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Ac15 help needed!!!

terrapin

Member
Messages
4,066
Last night my AC15 developed an odd little "phantom" noise. It is rather small, but there!!! It is like a little chimey higher frequency sound that just kind of floats there. No noise when not playing, but is there constantly when playing. It seems to be more apparent in Top Boost vs. ef86 channel.

My first thought is that I may have a microphonic preamp tube???

What do you guys think and how can I diagnose the problem???

Thanks in advance...................
 

wattfab

Gold Supporting Member
Messages
38
OK I'll "chime" in.....
I would start with preamp tube swap in the top boost channel, then I would check all the speaker mount screws to make sure its not simply a hardware rattle. They should be hand tight not cranked down. Sometimes its that simple.

If you have spare power tubes that would be my next step.
 

terrapin

Member
Messages
4,066
OK I'll "chime" in.....
I would start with preamp tube swap in the top boost channel, then I would check all the speaker mount screws to make sure its not simply a hardware rattle. They should be hand tight not cranked down. Sometimes its that simple.

If you have spare power tubes that would be my next step.
Sounds like the advice I anticipated. And yes, I do have more tubes.

Thanks,
 

Papajuice

Gold Supporting Member
Messages
3,147
Turn it on turn the volume up, nothing plugged it and then take a pencil and tap the preamp tubes. This will tell you which is bad because that is what you have wrong. Try not the hit the EF86 tube because this are naturally a little microphonic and will make noise.
 

terrapin

Member
Messages
4,066
Turn it on turn the volume up, nothing plugged it and then take a pencil and tap the preamp tubes. This will tell you which is bad because that is what you have wrong. Try not the hit the EF86 tube because this are naturally a little microphonic and will make noise.

Yes, I kind of know about this diagnostic. This is when I wish Vox amps were a little more user friendly as far as getting to the tubes. My ONLY issue with my AC15H1tv is that the cabinet is a bit "cheap" in construction and the screws always want to strip!!!

Thanks for more solid advice............
 

Papajuice

Gold Supporting Member
Messages
3,147
A friend just traded for one of those got this week and played it today and the top Boost side died. Probably a tube in it also, sounds like he is about to have some fun.
 

SatelliteAmps

Member
Messages
6,170
SO, NOBODY has any advice!!!!!!!!!!!!! I find that hard to believe!!!
The Amp Tech forum would have gotten faster replies. As for what to do, check the tubes as stated. If that isn't it, then there are a variety of possibilities. Could be a vibrating jack, could be a failing cap, could even be a loose connection or ground somewhere. Are you competent to poke around inside of a amplifier? If not, then if it isn't the tubes, then it should probably go to a tech.
 

terrapin

Member
Messages
4,066
The Amp Tech forum would have gotten faster replies. As for what to do, check the tubes as stated. If that isn't it, then there are a variety of possibilities. Could be a vibrating jack, could be a failing cap, could even be a loose connection or ground somewhere. Are you competent to poke around inside of a amplifier? If not, then if it isn't the tubes, then it should probably go to a tech.
Thanks, No I am not qualfied to dig around in a tube amp other than simple tube stuff. Lo and behold, I took the back off and there was a poorly seated preamp tube!!! Reseated it, and problem GONE!!!! I love it when things are simple!!!
 

smallcrap

Member
Messages
355
Assuming this is a combo and it has a speaker out jack and you can disconnect the combo's speaker - Connect to another speaker cabinet to determine amp/tube making noise vs. chassis, etc. making noise.
 
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