Vox AD15VT sound problem

[roslih]

Hi, I'm new here. The amp is about a year old. The problem I am having now is that the sound is distorted and does not sustain. it's fading off and buzzes after you strike a note. More obvious when you play chords. It gets worst when I increase the gain setting. Could it be just a simple pot problem ? It happens on all the modelling amps I select. Not so obvious when I play it clean. here's a sample of the sound. Take note of the tail end of each note. It starts buzzing Can anyone help......

http://www.imeem.com/people/L9wrJri/...m_a_ad15vtwma/

[Twist]

Sounds like your speaker is bad. Possibly the voice coil has warped and is rubbing.

[roslih]

Thanks for the tip. btw, I've tried using the headphone and it's the same problem. So, i guess the speaker is not the problem

[Mars Amp Repair]

Hello,
Well I guess the 'hold the amp up to the phone & I'll see if I can fix it' doesn't work too well here! yeah, kinda difficult to determine from the sound clip, but still helpful.

If the amp has a 'send' jack' you could pump that signal into another amp & see if the issue is before or after the power tubes. Be careful, as some of the line or amp out jacks are actually just 'emmulated' outputs derived from the speaker output which won't help you isolate the problem.

At the very least, you could try swapping the power tubes, if you have another set handy.

Also try balling up your fist & banging on the amp as the chord fades & see if that makes any difference. It could be a funky connection somewhere in the amp. glen

[Enzo]

Um...

This is one of those digital monstrosities. Has an LM3886 power amp. No power tubes. Ther is one tube in the preamp. Certainly worth a quick swap there to be sure.

I wish I could be more help, but I absolutely hate these things. The big AD60/120 most of all.

[acorkos]

i had one of those that sounded like the way yours does. like enzo suggested, it had a bad power amp chip. replaced it and it was good as new.

[roslih]

Hi guys, I m truly grateful and really appreciates all the kind advice given (although i wish I cud let you listen over the fone like suggested, but I am in SINGAPORE....now that's going tough isn't t...bearing the time difference and all...) anyway, I cranked the gain gain vol and master vol almost to the max (i lived in a flat (apartment)..so i didn't want my neighbors to call the police (it's pretty damn loud you know !!) and guess what ????...the problem's gone !!! Rite now although I am happy I have somewhat "solved" the problem, but is still clueless about what the problem was. Could it be just some loose connections or was it the preamp tube that is maybe "acting" up. I have an AD30VT. If the problem crops up again, I will try to swop the tube. Btw, can I use a 12AX7EH (taken from my Marshall AVT50 for this amp...sorry for this noob question, I am really new into all this stuffs)...anyway, thanks again....cheers

[petemoore]

Preamp output does this, amp/speaker follow suit of course, so it is in the preamp here.
After a while it replaces with mush when the signal fades, particularly with chords/low end.
THen after tube swaps [plural for certain], no difference...same fade out / mush in effect after a while playing.
So the preamp is prime suspect, and will get off scott free because the parts would be a tiny blur, if you could see them.

[petemoore]

Yepp, they used to work great, reliably enough to gamble on a second one, another ad15vt for her guitar needs was purchased used in good shape, and sounded great, ie ... in particular, the preamp was useful, when fed to a more substantial amp / speaker /cabinet.
Currently, both preamps [and the hardwired amps/speakers] seem to be unusable for the same reasons others are posting about failing ad15vt 'repair'. Tubes replaced multiple times on both amps here [I'm sure others have tried this too], and not really much else to look at except trying to further pinpoint the troubled circuit and replace the PCB or...
Because the fail rate puts tubes first, capacitors second [20+ yr. old caps], begin by replacing the most likely capacitor with = uf rating and >or= voltage rating and correct polarity, test, then repeat above on the next likely suspect cap until every cap has been replaced. That typed, I'm not certain a failing cap is a good match to the symptoms, but doing this would narrow the suspect list.
Though electro-mechanical hardware [pots, switches, plug-leafs etc.] can be as likely as tubes, these seem to work fine like new on both amps, and don't really fit the suspect profile of fading bass, "blurry" sound.
This scenario turned out to be much more common and less fixable than I'd envisioned when throwing down on the product.
That leaves power supply iron, which will all test good I bet, resistors which are unlikely to cause a problem unless heated excessively, which we'd probably have noticed, and then chips.
Shopping for replacement PCB's would be a likely step if chips were the last suspects, or simply replacing the advt15 with another.