Vox AC15HW1 Making weird low octave overtones?

Discussion in 'Amps and Cabs' started by pedalparty, May 12, 2015.

  1. pedalparty

    pedalparty Member

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    Hello, my AC15 has started to develop odd low bass overtones in the middle of the neck particularly on the "a" and "d" strings. It also does it on certain note combos. Everything else about the amp sounds great and has since the day I got it about 2 years ago. The overtones aren't overwhelming but very annoying and umpleasant. I record with this amp alot and obvioulsly can't record with it like this. I have changed all preamp tubes and that didn't fix it. I play it daily and get it pushing pretty hard frequently so I guess I should change the power tubes soon, especially since I never have. Any other suggestions? Caps? Thanks!
     
  2. Bobche

    Bobche Member

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    I would first with changing the power tubes. Many say that the vox eats tubes so it could be a bad power tube. Just make sure you get a matched pair of EL84s in there! If issue persists, take it to a tech.

    I have the exact same amp but havent yet experienced any similar problems.
     
  3. Cirrus

    Cirrus Member

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    Yeah, I'd think power tubes too - I've had a similar issue a couple of times with my AC30 where a power tube is going bad and produces low frequency mess on certain notes. Once it happened at a gig and it sounded like a digger was crashing into a pile of rocks every time I played a C.
     
  4. Bobche

    Bobche Member

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    Funny this - i was just playing my other vox which is an AC15C1, and it was making all sorts if strange humming/rattling sounds when playing certain notes or chords. Pretty certain its a tube gone bad!
     
  5. TheWayfarer84

    TheWayfarer84 Supporting Member

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    My AC15HW had a bad solder joint where that bright switch connects to the volume pot. It caused it to fart, fade or completely cut out occasionally. A few others have had the same issue with that exact joint. I would try moving that wire around with chopsticks to see if it goes away. If so, just reflow the solder.
     
    Last edited: May 12, 2015
  6. pedalparty

    pedalparty Member

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    There great aren't they Bobche. Thanks CiscoKid784. I hadn't heard that before.
     
  7. hacinador

    hacinador Member

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    It can also be a speaker cone cry. But I would try to change the power tubes at first.
     
  8. Bobche

    Bobche Member

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    Yeah they are very nice amps indeed. I have only had mine for around 2-3 months so still trying to work out how i like it set best. You get different tones using different wattage, inputs and MV bypassed.

    im mainly using a tele these days. At first i was always using the Normal channel with bright switch on, edge of break up, but lately iv been using top boost more, 7 watt setting on Low Input. Sounds great for home use.

    how do you like running yours mate?
     
  9. playthecray

    playthecray Member

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    I also have a Vox AC15HW1X that I acquired a couple months ago. Great amp. Jump the channels and bypass the MV. Has anyone experimented with different preamp tubes? Specifically a 5751 in the V1 slot? I am still running the stock Ruby tubes. I have a matched pair of Tungrams EL-84s that I want to install.
     
  10. J M Fahey

    J M Fahey Member

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    Try the tube swap first, hope you nail it.

    If not, my second suspect would be poor filtering, either somewhat dry main filter cap or it having a poor/dirty/corroded grounding.
    always make me suspect poorly filtered 100/120Hz being mixed with and intermodulating lower Guitar notes being played.

    To boot, classic Fender/Marshall type amps use "cascaded filtering" meaning the main voltage is filtered by a choke or resistor to have a lower and cleaner screen voltage, again for the PI and so on until you reach the first preamp tube, which gets *very* clean +B voltage, after 4 or 5 filtering stages.
    VOX on the contrary, "branches out" screen powering voltage into 3 independent filters.
    Not bad for a PI which handles a couple dozen volts of audio, definitely poor for a sensitive preamp which handles millivolts, so a VOX will start showing hum earlier than any other amp.

    Of course, it's part of "the recipe" and nobody even dreams of changing that.

    VOX AC15 preamp, notice how the top power line splits into 3 RC filters: R1+C5 , R6+C2 and R10+C4 , and the first, most sensitive tube in the whole amp, V5, gets *less* filtering (22k+10uF) than the PI (68k+10uF) , all coming from the same relatively "dirty" supply.
    [​IMG]
     
  11. pedalparty

    pedalparty Member

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    bobche, I have been running mine in the top boost low input with the bass around 12:30 o'clock, treble around 12 o'clock, and gain around 12 o'clock and tone cut around 3 o'clock. Its a great touch sensitive sound that takes pedals well. It also compresses just enough to keep everything at a similar volume level live, which is very handy for me. I hadn't considered cone cry hacinador. I've never experienced it before. Thanks J M Fahey! that was very informative. My guess was either power tubes or filter caps. I sure hope its not a filter cap. I don't trust myself with those voltages.
     
  12. RocksOff

    RocksOff Member

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    Coincidentally, have you checked your intonation?
     
  13. Bobche

    Bobche Member

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    Im pretty much using mine in the exact same way, except ive got the channel volume cranked. Even with it cranked it still retains clarity and sounds great with a HotCake pushing it.

    Im very keen to try a Timmy through it soon!
     
  14. eneyantia

    eneyantia Member

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    Many say that the vox eats tubes so it could be a bad power tube.[​IMG]
     
  15. kbgear

    kbgear Member

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    Yeah, this was my first (novice) thought, too.

    That schematic makes my brain hurt. Guitar feeds into V5 first? V1 is the PI? Feh.
     
  16. Cgkindler

    Cgkindler Member

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    Could it be cone cry?

    My Two Rock Studio Pro 22 is doing something similar (only at the high A- High D#...)

    Just a thought! :)
     
  17. pedalparty

    pedalparty Member

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    Yeah rocksoff, my intonation is good. Cool bobche. I haven't tried the hot cake but am wanting to. The timmy is amazing! I know a ton of people rave about it on here but I've never been so happy with a drive pedal. As soon as I heard it the first time I just felt like the search was over and I have been extremely happy with it ever since (2 years). The eq on it is just the best. For the transparent and smooth overdrive I am looking for, it is literally perfect with a vox. I started a thread like a year ago reviewing all the overdrives I had owned and played through the amp at that time. Alot of folks chimed in and there was some very helpful info in there for vox owning overdrive enthusiast. I think it was called voxish amp owners unite or something if you wanna check it out. Ive since tried alot more I need to add to the review haha.
     
  18. Adagietto

    Adagietto Supporting Member

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    +1 on checking the power supply ripple. I had an amp that developed some subtle octave tones on certain notes, very similar to this description. It was the filter caps.

    One of the big clues was that bending the problem notes produced a ring-mod-like effect. That's because ripple is a fixed frequency (120Hz in the US, between A#2 and B2). Bending notes against the fixed ripple tone produced varying sum and difference tones — even though the ripple tone was not noticeable on its own. If the problem is ripple, one may also notice increased background hum.
     
  19. FreeBlues

    FreeBlues Silver Supporting Member

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    Off topic, but yes please!
     

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