VOX AC30C2 mods

Discussion in 'Amp Tech Center' started by LightningPhil, Aug 1, 2019.

  1. LightningPhil

    LightningPhil Tele-Meister

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    Has anyone here been tinkering with the VOX AC30C2 amps?

    After a good while of owning one, it's turn under the soldering iron came up... So here's the schematics with a few notes on them. All mods are in red to easily identify the changes (and leave the schematic useful for troubleshooting a perfectly normal amp too).

    The goals were not to change the character of the amp much at all, but lightly change the way it breaks up. And to shove the noise floor as low as practicable.

    As to why:

    In my experience, the higher the note on the guitar, the lower the peak to peak electrical signal. Not only does that make the highs effectively quieter, but also distort less or even fail to reach distortion. The AC30 has small coupling capacitors between stages and so bins off a lot of bass - which sorts out the volume difference, especially so in the top boost channel.

    However, a small coupling capacitor has high impedance at low frequencies. So it doesn't load the valve driving it much. Thus the valve doing the driving sees a slightly higher impedance at low frequencies and has a little bit more gain there than at the highs than it would do if the coupling cap was much larger - by a little less than 1dB (small but still a thing).

    The character of the amp would change massively if the coupling caps were made much larger - so another method of lowering the driving valves gain was required. This is accomplished by using a much smaller cathode bypass cap. This reduces the gain of the valve around the lows by about 5dB and leave the highs at full gain. A damped capacitor smooths the response between the 2 bands, creating a slow rise in gain from 100Hz to 1000Hz of about 4dB. And then a slightly larger coupling cap to the next stage is used to get back some of the lost bass.

    So, in short, this transforms the amp from a bass boost followed by a high pass filter into a true top boost amp. For the clean channel, this happens once and for the top boost channel, twice. The net effect is similar sounding clean tones followed by more even breakup across the frequency range as its driven. For me that makes it more tactile.

    Also, by decreasing low frequency gain, mains hum is not amplified as much, which reduces noise there.

    Other more simple mods are:
    • Putting damping resistors in series with the treble caps on the volume pots. This makes it sound a little less bright and more uniform across the volume range
    • Increasing the amount of bass that can go through the FFX loop - which is where I run reverb pedals
    • Installing power rail bypass caps on a couple of op-amps - because why not
    • Increasing the size of the caps buffering the HT for the valves - which reduces noise by lowering the roll off frequency of the filter made by them and the resistors separating the B+ rails
    The net result is quite nice and tactile with a very low noise floor - easy to forget its turned on...

    And finally - it's all fully reversible.
     
    Last edited: Aug 1, 2019
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  2. LightningPhil

    LightningPhil Tele-Meister

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    Mods to pre-amp:
    PNG - preamp.png
     
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  3. LightningPhil

    LightningPhil Tele-Meister

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    Mods to power-amp:
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  4. LightningPhil

    LightningPhil Tele-Meister

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    Mods to FX circuit:
    PNG - FX.png
     
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  5. LightningPhil

    LightningPhil Tele-Meister

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    The images don't seem to be super clear, but give an idea of what's going on. So below are the PNG files, zipped to stop anything down-sampling them.
     

    Attached Files:

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  6. LightningPhil

    LightningPhil Tele-Meister

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    And for good measure, here's the schematic I started with:
     

    Attached Files:

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  7. HF1600ie

    HF1600ie Tele-Meister

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    What is really the target in terms of tone ?

    To be honest, the only mods I would do, would be :

    - Set the plate resistors for the normal (R12) and top boost (R14) channel at 220K each. R14 seems to be 220k already.
    - Jumper R13 (if you want more gain in the normal channel).
    - C23 = 47pF (correct capacitor based on old specs).
    - Remove C8 for darker tone at normal channel. I personally like C8 120pF in there. Increase to 220pF for an even brighter normal channel (probably too bright on a combo format).

    I did these to my Vox AC30CCH (head) and via a 2X12" cab and it is absolutely incredible. Both channels.

    - C82 is probably filtering very very high frequencies to ground. Shouldn´t hurt to leave it there and probably an evolution over the old circuits.
    - C13 seems to be out from the old circuits as well. Maybe there for stability purposes.

    This should let your Vox sing nicely. But it seems to me you want something else :)
     
  8. AJBaker

    AJBaker Friend of Leo's

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    I just removed the bright caps (on my ac15c1)
     
  9. tubelectron

    tubelectron Tele-Afflicted

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    Thanks LightningPhil for sharing your work ! :cool: I take a copy... ;)

    The AC30CC2 is an amp that I will buy one day - I missed one in Red covering :rolleyes:

    It is a much better beast than the 1976 AC30 Top Boost I had in the past (scanned paper pic) :

    [​IMG]

    -tbln
     
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  10. LightningPhil

    LightningPhil Tele-Meister

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    HF1600ie - the tone goals were mentioned - The goals were not to change the character of the amp much at all, but lightly change the way it breaks up. And to shove the noise floor as low as practicable.

    To put it another way - I love the AC30 sound (in a manly platonic way...) - but found I couldn't quite get the "vintage" sound I was after. I wanted it to be easier to control subtle breakup and light saturation by how hard I play. Before the mods, breakup would be easy to find on low notes while the highs took more effort. The mods flipped that and allows it to be slightly easier to play clean rhythm and enter breakup when desired - and a little easier to get the highs a little more excitable too.

    Most people would plug in a tube screamer which has a much more pronounced effect. And that would be the sensible option. And while I've got a couple of tube screamer like pedals - I don't do sensible.

    While I'm mentioning TS like pedals, I find the added noise, much reduced bass and limited bandwidth awesome sometimes. But it's not the same as keeping the distortion and gain all valve.

    Yes, I know the AC30C2 has op-amps in the signal chain, but they're low gain and high bandwidth, so quite transparent to the signal. Wish they weren't there but happy to accept them as it's a good implementation. Some other amps I have are valve all the way through, so I don't feel the need to bypass them.
     
  11. ERICBEEP

    ERICBEEP NEW MEMBER!

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    This is very cool, Phil! Thanks for sharing! I would love to hear a clip of your modded C2; I'm sure I'm not alone. I know you've already stated your goals for these mods, but as people use AC30s for quite a wide spectrum of distinctly different tones -from complex single-coil chime and jangle to roaring Brian May/humbucker saturation- what are you dialing in with yours?

    I personally aim for maximum single-coil chime/jangle/sizzle, as ultra-brilliant as I can get while avoiding harsh icepick ultra-high frequencies. It seems like your mods might suit me well based on your descriptions. I've owned quite a few amps over the years including various boutique Vox-inspired amps, but the C2 gets me the closest thus far. I prefer it over my 1964 AC30TB for this tone, just as a point of reference. Thanks!
     
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  12. LightningPhil

    LightningPhil Tele-Meister

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    Thanks Eric,

    It may well work for you. Usually use a Les Paul, but the pickups can be split and I think you’d like the sound.

    No recording stuff, so no clips. Also, it so happens that I’m much more of a tech than a musician, with some sort of education in electrical engineering and zip for music...

    It’s possible to reverse all the mods, so give it a go if you wish and as long as you’re careful, it won’t devalue the amp.

    I would say pop by and have a noodle, but there’s a large pond in the way.
     
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