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Vox AC4C1 schematic and mod ideas

Discussion in 'Amp Tech Center' started by PurpleStrat, Feb 2, 2014.

  1. Kingsangels

    Kingsangels TDPRI Member

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    Hello, So took apart stared at it dor an hour. Pushed down on tubes. Put back together and got sound. But very low.
    But there is sound. Volume and gain cranked and its at whisper volume. Should i change all tubes?
     
  2. sliberty

    sliberty Tele-Afflicted

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    I'd still change one at a time. Do you have any tubes n hand that are appropriate?
     
  3. Kingsangels

    Kingsangels TDPRI Member

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    Sure don't. Will have to buy all new. Will let ya know how it turns out. Thanks for everything
     
  4. sliberty

    sliberty Tele-Afflicted

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    You could start by buying 1 - 12ax7 and 1 - EL84. I feel like it is unlikely that you will need to replace all of the tubes. it is more likely that only 1 is bad (if it even is tube related). The amp has 2 - 12ax7's and 1 - EL84, so if you buy just one of the 12ax7's, you can try the new one in each position to see if it helps. But if you don't mind getting all new, then get all three tubes - they'll get used eventually.
     
  5. Bartimaeus

    Bartimaeus TDPRI Member

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    Well that isn't very punk, is it?
    Just dropping by to say that I tried swapping out the Celestion G10 Gold that I had in my AC4C1 for 10" Greenback. The difference is what you'd expect, a smoother tone but with less definition in the treble.

    IMHO, the Greenback better suits this amp than the Gold. The main problem that folks have with this amp is its high end, and the Gold speaker seems to accentuate how brittle and sharp it can get... I tried a bunch of component swaps to make the Gold work with it, but I could never get it quite right. TBH, I think I'll keep the Greenback and leave more treble in.
     
    asnarski likes this.
  6. MadJack

    MadJack Friend of Leo's Silver Supporter

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    If the G10 Gold is new, you really need to let it break in. They sound much sweeter once they're broken in.
     
  7. Bartimaeus

    Bartimaeus TDPRI Member

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    Well that isn't very punk, is it?
    I had the G10 Gold in there for about four years. It's a great speaker, and I could hear some of that sweetness, but there's a little harshness from the amp that I find difficult to remove – almost certainly due to the lack of a tone cut control.
     
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  8. haxxxor

    haxxxor TDPRI Member

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    I discovered this thread a few weeks ago and it's helped me fix up my little blue AC4C1-BL to a point where I'm super happy with it. My initial impressions were that it was a little brittle, top-end heavy, lacking in bass, small, chimey, and boxy sounding. Based on you guys' accumulated wisdom, I made the following small changes:

    • Replaced the stock 10" speaker with a Celestion Blue
    • Clipped C6 and C20

    This didn't fully address the issue, although it was tamed to a small degree - just not enough. The Blue was subjectively nicer to my ears, but the amp was still a bit bright/sterile and lacking in the warmth I was hoping for. So I did the following additional mods:

    • Replaced the stock output transformer with a Music Power Systems OT10SE
    • Added pink fiberglass insulation stuffing to the cabinet
    • Clipped C4
    • Replaced R1 with wire link
    • Replaced R4 with 33k

    Now THIS has given me the sound I wanted. It's full, warm, but still has all the glorious detail of which the Blue is capable. The amp doesn't get as dirty as it used to, even at full gain, but with a simple clean boost pedal it's easy to drive the AC4C1 into glorious, rich, creamy tube distortion.

    It's really, really good. I think part of the success was due to taking a hi-fi speaker builder trick to increase Q using a stuffed cabinet, thereby gaining the effects of a larger cabinet. It also cuts down the boxy sound of of the little amp by damping internal reflections. Here's some pics:

    The original output transformer:

    [​IMG]

    New output transformer fitted, caps clipped, resistors fettled, and some pink fiberglass glued in place:

    [​IMG]

    And the cabinet with its Celestion Blue and pink fiberglass:

    [​IMG]

    Thanks to you all for the helpful discussion, hints and tricks. I hope this post is of use to someone!
     
    3-Chord-Genius likes this.
  9. gusfinley

    gusfinley Tele-Afflicted

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    Doesn't your guitar have a cut control?
     
  10. sliberty

    sliberty Tele-Afflicted

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    A cut control and a tone control are not really the same thing. Guitars have tone controls. They roll off the high frequencies of a certain range by passing them to ground. A cut control rolls off high frequencies, usually much higher frequencies actually, by cancelling them in the power stage of an amp. The impact to the resulting tone is somewhat different.
     
  11. 3-Chord-Genius

    3-Chord-Genius Poster Extraordinaire

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    I wish I had seen this mod idea before I sold me AC4C1-12 (same amp as yours, different speaker and cab) because I wanted to love it. I clipped the first two caps that you mentioned, but that was all I did. Glad to hear it worked out.
     
  12. haxxxor

    haxxxor TDPRI Member

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    I found after some late-night playing that the top-end appears a little muffled at low volumes, which isn't surprising given that all of the bright caps were chopped out of the circuit. So I bought 3 SPST switches from Sparkfun and hooked them up to C4, C6, and C20. It looks like this:

    [​IMG]

    Internally the caps are wired to the switches like so:

    [​IMG]

    At day-time volumes when I can crank it a little, I turn off the switches to reduce brightness and it sounds great. At night when the kids are in bed, I turn on the switches to add the bright caps back into the circuit, making the amp playable/enjoyable at very low volumes. It works very well.
     
  13. dragonfly66

    dragonfly66 Friend of Leo's

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    Clipping of C6 and C20 are the bright caps to make the amp not sound so bright. And a speaker swap is a speaker swap.

    What do these other mods actually do?
    • Replaced the stock output transformer with a Music Power Systems OT10SE
    • Added pink fiberglass insulation stuffing to the cabinet
    • Clipped C4
    • Replaced R1 with wire link (is this the same as a jumper?)
    • Replaced R4 with 33k

     
  14. sliberty

    sliberty Tele-Afflicted

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    Transformer swap might add a little headroom and a little lower mid, but does little to nothing about bass response.

    Fiberglass is meant to provide less reflection inside the cabinet. This can alter freq response somewhat, but I never did it, so don't know the value in this amp.

    C4? Hmmm....might reduce some upper frequencies??? Might also allow radio stations to be picked up :) Not really sure.

    The R1 and R4 changes are to make the input more like a traditional input. I would suggest that you either do all of the last 3 mods or none of them as they all have to do with the way the input is configured.
     
  15. dragonfly66

    dragonfly66 Friend of Leo's

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    I bought the AC4C1-12 LTD MB last night because it was on sale, got a floor model for even less than the sale and am happy. After playing with it on a 2 humbucker guitar the distortion sounds terrible. So I went online and found a bunch of threads. Seems like everyone does the bright mod and tube swap. Most do a speaker swap and a few have done other mods.

    I already have tubes and a bunch of speakers to try so no brainer there. I clipped C6/C20, took out Chinese tubes and put in JJ EL84, Tungsol 12ax7 V1, TAD 12ax7 V2, and an 8 Ohm Celestion G12-35XC. I am digging the way it sounds now. Very clear grit with just enough bass when using the bridge pickup. With the neck pickup the bass gets a little muddy, but overall the mids and highs are crisp. I'll have to try it with my main axe that has P90s in it.

    I originally bought the G12-35XC to try out with my Fender amps, thus the 8 Ohm. I found an AC4C1 owner who had used an 8 Ohm speaker in the amp for over a year without any issues. Is there any cause for concern in using an 8 Ohm speaker instead of a 16 Ohm in the AC4C1-12?
     
  16. sliberty

    sliberty Tele-Afflicted

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    Using an 8 ohm speaker "probably" won't give you any big problems.

    Is it rougher on the tubes when you are mismatched? Yes. What is the impact? Shorter tube life? Maybe - maybe not. How short? Instead of lasting for 20 years they might last 15 years (what do I know?). What about my transformers? They are crap anyway. If you blow a transformer (which you almost certainly won't won't) it wouldn't be any great loss, and not very expensive to replace.

    I say don't give it another thought But of course that's easy for me to say since it's your amp :)
     
  17. dragonfly66

    dragonfly66 Friend of Leo's

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    After some time with the AC4C1-12 I just didn't bond with it. I've since purchased a Bugera V5 that works well for my purpose without mods. V5 is cheaper, warmer, has a very nice emulated spring reverb.
     
  18. sliberty

    sliberty Tele-Afflicted

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    Shame about the AC4. I liked mine just fine, even though I felt it lacked bottom end. But I sold it because I got a great deal on a barely used AC10. Then I sold the AC10 because I got a killer deal on a never used AC15.

    And no, I am not going to sell it in order to get an AC30 :)

    Actually, I have found that the Vox AC series amps don't really have enough headroom for. The AC15 is better, but I'd still like to be able to hit the amp harder without it getting all driven and compressed. In the end, I may end up with another Fender.....
     
  19. Ravemen

    Ravemen NEW MEMBER!

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    Hi! Can you explain how you've connected the OT10SE? Did you use the 5k or 7k primary? I saw some where that the AC4C1 uses 10k primary, do you think that 5k primary and 8oms secondary is a good idea? (Same ratio...)

    THANKS!
     
  20. Bartimaeus

    Bartimaeus TDPRI Member

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    Well that isn't very punk, is it?
    Earlier in this thread, there was some discussion as to which tone stack the AC4C1 amp used, since the original schematic erroneously had the bass pot grounded. This amp uses a modern Vox tone stack with an ungrounded bass pot, while the traditional Vox tone stack has a grounded bass pot.


    The Vox AC30CC2 and Vox AC30HW both offer a switch to select between the modern and traditional tone stacks, so I decided to mod my AC4 for the traditional response to see how it differed. (On the AC30HW, it uses the traditional tone stack on the “cool” mode and the modern tone stack on the “hot” mode).


    A straight-up conversion requires that you cut two traces and add two jumpers/wires. Really easy given how thin the traces are on this amp... Obviously, you need to cut the connection between the two lugs of the bass pot. You also need to cut the trace connecting lug 3 of the pot to the rest of the circuit. Then, you wire lug 3 to ground, and the center lug to the 10K resistor.


    On lower settings, the difference is basically indistinguishable. But at higher bass settings, you get those unusual interaction effects with the treble knob. With the stock/modern tone stack, the mids are removed in a very predictable way, but with the traditional stack you can end up with a lot less mids on several settings. The amp has a warmer (but not darker) feel to it on these settings.


    It seems that Vox thinks the modern/stock tone stack works better with gain, which is probably why they chose it for this amp and for the AC30HW’s “hot” mode. I modded my AC4 for low-volume semi-cleans, and I think the traditional tone stack offers more flavors for my use.


    I ended up wiring up a toggle switch to select between the two tone-stacks. I don’t know how much I’ll actually bother to switch between them, but I’m interested to find out.
     
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