1. The target date for the upgrade is August 11, 2020. We expect a few hours of downtime during that process. We will post on Twitter and Facebook to keep everyone updated on the progress.
    Dismiss Notice

vox ac30 tremolo not working

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by picbiff, Jul 4, 2012.

  1. picbiff

    picbiff Member

    Messages:
    20
    Joined:
    Dec 21, 2010
    Location:
    DFW, TX
    I've got a Chinese made late 2000 model ac30cc w/alnico blues. Recently the tremolo stopped working and I was wanting to know if replacing the tubes will fix this.

    FYI.. everything else seems to be working
     
  2. mcdes

    mcdes Member of no importance

    Messages:
    7,555
    Joined:
    Oct 9, 2007
    Location:
    New Zealand
    As far as I'm aware, there is a tube for the reverb, and another for the trem. So yea, changing the right tube 'could' fix it.
     
  3. Organtis

    Organtis Member

    Messages:
    591
    Joined:
    Jun 4, 2015
    Did you ever figure this out?

    I have a AC15CH with the same issue. I put the stock tubes back in and it is fixed but I see no obvious signs of wear on the ones I took out (Sovtek)

    It would be nice to pinpoint the right tube and not just chuck the whole $80 set. Or maybe there is some tolerance difference in the stock ones.

    Thanks
     
  4. pdf64

    pdf64 Member

    Messages:
    6,753
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    As amps age they develop bad connections, especially tube amps, especially combos, and especially those built using push fit connector systems to their circuit boards. Often, just moving things around a bit can re-establish a connection.

    Dunno how you might visually determine that a 12AX7 was worn? My experience is that 99% of weak/bad ones look the just same a good ones. The giveaway is that the circuit they're in doesn't work properly, eg preamp channels have low gain, trem oscillators don't oscillate.

    The way to identify a bad tube is to have a spare tube of the relevant type that you know is good, and substitute it into each socket that uses that tube type in turn. If no improvement of the symptom in question, take out the 'known good' tube, put the original one back in, and move on to the next socket.

    Here's a 2015 Vox C1/H service manual http://dealers.korgusa.com/svcfiles/AC15C1_Smanual.pdf
    The reverb and trem circuits use semiconductors. Maybe older versions are different?
     
    Steppin' Wolfe likes this.
  5. WillLane

    WillLane Member

    Messages:
    1,771
    Joined:
    May 11, 2015
    Location:
    Eastern NC
    Both the CC2 in the OP (from 2012) and your AC15CH do not have tube drivers for the reverb/trem. If I'm correct, each half of V1 drives the input for the Normal and TB channels, V2 drives the TB tone stack, and V3 is the phase inverter. This is true for the CC (2000's) and C (current) AC amps. Solving the problem with the stock tubes is probably coincidental, although I've been wrong before. :dunno

    As pdf64 said, check some of the connections around the reverb/trem controls. However, tube amps can contain lethal voltages even when they are off and unplugged, so consider taking it to a technician if the problem persists.
     
    pdf64 likes this.
  6. Organtis

    Organtis Member

    Messages:
    591
    Joined:
    Jun 4, 2015
    I assume you are right, I didnt think it was a tube but maybe something weird like a slight drop in power maybe not driving the trem circuit properly.

    I need to at least visually inspect it, maybe use some contact cleaner.

    I'll report back for sure.
     
    WillLane likes this.
  7. HotBluePlates

    HotBluePlates Member

    Messages:
    8,809
    Joined:
    Mar 27, 2017
    +1.

    The trem in this amp is created by a MOSFET, whose output is applied to bias-vary the output tubes somewhat like a Princeton Reverb. The trem is turned off when the footswitch shorts its Ring contact to ground, which turns off Q2, and Q2 then shuts off the MOSFET.

    If you're the footswitch is plugged in when your trem troubles are happening, try unplugging it. If the trouble continues when unplugged (and voltage at the base of Q2 remains solidly positive), you're probably looking for a bad connection from the Depth pot wiper back over to the board near the power tubes (the connection goes on to R86 & R89, the 220kΩ grid reference resistors for the EL84s).
     
    WillLane, Rob s and pdf64 like this.
  8. Rob s

    Rob s Member

    Messages:
    921
    Joined:
    Feb 13, 2011
    Location:
    Los angeles
    Wow I had no idea the trem was solid state in these.
    a cost cutting measure?
     
  9. WillLane

    WillLane Member

    Messages:
    1,771
    Joined:
    May 11, 2015
    Location:
    Eastern NC
    Cost-cutting and probably some sense of increasing reliability as well. With a normal 6 input AC30, if you don't use the vibe/trem sections you only use 3 of the 6 preamp tubes. If you take out the tube vibe/trem (or reverb), that is 3 less tubes to go bad.
     
    HotBluePlates likes this.
  10. picbiff

    picbiff Member

    Messages:
    20
    Joined:
    Dec 21, 2010
    Location:
    DFW, TX
    No I never figured it out. I played it like it was for a while and then traded it off for another amp.

    Sorry I couldn’t help
     
  11. HotBluePlates

    HotBluePlates Member

    Messages:
    8,809
    Joined:
    Mar 27, 2017
    Cost cutting and ...

    As @WillLane said, if you never use the vibrato in the AC30, you can save all kinds of circuitry. There are literally more tubes & as many parts used to create the vibrato as used in the whole rest of the preamp!

    The Vibrato/Tremolo channel has full-up circuitry for true pitch-shifting vibrato. It switches between the effect types by crippling one part of the vibrato circuit, so that only trem occurs.

    While there has been something of a resurgence in popularity of Fender brownface harmonic trem, and even some folks enthralled with Magnatone vibrato, most players who use an onboard modulation use tremolo. That would only require a single tube section and a handful of caps & resistors. So there's a great opportunity for simplification & cost reduction if one removes the full Vox vibrato circuit & only retains the oscillator. Like I said, it will literally cut the number of tubes & parts for the entire amp by about half. Quite a good trade if few players use on-board vibrato.

    The tube can be replaced with a FET/MOSFET easily with no tonal impact, because all it's doing is generating the sine wave that is modulating output tube bias. I'm kind of a tube-snob, and I have zero issue exchanging a tube in an amp for a MOSFET tremolo oscillator.
     
    Last edited: Aug 23, 2019
    Rob s and WillLane like this.

Share This Page