Vox AC15 "sag"

Discussion in 'Glowing Bottle Tube Amp Forum' started by marcflores, Aug 2, 2019.

  1. marcflores

    marcflores Tele-Meister

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    I've had my AC15 for a month now and one thing I really started noticing is that when I crank it (MV dimed, volume around 2 or 3 o'clock) it sounds like I have a compressor pedal on. It *feels* like there is also the slightest amount of delay between my pick attack and the sound coming out of the amp.

    I'm normally plugged directly into the amp. I just let the amp overdrive on its own and I use the onboard reverb (sometimes the tremolo, but rarely).

    What's also interesting is that after about 1 o'clock, the volume doesn't get too much louder, but the amp just starts to saturate a little more. It sounds pretty cool. It definitely doesn't break up like my DR did, but I wonder if the 75-watt Warehouse speaker has a lot to do with that.

    From what I understand, most tube amps have this sag or squish? It was less noticeable in my '68 DRRI.

    Maybe this a dumb question since I'm assuming this all happens inside the amp, but would any boost pedals or anything reduce the sag? I can live with it if it's a "feature not a bug" sort of thing. Just wondering if there's a way to reduce it.

    Thanks.
     
  2. corliss1

    corliss1 Friend of Leo's

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    That all sounds like the amp is operating correctly to me. A lot of players would kill to be able to crank amps to get to that spot on the dial.

    The sag is caused by the amp having to quickly respond to the demands of amplification, and, without getting too technical, it takes the electronics a second to "catch up"

    Doing things to the preamp, such as a boost, wouldn't normally affect sag, in my experience, since sag is more of a power supply/output thing than a preamp thing. If anything, you'd probably be turning the amp down since you're boosting the input more, which could create less sag.
     
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  3. scelestus

    scelestus Tele-Meister

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    Handwired or the regular one?
     
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  4. Dismalhead

    Dismalhead Poster Extraordinaire

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    So did you buy it used with the WGS speaker? I'd think that may have a lot to do with it not breaking up. I know it's not a direct correlation, but I'd think that a 75-watt speaker would give you a ton of clean headroom.

    My AC15C1 starts breaking up around the volume you need to play with a drummer; probably 75 dB or so. As I remember though it didn't happen until the speaker had broken in - probably after 50 hours of playing through it. Then it was like magic - all of a sudden there was this magical beautiful break up that wasn't there before.

    Also, the tubes may have a lot to do with it. My amp came with junk Chinese tubes. I put in some Tung Sols (preamp) and JJs and it now sounds like a proper Vox should.
     
    Last edited: Aug 2, 2019
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  5. marcflores

    marcflores Tele-Meister

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    I have no idea. The model is AC15C1 G12C.
     
  6. marcflores

    marcflores Tele-Meister

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    I bought it brand new from GC. They had it in the store for 2 weeks before I bought it. It's called Vox AC15C1 G12C.

    I also think it needs a lot more break in time as you've said. I've only been able to play it at gig volume for maybe 15-17 hours.

    Maybe I'll change the tubes in a month or two. I don't have any super important gigs or weddings until October anyway.
     
  7. PC_Hater

    PC_Hater Tele-Meister

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    Does the amp have a tube rectifier? If so the sag is built-in and a 'feature'.
    Different rectifier tubes will give different mounts of sag.
    Many tube amps use solid-state rectifiers to reduce sag.

    It is not beyond the wit of man to replace the tube rectifier with a solid state one - but do be careful!

    Being a modern tube amp it _might_ have some interesting cleverness to simulate a tube rectifier...
     
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  8. MilwMark

    MilwMark Doctor of Teleocity Ad Free Member

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    Boosting the input will make it sag more, but differently.

    You could put in a more efficient speaker. Note, the wattage of a speaker has nothing to do with how loud the speaker is.

    But, most people price the compression that amps deliver.

    It's a feature, not a bug.
     
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  9. marcflores

    marcflores Tele-Meister

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    That all makes sense. Especially about turning the dial down if I boost the input even more.
     
  10. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    "Sag" is the result of a tube rectifier trying, and failing, to keep up with the demands of a large signal. No tube rectifier, no sag. You may be experiencing compression caused by the large signal hitting the speaker, or that loud signal hitting your ears.
     
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  11. corliss1

    corliss1 Friend of Leo's

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  12. marcflores

    marcflores Tele-Meister

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    It has an EZ81 tube rectifier.
     
  13. Dismalhead

    Dismalhead Poster Extraordinaire

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    Just looked it up and they do indeed come brand new with the 75-watt WGS rather than the Celestion Greenback mine came with 5 years ago. It does say it "enhances headroom". I didn't know they'd made the change. Not that it's bad, I've put a couple of WGS speakers in other amps and like them a lot.

    I'd give it some more time for the speaker to break in, and yes, you probably want to swap those tubes. It's the one place they really cheap out on an otherwise spectacular amp. Mine had a "fizz" sound on note decay. All my friends would be like "what is that noise?".
     
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  14. marcflores

    marcflores Tele-Meister

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    That makes sense. The manual says EZ81 tube rectifier.
     
  15. corliss1

    corliss1 Friend of Leo's

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    Interesting....................................it's not listed on the site. Wonder if they just forgot to put that in the description.
     
  16. AAT65

    AAT65 Friend of Leo's

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    Hmmm... if so that’s a design change: the AC15C1 had a full-wave diode rectifier when launched, and mine (made in 2016 I think) has a diode (solid-state) rectifier. The version of the Vox website I can see doesn’t mention rectification at all that I can see, for either the normal AC15C1 or the G12C version:
    https://www.voxamps.com/ac15c1
    https://www.voxamps.com/AC15C1G12C
    (The handwired models do have an EZ81 rectifier, but then they are 2.5x as expensive...)
    Mine sounds great though, diodes or no!
     
  17. scelestus

    scelestus Tele-Meister

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    Other people have mentioned it and I asked because of the tube rectifier. Yours must be a really new one because up until very recently the regular AC15s have had SS rectifiers. Yours, like the handwireds, has a tube rectifier.

    You can get a solid state rectifier that plugs into the tube slot. They're inexpensive - under $20 and maybe even down around $10. That should stiffen the response up a bit if you want.

    Good luck! They're sweet amps and worth putting your time and effort into.
     
  18. marcflores

    marcflores Tele-Meister

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    Huh, yeah interesting indeed. Vox doesn't say ANYWHERE on the website what types of rectifier is used. In fact, you can't even find the word "rectifier" on those pages either on the description page or the specs page. Kind of a weird detail to leave out.
     
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  19. zombiwoof

    zombiwoof Tele-Afflicted

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    There is also no rectifier listed under the tube compliment, meaning it probably has SS rectification. If that is the correct listing for the amp the OP has.
    Al
     
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  20. gusfinley

    gusfinley Tele-Holic

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    Yes the "C" Series amps do not have a tube rectifier. This is likely a typo in the manual. They also have a "ladder" style power supply with a chain of RC filter stages and higher filter cap values.

    The vintage amps, of course, had tube a rectifier, choke and all of the B+ taps were fed by the same point. The also have quite low filter cap values.

    If you like the "sag" you're getting on this amp - you ought to try one with a vintage power supply like the AC15HW1!!
     
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