VOX AC15 C1 Loud Noise. Any ideas?

[jczeroman]

I purchased a Vox AC15 C1 new from a USA dealer in 2017. About a year ago, the amp developed a loud noise which occurs at random times. Once it starts, it doesn't quite stop. I have tried different guitars: no effect. I have tried different cables: no effect. I then replaced the pre-amp tubes: no effect.

You can hear the noise here: https://www.youtube.com/watch?v=TgY-LkikuEA.

The noise will increase and decrease in volume as I move the master volume knob.

Any help you can provide is appreciated!

[Chuck H]

Well it sort of leaps into it. Like a trigger as you increase the setting of the master. This is usually indicative of an oscillation. It sure doesn't sound typical though. Some questions...

At some given master volume setting, just below where the noise starts, do any other controls induce the noise as they are increased? ie: can advancing the channel volume, treble control or decreasing the cut control cause the problem without using the master?

I can't see in the video whether or not you have a guitar plugged in. Does the amp do this without a guitar plugged in? And, does the amp do this with a guitar plugged in when the guitars volume is set to zero?

Does it happen on both the normal and bright channels?

Does it happen when the channel volume on the amp is set to zero?

[jczeroman]

Thanks for your patience while I tested all of these out. And thank you for helping!

First the effect comes at random. I just spent 30 minutes messing around and couldn't get it to produce. Last night, I could get it to show up in less than three minutes.

It comes on with a kind of crackle, intermittently. Eventually it just starts going and wont stop. It seems to be triggered by playing a guitar, but it does not require a guitar in order to continue. In other words, to answer several of your questions, even if the guitar is turned to zero or the guitar is unplugged or even if I pull the guitar plug out of the amp--the noise continues. Turning other knobs (bass, treble, etc.) have no effect on the noise. As you see in the video, turning the volume knob does have an effect.

It happens on both the normal and top boost channels.

I can even get the effect to come in very briefly, without anything plugged in, if I smack the back of the amp a little.

[Chuck H]

There are a lot of plug and ribbon cable connections in that amp it seems. I think taking the time to de-ox and clean all non soldered contacts could solve the matter. There are products for this available on line or, if you still have one in your area, an electronics store. This includes tube socket pin holes and tube pins. The tests I proposed were based on what I witnessed in the video, where it seems that higher master settings would bring it on.

Another slim possibility is that your power tubes have become microphonic. Though the static noises wouldn't be typical of that condition. Mechanically compromised power tubes will lurch into ugly oscillation in mid frequency ranges and stay there until input is removed. This is a common condition for el84's in combo amps.

[Mick Bailey]

Have the power amp tubes been replaced? If not, I would get a new matched pair so that you've got a baseline to work from. If it is the tubes it will save you a lot of work. If not, well... you're starting from a point where you've eliminated the tubes. And you have a new fresh set.

[jczeroman]

Another slim possibility is that your power tubes have become microphonic. Though the static noises wouldn't be typical of that condition. Mechanically compromised power tubes will lurch into ugly oscillation in mid frequency ranges and stay there until input is removed. This is a common condition for el84's in combo amps.

I wonder about this. I tried tapping the power tubes with a pencil and got the noise instantly (plus the tube tap seems to reverberate through the amp). I'm going to go ahead an replace the power tubes.

[Chuck H]

I tried tapping the power tubes with a pencil and got the noise instantly

I would say that's a sure sign. But it's still possible there's an intermittent contact on a non soldered connection. I hope new power tubes resolve the issue.