Fellow amp modders, I have a weird problem....

[duhvoodooman]

My recently modded Valve Jr. is giving a nasty buzzy distortion on one particular note--between C4 and C#4 (about 270Hz). It's the worst when I play middle C on the G string and bend the note up a half step--horrendous buzzy distortion! A half-step further up or down, and it virtually disappears. I can get the same thing by playing this middle "C-and-a-half" note farther up the neck on the D, A or E strings, too, so it's not string specific. An octave lower (i.e. between C3 and C#3) and it's also there, but to a much lesser degree. An octave higher (C5+), and I can't hear it at all. So it's very pitch dependent, which I would think suggests that it's some kind of harmonic/resonance behavior. In my limited experience with amp modding, this is the first time I've encountered behavior like this.

I thought that perhaps it was a microphonic tube, but I separately changed both the 12AX7 and the EL84 tubes in my VJr, and neither change affected this behavior. Neither did switching between the onboard speaker and an external cabinet, so it doesn't appear to be a sympathetic vibration thing. And it happens with any guitar I use, whether single coils or humbuckers, and with any of the individual or combined pickup selections.

Has anyone here ever run into this? Any ideas on what causes it, and, more importantly, how to get rid of it?? TIA!

[M29]

Hello duhvoodooman,

My first thought was a microphonic tube as well. Have you changed any caps lately? Might be a cap. It may also be a solder joint. Might try reflowing any recent solder joints.

M

[mrmudcat]

I have been thinking on this and a cold solder joint came across the vast space between my ears a few times. Yep my reply came out sounding pretty much how I expected:whatever: (weak and inadequate):beer:

[Tone2TheBone]

Voo...yes I've had this happen to me cept it was on the D#. I have no clue as to what causes it.

[tot_Ou_tard]

Do you still have a problem when running clean?

If it was a rattling sound, I'd put a damper on the tubes. Maybe you should try it any way. I used to have note-specific rattling on my Champion 600 until I used a damper on the 6V6.

[duhvoodooman]

Voo...yes I've had this happen to me cept it was on the D#. I have no clue as to what causes it.
Were you able to fix it, Tone? Which of your amps did it happen on?


Do you still have a problem when running clean?

If it was a rattling sound, I'd put a damper on the tubes. Maybe you should try it any way. I used to have note-specific rattling on my Champion 600 until I used a damper on the 6V6.
Good thought, Tot, but I already checked that. This is not a mechanically-related sound--definitely coming from the electronics. And, AAMOF, it's most noticeable when playing clean.

[F_BSurfer]

DVM I have read your post here and your post over at sewatt and although I have not had this problem with a amp did have a similar problem with my SD-1 and it turned out to be the micro switch it was injecting note specific noise weather it was on or off just at a different level. Don't know if your switches in r6-7 area are micro or not ,there is where I would look first.I have found that those switches are very sensitive and not very reliable.

[tot_Ou_tard]

Good thought, Tot, but I already checked that. This is not a mechanically-related sound--definitely coming from the electronics. And, AAMOF, it's most noticeable when playing clean.
Well, that exhausts my vast amph knowledge. I'll leave it for the professionals. Good luck Vood!

[Tone2TheBone]

Voo. It happened just yesterday with my Marshall. I moved it into the livingroom to record some stuff for Robert and I was playing it pretty freakin' loud (master at 8) and anytime I fretted a D# on the low E string I'd get that weird raspy noise that you describe. I got new tubes in it...the speakers are questionable though cause they're the original ones since 1987. Get this though...it stopped doing that after I'd been playing for a few minutes more. Don't know what causes that. It didn't do that today though. *shrugs* Marshalls can be temperamental.

[duhvoodooman]

Well, to quote Brian Wilson, "God only knows...."

I took the amp back apart (I still had a couple of original carbon film resistors that needed to be upgraded to metal oxide type anyway) and poked around, but couldn't find anything wrong with any connections or solder points. I reflowed a handful of the solder connections, though they looked perfect visually. Before starting to change any components, I decided to put the PCB back in the chassis and test it again.

You guessed it--no buzz whatsoever. So, just like Tone & his Marshall, I have no idea what was causing this odd & annoying behavior. While I suppose anything is possible, I really doubt that anything I did had any effect on this behavior. But it's working fine now, and I'll obviously monitor the situation closely to see if it returns. Weird....

[Bloozcat]

"You're traveling through another dimension, a dimension not only of sight and sound but of mind. A journey into a wondrous land whose boundaries are that of imagination. That's the signpost up ahead - your next stop, the Twilight Zone!"

Cue music...di di di di...di di di di....