Express 5:25. Buzz from speaker with certain notes.

I have this problem with a noise coming from the speaker. Let me wxplain: The amp was purchased used (e-bay), not sure how old it is or if it is still covered under the 5 year warranty. Serial number is E25-2731. Anyone point me to a serial number decoder or can tell me if its still covered?

The noise is definately coming from the speaker, and is a "buzz" like a very low volume distortion that kinda peaks as the note is fading. This will happen on only certain notes. I did replace the power tubes with some Groove Tubes EL84-S with a 6 rating. This stopped the problem for a bit, maybe 2 months, although my son reminds me that the original noise I was hearing was more of a rumbling/wavering noise on some sustained notes... Now this buzz noise is back, prompting me to think that something else is going on. I grabbed a full set of pre-amp tubes from my son's collection and swapped them out, noise is still there.

My son says I'm crazy, but it really sucks to hit a note that you want to sustain, and all I can focus on is that friggin noise that slowly develops, peaks, and fades away.

It reminds me of those cicada bugs you here in the summer months buzzing up in the trees.... Not as loud and under the note, but Really annoying.

Someone mentioned a grid resistor that may be bad. Where can I get a schematic for the board? And I wonder if this is the problem (bad resistor) that it would reappear 2 months after replacing the tubes. I would have expected that the noise would be there all the time, even with new tubes....

Any help would be really helpful.
Thanks

Try it with another speaker.

I had a buzz everytime I hit an open A, I took it back to the shop, they told me it was normal, but tried to find out what it was, they couldn't.
Are you sure that it is coming through the speaker?

No, I'm not sure, that's why you do tests. :roll:

Does it do it with a different guitar? Have you checked for any possible loose items in the amp? May be a screw not bottomed out. It almost sounds like a sympatheic vibration since it is related to a certain note/frequency. When you change out the speaker for a test you could make sure amp is unplugged and look for anything loose or very close to something else that could cause a vibration.

Hello, thanks for the replies and follow-up questions.

I am sure that it is coming through the speaker. It does this with any guitar too. As far as loose parts, I checked all the obvious and accessible srews, nuts, bolts, whatever. I checked the speaker for cracks, splits, holes.

I've also noticed that it takes about 4-5 minutes of playing before the noise starts to develop. Once it starts, it's there until I stop.

Once it starts it exists on all channels; clean, blues, burn, etc. and on 25 or 5 watts.

Tonight I'll do two things. I'll try another speaker (very doubtful this will fix it) and I'll record the noise and post a link to it so that you can hear what I'm talking about.

What about a schematic for the amp and a Serial Number decode? I'm curious to know if it's still under warranty.

More than likely a nasty tube. Mine did that when I first got it. Was a bad power tube.