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Thread: Vox JMI AC30 Tagboard Layout

  1. #1
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    Vox JMI AC30 Tagboard Layout

    With the misdirected zeal of youth, I'm afraid the tagboards in my old 60s Vox JMI AC30 got stripped of the original circuitry and repurposed for various overdrive / channel switching mods back in the 80s
    I'm now very much wanting that fantastic Vox vibrato, so it makes sense to reinstate things back to stock.
    A copy of the drawing for component layout on the tagboards, and crucially, the wire links between tags, would make that much easier.
    It's the basic 6 input, non top boost model, as per https://el34world.com/charts/Schemat...x_ac301960.pdf which helpfully has component numbering.
    Thanks for any assistance!

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    Supporting Member mozz's Avatar
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    On my build list, when, i have no idea. You don't see many old Vox's here in the US, at least not cheap in price. Have you posted a question there on the el34 boards? They might have a board or layout.

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    Last edited by mozz; 10-14-2018 at 07:56 PM.

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    Senior Member Pedro Vecino's Avatar
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    Here is this layout. I dīont know if it is the one you are looking for, but it seems to coincide with some pics that I keep.

    http://turretboard.knucklehead.dk/wp...normalrev7.pdf

    http://www.pedrovecino.com/amps/ac30x.jpg

    For these things and others I use the ampmaker tag boards. Like the ones Cornford used.

    http://www.ampmaker.com/store/Tag-board.html

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    Fantastic, thanks Pedro, that turretboard drawing looks perfect, it even seems to have accurate component numbering.

    Regarding your photo, sorry if this is old info to you, but the black Hunts coupling caps used in UK amps of that era are very unreliable, way more so than any ecap!
    I think they are generally the reason that the AC30 vib/trem effect of the 60s models rarely work.
    The first job with any 50s/60s kit that lands on my bench is to check for those and mark them as likely suspects for any operational issues, and campaign replace them all before I'm finished with it (if the owner wants it to be reliable).

    Thankfully my AC30's tagboards are in place and intact; just as well, as Barry (Mr Ampmaker) has just started a 12 month sabbatical!

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    Last edited by pdf64; 10-14-2018 at 12:08 PM.

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    Senior Member Pedro Vecino's Avatar
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    Iīve never seen those capacitors but I have seen Hunts electrolytics in several old British amplifiers.
    I didnīt know about Barry's long vacation. I still have a large box with transformers, tag boards, resistors and coaxial cable from Ampmaker.

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    This is a great book - has layouts, schematics and component values for every conceivable flavour of vintage AC30.

    http://www.thevoxac30guide.com

    And yes. Those black or brown Hunts capacitors are very unreliable - they are the usual reason why tremolo oscillators don't work on old british amps. They sometimes fail open circuit, but sometimes they just leak which is often a cause of premature EL84 death where a leaky coupling cap from the previous stage allows their grids to go positive. (You'll note that AC30s usually have poly caps in this position).

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    Pedro, in that AC30 photo from your site, there's a black Hunts cap lurking on the board under the gold Tropifol C38, just by the Vib/Trem vol control; it looks to be C27 on the schematic, 2n2f, part of the phase shift network.

    Ted, thanks for raising that book, it's only a tenner! I came across a very expensive / extensive Vox book a few years ago and decided against it. EDIT Book now ordered!
    Regarding the black Hunts caps, I've also found some used as coupling caps in a Watkins Westminster that had failed open circuit, no ac getting passed to the next stage. Horrible sticky things, all conceivable failure modes exhibited!

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    Last edited by pdf64; 10-15-2018 at 12:40 PM.

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    Senior Member Pedro Vecino's Avatar
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    I would never have seen it. Thank you!

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    I was on the lookout for it, as it was a black Hunts in C27 that had failed on the last AC30 I fixed up, a few months ago

    I've seen this https://www.ebay.co.uk/itm/Replaceme...W/263948476845 and it set me thinking; I'm now suffering from mission creep.
    I was thinking about adding a top boost on the back panel, but they seem such a kludge.
    As I'll be rebuilding the preamp boards anyway, and as the integrated top boost preamp chassis and faceplate is available, I may as well replace them on my amp, and so end up with a proper integrated top boost, which is what I's always wanted. But then the amp would still be a crime against the originality police.

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    Last edited by pdf64; 10-15-2018 at 01:04 PM.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by pdf64 View Post
    Regarding the black Hunts caps, I've also found some used as coupling caps in a Watkins Westminster that had failed open circuit, no ac getting passed to the next stage. Horrible sticky things, all conceivable failure modes exhibited!
    Any Hunts caps no matter the color, make a quick exit when I find them.

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    Enjoy. Every. Sandwich.

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    Just to note that if anyone is looking, Mike at Modulus in Scotland (guitarmike2107 over on TAG) has the 2x18 tagboards https://modulusamplification.com/Gen...aspx?sid=12886

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    Quote Originally Posted by Ted View Post
    This is a great book - has layouts, schematics and component values for every conceivable flavour of vintage AC30.

    http://www.thevoxac30guide.com
    Yes definitely. I bought this book a couple of years ago, very detailed information. The author did a really good job. In the revised edition I got, it also includes schematics, layout and parts information for AC15, AC10 and AC4/AC2 amps.

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    C41 value?

    Yes, the book is really good, includes the tagboard info I need, including that for the integrated top boost version.

    One query though; the schematics for 60s AC30s show C41 (eg of the AC30/6) as being 0.005 (it bypasses R41 in the shared plate circuit of V8 the ECC82 vib/trm modulator https://el34world.com/charts/Schemat...x_ac301960.pdf )

    However, the few the examples I've seen use a 0.047uF cap there, usually a Tropyfol or 'mustard' type.

    And the later 70 and 80s schematics show that value there also, eg https://el34world.com/charts/Schemat...x_ac30dals.pdf

    Interestingly, the Korg / Marshall version shows it (C26 in this case) as being 4n7F (dunno wht they actually fitted), see https://el34world.com/charts/Schemat...x_ac3093pr.pdf

    Functionally, if it is 0.047uF, I think it acts as a shelfing filter (lower corner freq ~72Hz) to boost the low end, which is attenuated by the following series of 4n7-1M HPFs (C12-R35C13-R36, C14-R37, C15-VR4), along with the modulation signal.

    If it's 4n7F, it would be boosting the mids too, and I can't think why that would be needed?

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    Last edited by pdf64; 10-25-2018 at 05:29 PM.

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