise whose output tubes could be swapped into the suspect amps?

[osing]

HBP - the speakers are indeed powered even when I am recording direct this way, and the sound levels are very high as I like to play these amps at 5-6 on the volume control (I play with ear plugs!). Your post gave me some additional ideas:


Both amps have solid state rectifier options - I can simply try these instead of the rectifier tubes to potentially eliminate the rectifier tubes as the source.


I can connect two dummy loads to the Radial CabLink so I can still record direct but this time with no sound coming from the speakers


This should allow me to determine if the actual speaker volume is causing an issue with the tubes


I don’t have any other amps, but each amp does have its own set of tubes. I have a new set of 7581A tubes I could try. I cannot guarantee that they are known good, but it might be worth trying them.


I will also be trying a PRS dual humbucker guitar on both amps and try to get a better sense for what specific frequencies are causing the problem with any of the guitars. Hopefully, I can isolate the problem to be either guitar or amp related.


I won’t be able to do any testing until Tuesday/Wednesday but will report back.


Thanks!

[osing]

Hi again,


It took longer than expected to get around to doing some additional testing:


Solid state rectifier - same problem


Plugged into dummy loads and recorded silently and direct with no improvement


Swapped all tubes to no avail


I then remembered that I actually have an EL84 based Peavey Classic 30 that I hadn’t played in years, so I cranked that up and noticed a very similar noise.


At this point I am pretty convinced the guitars are the problem, which is similar to the experience ac427v had mentioned earlier in this thread.


I am going to focus on the bridge, saddles, trem springs, and nut first. I should have sufficient neck relief and have fairly high action, but I’ll check out if there is an issue with the frets as well. I’ll report back when I figure this whole thing out, which I hope to accomplish before going crazy!