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Thread: AC30C2X Caldwell mod

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    AC30C2X Caldwell mod

    Hello everybody, I was looking for a mod that would bring my AC30C2X more into a TBX territory, I'd find out about the Caldwell mod some time ago specifically for the CC2 but apparently C2 and CC2 share some cap values so it should benefit from it too, anybody tried it on the Custom series already?

    Anyway since the mapping is different between the 2 amps, would you confirm that this mapping convertion is correct?

    Plate resistor mod:
    R6 & R7 = R14 & R12 respectively. However, R14 is already 220k so all you have to do is change R12 from 100k to 220K

    Normal channel gain increase:
    R49 = R13 – jumper this for more gain. The treble bleed cap on the normal channel is C8, stock is 120pf if you wish to muck around with that. (Mod changes this cap to 100pf, and has a "brilliance" switch that puts a 68pf cap in parallel with the 100pf)

    Top Boost treble cap:
    C13 = C15 (stock 120pf) tweak to taste (68pf?) or remove

    Tone stack tweak
    C16 = C23 (stock 56pf) -> 47pf


    I still have to find the C2 corresponding value of R49, C12 to complete the list

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    Old Timer J M Fahey's Avatar
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    Please post the original schematic or we don´t know what C** , R** ,etc. mean.

    That said, read this thread:
    https://music-electronics-forum.com/...ad.php?t=40026
    which tells you more than you wanted to know about the Vox tone stack, and why you should not mess with it.

    In any case, those so called "Caldwell mods" do very little, close to nothing; definitely nothing to brag about.

    Tone stack tweak
    C16 = C23 (stock 56pf) -> 47pf
    Guess you mean the Treble cap: both values are valid and have been used in different versions; point being that to begin with with 20% tolerance used way back then, both can very well measure the same.
    And more important, being such a small value and considering the high value pots used (1M vs. typical 250k found in Fender), parasitic wiring capacitance can easily add 10 to 20pF (or more) so shifting effective value big time.

    Besides:

    Top Boost treble cap:
    C13 = C15 (stock 120pf) tweak to taste (68pf?) or remove
    Maybe you mean some "Bright" cap?
    Messing with it will kill some of typical VOX chime/sparkle ... you sure you want that?
    Plate resistor mod:
    R6 & R7 = R14 & R12 respectively. However, R14 is already 220k so all you have to do is change R12 from 100k to 220K
    You may get extra 20% gain ... about 0.1 or 0.2dB not noticeable to Human ear.

    Of course, after you Mod something, "it always sounds better"

    And if mods mean Brand change ... think again.

    Maybe there is a reason Schematics show parts Value and not Brand.

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    Juan Manuel Fahey

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    +20% corresponds to 1.6dB. Not much but might just be noticeable.

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    Old Timer J M Fahey's Avatar
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    I was half expecting this answer, always nitpicking on numbers but missing the main idea.

    Which is: the Mods are useless, best case doing nothing, worst case damaging a classic design tested and approved by millions of guitar players.

    Do you have any comment on the Mod itself, its effect on sound and playability, etc?

    Now I expect some nitpicking on the "millions of guitar players" accompanied by calculations on how many they really are, preferrably within 1%

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    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    I was half expecting this answer, always nitpicking on numbers but missing the main idea.

    Which is: the Mods are useless, best case doing nothing, worst case damaging a classic design tested and approved by millions of guitar players.

    Do you have any comment on the Mod itself, its effect on sound and playability, etc?

    Now I expect some nitpicking on the "millions of guitar players" accompanied by calculations on how many they really are, preferrably within 1%
    Classic design? These amps really have nothing classic, they're just full of useless transistors parts that muck the whole circuit + wrong values, the Caldwell mods help to bring some components to vintage values, might not be too much but still noticeable, I replaced today R12 with a 220k and not only did the gain improved, but also some mid highs showed up, natural saturation now sounds less scooped with less fizzy highs on the normal channel,

    As for the tone stack and the bright cap on the top boost, I left it alone since I hardly use the TB channel, but for those who don't crank the amp and use pedals the bright cap mod apparantly helps a lot for a more organic and less fizzy sound, takes pedal much better.

    Are you sure old Vox ACs were that chimy? Cause I think the chimy AC30 is just a myth, old AC30s were dark almost dull sounding, and shone when cranked with a treble booster, but still sounded on the fat side and very midrangy with little clean headroom, modern ACs sound scooped, bright and with lot of clean headroom (especially the CC2), so some mods to bring it into the TBX territory is something I'd look into if it's really the classic amp you're after.

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    Last edited by kevin mask; 09-25-2019 at 03:17 PM.

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    Lifetime Member Enzo's Avatar
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    Is anyone going to post the schematic, and a listing of whatever the Caldwell Mod might be? That way we can follow along and possibly contribute.

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    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Is anyone going to post the schematic, and a listing of whatever the Caldwell Mod might be? That way we can follow along and possibly contribute.
    Here's the list from the man himself. Schematics below.


    "Here is how to make the amp sound/behave like a “proper” AC30."

    1 – R6 and R7 are the plate resistors for V1 (Top Boost and Normal channels). In the Custom Classic they are 100K resistors. Change these to 220K resistors.

    2 – there is a 120pf bright cap (C13) across the Top Boost volume knob. The “vintage” spec for this cap was 100pf, but to sound more like a 40 year old AC30 with 40 year old speakers I find that 68pf is a more pleasing value.

    3 – Vox went with a 56pf treble cap in the tone stack (C16). Changing this to the “vintage” 47pf value very slightly reduces the treble.

    4 – they have made the Normal channel fairly useless in tone (way too dark or way too bright) and limited its max gain. Jumper the 330K resistor ahead of the Normal volume pot (R49), change the 220pf bright cap (C12) to a 100pf cap, and then put a 68pf cap across lugs 1 and 2 of the Normal volume pot. This will allow the Normal channel to have more gain and go from a warmer but not too dark sound to a brighter but not too bright sound.

    5 – Vox effed up the effects loop due to a typo. In many Custom Classics there is a drastic loss of low end when the effects loop is on. Many people think “the loop sucks” as a result. It’s actually a fantastic effects loop once one capacitor is changed.

    The schematic had a typo – they put .22uf where they meant to put 2.2uf, and the factory followed suit. So all the earliest runs of Custom Classics have .22uf caps on C109. If you change this one cap to 2.2uf or larger (4.7uf, 10uf, whatever) all the low end comes back and the effects loop is absolutely great.

    I called this cap to the attention of Vox a few years back, and for a while the factory was using the correct larger cap here. Then someone at the factory messed up a memo and the larger cap was being used for C108, not C109, and the low end problem returned. The larger cap is fine for C108, but C109 has to be larger than 1uf for the lows to be unaffected.

    So, look at the board on your amp that has the effects loop jacks/switch. Find C109. If it is a yellow box cap, it needs to be changed. If C109 is an electrolytic (looks like a battery, not a box) you’re fine.

    These are just a part of the mods I perform on these amps, but these are simple, inexpensive, very important mods that many AC30CC owners could perform themselves. So here they are.

    Author: Lyle Caldwell

    http://psionicaudio.net/

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    Quote Originally Posted by Dave H View Post
    Here's the list from the man himself. Schematics below.


    "Here is how to make the amp sound/behave like a “proper” AC30."

    1 – R6 and R7 are the plate resistors for V1 (Top Boost and Normal channels). In the Custom Classic they are 100K resistors. Change these to 220K resistors.

    2 – there is a 120pf bright cap (C13) across the Top Boost volume knob. The “vintage” spec for this cap was 100pf, but to sound more like a 40 year old AC30 with 40 year old speakers I find that 68pf is a more pleasing value.

    3 – Vox went with a 56pf treble cap in the tone stack (C16). Changing this to the “vintage” 47pf value very slightly reduces the treble.

    4 – they have made the Normal channel fairly useless in tone (way too dark or way too bright) and limited its max gain. Jumper the 330K resistor ahead of the Normal volume pot (R49), change the 220pf bright cap (C12) to a 100pf cap, and then put a 68pf cap across lugs 1 and 2 of the Normal volume pot. This will allow the Normal channel to have more gain and go from a warmer but not too dark sound to a brighter but not too bright sound.

    5 – Vox effed up the effects loop due to a typo. In many Custom Classics there is a drastic loss of low end when the effects loop is on. Many people think “the loop sucks” as a result. It’s actually a fantastic effects loop once one capacitor is changed.

    The schematic had a typo – they put .22uf where they meant to put 2.2uf, and the factory followed suit. So all the earliest runs of Custom Classics have .22uf caps on C109. If you change this one cap to 2.2uf or larger (4.7uf, 10uf, whatever) all the low end comes back and the effects loop is absolutely great.

    I called this cap to the attention of Vox a few years back, and for a while the factory was using the correct larger cap here. Then someone at the factory messed up a memo and the larger cap was being used for C108, not C109, and the low end problem returned. The larger cap is fine for C108, but C109 has to be larger than 1uf for the lows to be unaffected.

    So, look at the board on your amp that has the effects loop jacks/switch. Find C109. If it is a yellow box cap, it needs to be changed. If C109 is an electrolytic (looks like a battery, not a box) you’re fine.

    These are just a part of the mods I perform on these amps, but these are simple, inexpensive, very important mods that many AC30CC owners could perform themselves. So here they are.

    Author: Lyle Caldwell

    http://psionicaudio.net/

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    Yeah but watch it cause he refers to the CC2 model on that guide, mapping is different, here's the C2 schematics sent by Lyle https://www.sendspace.com/file/b4mlau

    Another mod I was recommended for a more classic amp and a better sound is adding a choke since this model doesn't have one, and do away with all the transistor parts like FX loop, tremolo and reverb etc, and just keep normal + TB channels (going from phase inverter directly to power stage), that should turn this amp into a real more classic Vox, my techs said that 20% of the tone would benefit from this, what do you think?

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    Are you sure old Vox ACs were that chimy? Cause I think the chimy AC30 is just a myth, old AC30s were dark almost dull sounding
    That's how my original 1970 AC30TB sounded before I changed the speakers to mid 60s Vox Alnicos. As so often changing speakers was the most effective mod.

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    Quote Originally Posted by Helmholtz View Post
    That's how my original 1970 AC30TB sounded before I changed the speakers to mid 60s Vox Alnicos. As so often changing speakers was the most effective mod.
    I've got a pair of correct H1777 Blue Alnicos on mine, no need to replace those I guess

    Here's what Caldwell has got to say about the chime and jangle AC myth:

    "First, there was so much chattering on the net about “jangle” and “sparkle” and “Beatles” that Vox voiced this amp differently from other AC30s, to appeal to those who thought the amp should sound a certain way (even though AC30s don’t sound that way at all).

    History – the Beatles didn’t use AC30s after the first record. The “jangly” Beatles sound was AC50s, AC100, various solid state Voxes, and Fender amps. The Byrds used a combination of DI and Twin Reverbs for their “jangle”.

    The AC30 has never been a “clean jangly” amp. In their natural state they are a wonderful amp when clean but they have low clean headroom and the real magic is when they are at the edge of clean and overdrive, where they break up and clean up responding to playing dynamics. Don’t think Beatles/Byrds – think REM, U2, Yardbirds, Zeppelin. Not too dissimilar from the cleans coming from a Plexi, if the Plexi was being run into an open-backed 2×12.

    Anyway, Vox voiced the Custom Classic to have a lot of clean headroom (too much IMO) and to be bright (too bright IMO). This especially makes the stock amp not take drive pedals too well as the overdrive sounds fizzy into such a clean bright preamp."


    And I have to agree, most of the classic rock Vox users I know never had such a jangly tone, but more of a midrangy fat crunch, and newer Chinese ones tend to have fizzy highs when saturated, when the master is fully opened it gets much better, but still fizzes a bit, if the venue has a good acoustic and resonance they still sound fine even stock, but the amp alone sounds more scooped than classic AC30s, even with a Les Paul it often lacks of mids.

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  11. #11
    Junior Member Matt R's Avatar
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    The CC2 was decked out with bells and whistles. Unlike the originals. Made for the masses to be marketable.

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