Horrible sounds at certain frequencies.

[hammeroff]

Around the note B flat my amp spurts out a very metallic shrilly noise.

The shrilly noise only lasts briefly, and gets louder as the output of the amp gets louder. It's especially noticeable when the note is picked harder.

The amp itself is a '65 Fender Twin Reverb (reissue)

Here are the variable's I've eliminated:

-Environmental noise => I've played in different rooms of 2 different houses.

-Guitar noise => I've played 3 different guitars through it, all 3 guitars don't make the shrilly noise through my back-up amp.

-Cable/Pedal noise => The shrilly noise is there, without any pedals in the chain, and it's there with every different cable I own.

Here's a brief history about the amp:

The amp has been in this house for 4 years, it's never had any problems with extensive noise or funky sounds.

For about 3 years she was played with no effect pedals, till about a year ago I got a Tube Screamer.

About 1 year ago, she started getting dragged around to different places to jam with buds. Until 1 week ago, no issues.

Here's what probably caused the problem:
Last week I built a fuzz face clone, and was doing some extensive testing of transistors and trim pot adjustment. The amp was exposed to more distortion than she has ever pumped out. It wasn't for a very long time, but some of the settings I was trying caused some pretty....bad sounds to come out.

Last, but most importantly:
Is there anything I should know before I put my favorite piece of equipment on the work bench and start taking it apart? Are there lethal capacitors in her?

Sorry about the lengthy post, and thanks a million in advance for any help.

--Matt

[sumgai]

Matt,

Yes, there are most definitely lethal voltages inside the chassis of your amp! Use more caution than you might believe you possess, that's all I gotta say.

Your symptoms point towards a couple of things. In order of ease and least costly to check out or fix:

1) a loose speaker mounting nut (or nuts);

2) something gone bad with the cabinet itself.

3) a bad tube; or

4) a bad speaker.

Do the easy and inexpensive things first. Check the tension on the speaker mounting nuts (that should be an 11/32 socket). You just want them a bit more than snug, and pretty equal all the way around.

Next, make sure that all screws on the cabinet are tight.... the back boards, the metal corners, the rubber feet, the casters (hey, what happens if you pull them off?), the handle, the tilt-back legs, the baffle board, and finally, the chassis itself. Things loosen up when they'be been pounded up and down the road for awhile.

And while you're at it, clean out all that accumulated trash from under the reverb pan, ya never know..........

Next, it's a toss up..... the speaker is easier to test, just plug something else in, in place of the standard speakers. But they are the most expensive thing to have to replace, that's true. Still, I'd do this before messing with the tubes. Again, it's a personal choice, but I happen to have a few extra speaker cabinets laying around. Plus, it's a fact that speakers don't get hot, unlike them tubes that you have to pull out and then replace for each test. ;D

If you still haven't found the bugaboo, let us know, and we'll line out a tube testing regimen for ya.

HTH


sumgai

[Ripper]

Sumgai you have me worried now...
Im using a Marshall Valvestate 2000AVT/50w
As you know its a hybrid. Just 0ne 12AX7 tube.
When the tube wears out, and I go to change it, are the chances of me getting ZAPPED!...that great?

[hammeroff]

Thanks sumgai! (if they're 11/32, you're totally getting a +1)

Here's a link I just found for some safety when working on amps:

aikenamps.com/SafetyTips.html

I'm goin in!!!

[sumgai]

Matt,

I concur with everything on Aiken's page of Safety tips!

Good find, and always appropriate, so +1 for you! ;D


sumgai

[sumgai]

deep,

Sumgai you have me worried now...
Im using a Marshall Valvestate 2000AVT/50w
As you know its a hybrid. Just 0ne 12AX7 tube.
When the tube wears out, and I go to change it, are the chances of me getting ZAPPED!...that great?

Why would you get zapped just because you changed a tube? But I wonder even more, why would my suggestions about looking for phsycial causes of the "shrilly" noise get your hackles up? I think I said that tube are also a potential cause of such problems, and left it at that....... didn't I?

Hmmmm, was it the fact that I said "tubes are hot"? But you already knew that, right? So like Clara asked, "Where's the beef?"

Still, a little review never hurt anyone, did it?

In summary, to change a tube, you should turn off the amp first. Regardless of any potential for broken glass, the more pressing concern is, what if you get the tube twisted around, and force it into the socket, incorrectly aligned? There might be smoke and fire, or there might be worse....... Do it safe, and power down first. After all, how much time does it cost you to flip the power switch?

Thought so.

HTH


sumgai

[hammeroff]

11/32...you were right +1

I just went around and starting tightening stuff. Every thing I got a hold of was loose to some degree; a little alarming, but oh well.

The problem was actually metal tube sleeves:



Again thanks for the help guys.

On a tangent...what do these sleeves do? Disperse heat evenly?

--Matt

[ChrisK]

No, they're noise shields (or in your case, a noise enhancer).

They shield the tube from electromagnetic fields.


I'm glad that you found the problem, I was going to suggest that you just avoid playing any Bb's.