V4 scratchy noise

[Cates]

Hey guys. I'm getting a noise coming out of my V4 that sounds a lot like somebody is sanding something, kind of a scraping sound. It's kinda faint. The amp still plays though.

The volume does not effect the amount of scratching or the volume of the scratching. I already checked the tubes to make sure they aren't micro phonic...they aren't.

So what do you guys think is the issue? Pre-amp tubes? Power amp tubes? How much do ya think it'll cost me?

 

[Rick Auricchio]

It could still be a noisy preamp tube. No way to tell without substituting a known-good one.

 

[Cates]

Alright, preamp tubes are pretty cheap anyway. thanks for the advice. Should I have it taken in to get them replaced? Is there a big risk of shock?

 

[Cates]

Also, is there a way to know which tube is malfunctioning or do I just need to buy one of each kind of tube?

 

[wallybill]

You could take the tubes to someone with a tester. But I always keep a set of back up tubes. Years ago, I had a preamp tube go out on me at a gig and I played straight into the PA for the rest of the night. Never had a tube go bad since, but I still have an extra set - just in case. That way you can switch out each tube one at a time and narrow down to which tube (or if it is a problem in another part of the circuit). I always switch out all the power tubes if I'm testing those - just because someone once told me to do that.

 

[Rick Auricchio]

As long as the amp is off and you're not reaching under the chassis, you should be fine. If your output tubes have metal caps, avoid touching those caps.

Many amps have a few of the same type of tube in the preamp. For example, if your amp has three 12AX7s, you only need one new one. Try it in the first position. No improvement? Put the original back and try the second. Etc.

When changing preamp tubes, there's no need for rebiasing or adjustment.

 

[Cates]

As long as the amp is off and you're not reaching under the chassis, you should be fine. If your output tubes have metal caps, avoid touching those caps.

Many amps have a few of the same type of tube in the preamp. For example, if your amp has three 12AX7s, you only need one new one. Try it in the first position. No improvement? Put the original back and try the second. Etc.

When changing preamp tubes, there's no need for rebiasing or adjustment.

Cool man, thanks a lot.

 

[Rick Auricchio]

If your amp is a V4BH, the tube complement is (2) 12AX7, (2) 12AU7, (4) 6L6/5881.

The 6L6/5881s are the output tubes; if those need changing, have a tech do it so he can rebias the amp if necessary.

Looks like one each 12AX7 and 12AU7 will let you troubleshoot and have spares. Those are available at any music store and aren't expensive. There are a dozen brands.

 

[Cates]

Its the V4. JUST V4, not the V4B or V4BH.

 

[Rick Auricchio]

No info for the V4 on Ampeg's web site, so you'll have to look at the chassis to see the tube complement. The preamp section probably won't be too much different.


EDIT: I was way off on my guess about the tube complement. See the schematic at this site for the list of tubes. They may not be easily available at every music store.

 

[B-string]

A tube tester will more than likely tell you nothing, swapping out is your best bet. Other than that scratchy pots can cause this as can leaking coupling caps.