VOX AC15 tube advice needed

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Re: VOX AC15 tube advice needed

Postby (brammy) » Sun Dec 09, 2012 2:31 am

Do your 12-strings have toasters or higains?


I dont have any Ric 12-strings.... I have two 360s (hi-gains) and one 350v63 (toasters)
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Re: VOX AC15 tube advice needed

Postby (Folkie) » Tue Jun 04, 2013 10:22 pm

Well, after several months of trouble-free use, the problem has come back. This time I have my 360/12c63 plugged directly into the Top Boost channel. As soon as I take the amp off standby, I get unruly static and squeal. Nothing else seems to be causing any interference. Could this be a microphonic tube?

I do have a spare 12AX7 given to me by Gary at North Coast Music, who sold me the amp. But I'd be more comfortable having an expert replace it. On the other hand, the amp is so heavy that it's an ordeal to get it back to Empirical Sound, so I want to exhaust all other avenues first. I've tried turning off the overhead lights and fan, and changing cables, to no avail. I haven't been having any noticeable problems with my solid state Fender amp.

I was planning eventually on upgrading the tubes in my AC15, so maybe now is the time to do it. I'd appreciate any advice from tube-experts here.

Thank you,

Robert
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Re: VOX AC15 tube advice needed

Postby (soundmasterg) » Tue Jun 04, 2013 11:34 pm

Folkie wrote:Well, after several months of trouble-free use, the problem has come back. This time I have my 360/12c63 plugged directly into the Top Boost channel. As soon as I take the amp off standby, I get unruly static and squeal. Nothing else seems to be causing any interference. Could this be a microphonic tube?

I do have a spare 12AX7 given to me by Gary at North Coast Music, who sold me the amp. But I'd be more comfortable having an expert replace it. On the other hand, the amp is so heavy that it's an ordeal to get it back to Empirical Sound, so I want to exhaust all other avenues first. I've tried turning off the overhead lights and fan, and changing cables, to no avail. I haven't been having any noticeable problems with my solid state Fender amp.

I was planning eventually on upgrading the tubes in my AC15, so maybe now is the time to do it. I'd appreciate any advice from tube-experts here.

Thank you,

Robert


It could be a bad tube, but it could also be a dirty socket or pot, or perhaps the pins on the socket might need re-tensioning. It could also be other things too, such as a bad guitar cord, so check that first with a known good one. Aside from that, or problems with the guitar jack, the first place to start with problems like this is the tubes themselves. They do go bad from time to time and need to be replaced, which is why they are mounted into sockets. That said, preamp tubes, unless they are poorly made usually last many years without problems. Replacing a smaller preamp tube like a 12AX7 in an amp is very easy. First rule is to realize that it fits into a socket and the pins must be lined up correctly before it will go into the holes. You carefully pull the existing tube out in a gentle rocking motion, and don't squeeze the glass too hard or it breaks. Its similar to a light bulb in that respect. As you take it out you take a mental note about the resistance that you feel as the tube comes out. If it is extremely easy then perhaps the socket pins need to be re-tensioned. If it seems to have a bit of resistance, then it is probably ok, and you can put the new tube in. If it is hard to see the socket, then you can gently push down on the tube as you turn it around in a circle. As you rotate it, eventually the pins will line up and it will go into the socket. Once it is in the socket, turn the amp on and try it. If the tube fits loosely into the socket, then you should take it to a tech to have the socket re-tensioned. Likewise, if changing the tube(s) doesn't fix the problem, then you should probably take it to a tech too as anything inside the amp that might have to be troubleshot is operating under high voltages which can be dangerous to work around unless you know what you are doing. Having the power tubes changed in some amps involves setting the bias, but in this amp it is cathode biased so it sets the bias on its own. All you have to do to swap tubes on the AC15 is to put the proper tube into the socket and make sure it is tensioned correctly so all the pins make adequate contact and there is nothing else left to do. Do not try to tension the sockets on your own until you verify that the caps in the amp are discharged, AND learn how to do it properly yourself but learning from someone who knows how perhaps. High voltage can be on the socket pins even months after the amp has been played. best to be extra safe around high voltage so you don't get shocked as it can be very dangerous. It sounds like you don't have much experience with electronics or tube amps so you may be visiting a tech for awhile until you learn some things. Tube amps are always noisier than solid state amps, but they do things tonally that solid state just can not do, which is why most guitarists use tube amps still. Anyway, hope that helps a bit.....

Greg
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Re: VOX AC15 tube advice needed

Postby (Folkie) » Wed Jun 05, 2013 12:25 pm

Greg,

Thanks so much for the detailed and helpful response. You clearly know your stuff!

Robert
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Re: VOX AC15 tube advice needed

Postby (soundmasterg) » Fri Jun 07, 2013 2:39 am

Folkie wrote:Greg,

Thanks so much for the detailed and helpful response. You clearly know your stuff!

Robert


You're welcome! Hopefully it will help you to troubleshoot it.

Greg
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