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VOX Valvetronix

Discussion in 'Modeling Amps, Plugins and Apps' started by blowtorch, Feb 28, 2013.

  1. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    I read on another forum that this is more complicated than it seems. Something to do with the grounding scheme on the amp. Your output jack needs to be "isolated"... and that is as far as my knowledge extends, alas. So do a bit of research before you start clipping wires.
     

  2. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    I have schematics somewhere, and so would refer to that anyways, but thanks for heads up.
     

  3. Joe Sailor

    Joe Sailor Tele-Afflicted

    Jan 18, 2011
    Longmont, CO
    I just bought an AD30VT with a 12 inch speaker! From a man who "replaced the power transformer and the output FET...Upgraded to a 12" Celestion Rocket 50 speaker...Vintage 1967 VOX logo"
    He put it in a larger Fender? cabinet, about 22 by 20 inch, the Vox grill is gone (too small) replaced by black cloth and the old logo. He had a AC15 and a vintage Vox, sure the AD was outclassed but it held its own.
    I am not sure what a FET is, but it sounds good to my ears.
     

  4. jefrs

    jefrs Doctor of Teleocity

    Nov 20, 2007
    Newbury, England
    I did try-fit a 12-in but it was never going to get in, the box is not big enough on height, you would have to remove the chassis. As in fact your chap did.

    A FET is a Field Effect Transistor. They are generally used on input stages

    I do have the schematic. The output is a LM3886 with a feedback for power level through a NJM2082D
    The LM3886 is a "68W Audio Power Amplifier" module (the AD15VT uses a LM1875). Basically a power transistor package, mounts on a heat sink. Looks like a good unit, plenty of overhead for the 40W they are asking from it on the AD30VT (yes, it is a nominal 40W amp).
    http://www.ti.com/lit/ds/symlink/lm3886.pdf
    The NJM2082D is a "dual J-FET input op-amp"

    The OEM speaker on mine was/is stamped "AD-30" otherwise unbranded but looks to be a Vox/Wharfedale GSH10-30. It is a decent speaker for its price (cheap ;). The Celestion Rocket 50 is one of their cheapest 12-in and often used for OEM in ext cabs. That's going to be one of the big differences compared to an AC15, that gets a G12-15 Blue or a Greenback if you don't want it to blow ;)
    The <15W Blue is never going to work in an AD-VT hence my choice of the 30W G10-30 Greenback (which may itself be under-rated for the AD30VT)

    The OEM speaker on the AD15/30VT schematic is 8ohm and I can confirm this.
    The LM3886 appears to be able to drive a 4ohm load so an 8ohm ext cab in parallel should be possible. But Note! - I spotted something about equalization and reactive loading in the design notes so this may not be a good idea unless your really understand what you are doing with it.
    Mine does have an ext cab socket but it shuts off the internal speaker i.e. 8ohm only.
     

  5. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    Not sure about UK, but here we buy our round tuits, sky hooks and left-handed washers at Home Depot...
     

  6. Abu Twangy

    Abu Twangy Friend of Leo's

    Mar 16, 2012
    Rocky Mount, NC
    Just make sure that you total speaker load is not less than the recommended load. The little power amp on a chip might not like it.
     

  7. randomhitz

    randomhitz Tele-Meister

    330
    Jan 29, 2008
    Missoula, MT
    You can get isolation washers for the speaker output jack that you would install but you would want to make sure you got a switching jack that would disconnect the internal speaker so the impedance load didn't get to low when your external speaker is plugged in
     

  8. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    Kiss The Hiss... goodbye.

    I have an old Sano accordion amp - not functional - with a 15 and two 8s. I am taking it apart, with an eye to having the amp, pre-amp and reverb serviced and repaired. At that point I will build a head cabinet for it. In the meantime, I those 2 8" speakers, so...

    [​IMG]

    Disemboweling is never pretty.

    [​IMG]

    Mid-20th century tech replaces 21st century tech. That's a CTS speaker on the left. It weighs about 2 lbs more than the stock Vietnamese speaker on the right. (Lord, that brings back bad memories.)

    Upon re-assembly I can say that not only is the hiss gone, even with the treble all the way up, but the amp sounds clearer and with more chime on the blackface settings. Since accordion amps were meant to be as neutral as possible (hence the brand Sano "High-Fidelity" amp) I am guessing this speaker is designed with that in mind.

    The CTS is also an 8 ohm, so I will have adjust my settings a bit, but so far this is a vast improvement. Therefore, if you happen to have a half-way decent 8" speaker you aren't using, swap it in.
     

  9. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    Abu - The LM3886 output chip datasheets show load impedance figures for 4 to 8 ohms at 68W and 38W respectively, so if you stay at those figs for total impedance it should be fine. Plently of configurations of 4, 8 and 16 ohm speakers can be had.

    Pump - we don't have Home Depot here, but do have similar (B&Q). Unfortunately they don't seem to stock round tuits, tartan paint however.....
     

  10. jefrs

    jefrs Doctor of Teleocity

    Nov 20, 2007
    Newbury, England
    If you understand the spec sheet than you may be able to add a parallel ext cab socket dropping impedance down to 4ohm. The internal AD30VT speaker is 8ohm.

    My quick glance through the specs made me think that one of the pins had to be altered to make it happy on 4ohm. :?: Something about altering the gain. I believe it is entirely possible to drop straight down to 4ohm but I don't want you any anyone else to do it on my say-so. Is the heat-sink up to running at over 60W?
    Do check! - read pp.22-23

    Supply to the LM3886 in the AD30VT is +30Vdc/-30Vdc easily enough for 40W into 8ohm then (did I mention that the AD30VT is not a 30 watt amp?) The feedback loop and the "power level" gubbins limits and reduces the actual output.

    The LM3886 must be fairly bullet proof because the Korg/Vox schematic says "if output power is not high enough, please delete both R67 and R68". Reduces NFB. You got to love them for putting "please" into an assembly instruction.

    Otherwise an insulated plastic switching jack on the speaker takes care of switching the internal driver off when using an 8ohm ext cab. Doesn't have to be insulated, the speaker "return" is grounded (pin 2). Done that, easy.
     

  11. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    Understand the spec sheet would be pushing it - I know roughly what they are talking about when I read it. To be honest I only scanned it to look at the outputs listed - I'd want to take a closer look before plugging something in that'd change the impedance.

    I learned that the hard way with an old Carlsboro Viper bass amp that I blew about three or four times before finding out that despite having an extension socket labelled for 4 ohms, you were pushing your luck with an 8 ohm cab... (That's another round tuit job to rebuild that amp using one of the ROG circuits feeding in to a 100W o/p chip I have stashed somewhere.)
     

  12. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    Another problem with modding the VT+ line is that the wattage is more than you are led to believe. If I have a VT20+ why does the power level knob go up to 30? Sort of like an amp with a volume knob that goes to 11. (And thanks to that movie for adding a new metaphor to the language. :D )

    That could be an issue if you decide to swap in a speaker that is rated at 25 watts like the Jensen CR8. Or not, assuming the speaker rating is nominal. Since I have actually taken mine to gigs I keep forgetting that this amp is marketed for practice. I am sure the majority never get anywhere near full power but I personally know several guys who gig with a Valvetronix amp of some type or a Fender Mustang, plus a few more who have one in the car as a backup.
     

  13. jefrs

    jefrs Doctor of Teleocity

    Nov 20, 2007
    Newbury, England
    The VT20+ is not an AD30VT

    For a start they have a 4ohm speaker. That meant they could use a lesser output module, to squeeze the watts out it is already maxed out on that load. It is more than 20W but I doubt it really goes to 30. Did they not use the chip from the AD15VT or was it new and cheaper one?
    The AD30VT power level knob doesn't have graduations.
     

  14. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    Played out last night and ran the thing up to where the power level dial goes from a black line to a gray line (I assume at or about 20 watts). I did not push the power into the 30w Phantom Zone. Could still get a mostly clean tone if needed with single-colis (Am Std Strat), could not with higher output pups (Tele with hi-output SD bridge and humbucker neck). Small room but we had a drummer.

    Keep in mind I replaced the speaker to a very efficient CTS from the 1960s with a very clean top end. The high frequencies kept me in the mix (I played mostly rhythm). If I were to do it again with my current equipment I'd also pack my Carvin V16 or the Excelsior and a A/B/Y box and run the V20+ at lower volume. It would give that poor pre-amp tube a bit of a break and the plentiful low end from either of those amps would be a nice balance.
     

  15. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    I may be wrong on this (and actually it contradicts my earlier posts relating to changing speakers) but my understanding of modelling amps like the Valvetronics series is that the stock speaker is actually meant to give a clean undistorted reproduction of the amps output, and all the tone including speaker distortion comes from the amp modelling. That being the case then I would expect to be able to run my AD30VT at full power on the clean amp models and for it to be clean (or at least as clean as the amp model settings allow).

    My reasoning: A Tonelab (or Pod or whatever) is best served running direct to a PA (or other completely clean amp) so that the amp / speaker models are not distorted by a guitar amp trying to enforce its own tone on top of the carefully constructed models. By that reasoning then, if you think of the Valvetronics amp as a Vox Tonelab with a power amp and speaker built in then the power amp and speaker needs to reproduce cleanly the sounds of the modelling section of the amp.

    Maybe then referring to my thoughts above I need to plug an efficient PA speaker in to the amp to get the best results....

    ..... Or maybe I'm talking out of my rear end again - in which case apply the usual sarcasm and wit....
     

  16. jefrs

    jefrs Doctor of Teleocity

    Nov 20, 2007
    Newbury, England
    The clean amp models are quite faithful and that means lower output volume than the high gain models, so yes one can run the AD30VT at full chat on the clean models.

    I forget what I paid for my AD30VT, about £120 perhaps maybe less. That does mean we don't get a particularly good OEM speaker even if it is intended to be matched to the amp. The speaker in the CubeX amp is far more "matched" to the little box than the Valvetronix one is.

    I have fitted a Celestion G10-30 Greenback to my AD30VT and it does sound better than the stock speaker, more like a guitar amp should. However that speaker is about £60, half the price of the amp.

    The AD30VT is far more of a working guitar amp than a purely modelling Tonelab/Pod/what-have-you is into a desk or PA. I have added an ext cab out to my AD30VT and yes it does change its voice on another speaker (but the G10-30 doesn't need help). For comparison I also have a Korg AX10G with much the same models as the AD30VT, no contest the amp wins.
     
    Last edited: Mar 11, 2013

  17. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    My post above was just some thoughts given a small amount of knowledge of how SS amps work, so you're probably right.

    Once I do put the jack in, I have several speakers around that I can try out which'll e interesting....
     

  18. PumpJockey

    PumpJockey Friend of Leo's Ad Free Member

    Oct 12, 2010
    New Joisey
    I can testify that a different speaker gives you different sounds to the models. A more efficient speaker gives you enhanced cleans, reduces overdrive etc.
     

  19. jefrs

    jefrs Doctor of Teleocity

    Nov 20, 2007
    Newbury, England
    A given speaker will sound quite different in the box to out of it or in another cabinet.

    The AD30VT is quite loud enough on its own. It goes plenty loud enough to gig. So despite my added spk out I have not played with ext cabs since fitting the G10-30.

    What I would like is a much higher level on the headphone output, the thing is only "consumer line level" (0.8-1.2V) and not enough to drive studio cans.
     

  20. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Sep 8, 2009
    Horsham, UK
    Hmm - with what you are all saying, that round tuit might just get here this weekend.... The weather still isn't looking good enough or me to spray the two guitar bodies I have hanging in the workshop so I may have a little time to spare....

    And if it is as good as advertised we may end up with yet another plan added to the list...
     

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