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    MKCarnut Custom M125 Build

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    mkcarnut

    Posts : 10
    Join date : 2016-11-13

    MKCarnut Custom M125 Build

    Post by mkcarnut on Sun Nov 20, 2016 10:14 pm

    Hi all - I am working on a design, and then a build, of some custom-built M125's.  I would love some input as I design and build the amps.

    To start things off, I will share that I have custom built Hypex nCore Class D amps for my fronts (Maggie 1.7's) that I listen to in 2-channel mode with my pre-amp being an Emotiva XMC-1 (via an LH Labs DAC).  I will continue to use the nCores for multi-channel use and the M125's for 2-channel.

    I am opting for a custom-built case because:  a)  I can.  b) It's fun.  c) Why not?  Other than - in doing so, I could screw something up - which I don't want to do!

    Some basics...
    1) I don't like having the bias test points and power switch on the front for aesthetic reasons.  So, I want to change that.
    2) We have cats.  They like to lay on top of stuff.  I want to protect my e-gear from that.
    3) My whole setup (my stereo stand, by nCores, etc) uses a mix of poplar, aluminum, and smoked glass...so new stuff will fit the "look"

    So, I have laid out the existing M125 design and an inital cut at my own design.  I am open to change based on input to avoid aesthetic desires overcoming my #1 objective - great sound.

    With that, here's an approximately to scale layout of the M125:

     photo Stock.jpg

    And, here is what I am looking at doing.  A few comments...  1st - I want the output tubes displayed across the front to show them off. 2nd - I want the tube glow to be visible, but a little subdued.  3rd - I want to "hide" the other gear behind the output tubes.  4th - I want the power switch and bias test points to not be on the front.  5th - I want the poplar wood/smoked glass look to continue to match my gear.  6th - I want a removable cover to keep the cats off.

    As such, I plan to modify the layout to have the output tubes across the front behind a smoked glass "wall" that is across the front and surround the whole amp.  Glass will be held in place by aluminum brackets commonly used on custom shower enclosures.  And, instead of the standard case, the case will have front/sides made of poplar with a top and bottom panel made of the same gauge stainless steel as the Bob Latino kits with the overall size increasing to 12" x 12".  Power on switch moves to the side.  Bias test points move to the top plate where shown on the below.  I MAY make a right-left specific version, with the only difference being the left/right positioning of the driver board.  And, I will have a removable cover (built of poplar and black cloth ) to keep dust out and cats off the main parts when not in use.  It will come off once I power up.

    Layout would be as follows:
     photo Custom.jpg

    The fixtures that attach the glass to the poplar would be like the following...90 degree ones to attach the glass to the glass and would be vertical.  Other ones would attach glass to wood on at least 4 points to provide a gap between the wood and glass.  Exact finish can vary but will either be dark to match the glass or raw aluminum to match some other aspects of my gear.

     photo Corner Braces.jpg
     photo Base Braces.jpg

    Some have voiced a concern about:
    1) noise from changing the layout.
    2) hear from enclosing the tubes in glass on the side.

    Regarding #1 - I would follow as closely as possible the wiring layout used - with appropriate twisted pairs and orthagonal layouts of wires and avoiding crossing of wires with various parts.

    Regarding #2 - I read that the tubes can get "hot", and some advised that putting them close to glass would trap the heat.  So, I searched and found temps of typical 100 watt light bulbs and found them to be in line with the tube temps of a tube amp.  And, I used my IR temp gauge to find that my halogen bulbs in by kitchen run comparable temps to tube amp tubes.  And, despite glass fixtures being in intimate contact with the bulbs - the outside temp of the bulbs is 100 degrees less than the bulb temp.  Net, I am thinking the heat impacts of having glass about 1" away from the tubes is negligible.

    Thoughts on the design?

    Thanks,

    Mark
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    wgallupe

    Posts : 112
    Join date : 2014-05-18
    Age : 63
    Location : Central Mass.

    Re: MKCarnut Custom M125 Build

    Post by wgallupe on Mon Nov 21, 2016 10:51 am

    Hi Mark,

    I found your post and 'project' interesting as I am in the same stage of designing custom chassis for a pair of Mark III monos. A couple things that came to mind:

    1) I want the tube glow to be visible, but a little subdued. I would go with a very light smoked glass or no smoke at all because the tubes don't glow all that much. The M125 pics on Bob's site exaggerate the brightness/glow.

    2)I want to "hide" the other gear behind the output tubes. Consider mounting the quad cap and driver board under the stainless steel top plate. Just provide holes for the driver tubes to fit through, holes to access the bias trim pots and mount the tall caps on the bottom side of the PCB.

    3) Take a look at Front Panel Express for designing your top and bottom plates. They are available in aluminum. I found it to be very difficult to drill holes in the VTA/ST120 stainless chassis when I customized it.

    Good luck and have fun!

    Wayne
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    Peter W.

    Posts : 411
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: MKCarnut Custom M125 Build

    Post by Peter W. on Mon Nov 21, 2016 11:21 am

    My take:

    a) Noise could be addressed by some additional shielding (if even necessary) underneath. Perhaps some bent sheet-metal with grommeted penetrations for wiring between the power-supply and the audio sections. Something that would screw to the top-plate and bottom plate for proper grounding. With a little bit of care and some chassis depth, you could get pretty much everything but the tubes under the chassis - mount the quad-cap horizontally.

    b) Heat is the enemy of all electronics, especially tubes. And fans are extraneous noise, however quiet they may be in operation. Smoked glass/plex/Lexan are excellent reflectors of heat, with any of the polycarbonates being insulators as well. And if you have a solid top chassis plate, you have limited opportunity for convection cooling as well. When considering halogen lamps, waste heat is not really an issue. When considering a vacuum tube, overheating them can be a problem leading to shortened life at least. A 20% reduced service life on a halogen lamp is not significant. On a $40 tube - it just might be. Further to this the heat must go somewhere - and what is big, black (heat-absorbent) and unhappy when overheated living right nearby - the transformers.

    I suggest perforated sheet metal on three sides if you must, and the smoked material in front on the 'display' side. More-so, if you could float the glass such that there is some ventilation above and below to aid the convection process, you may not need a fan.

    I applaud your desire for a cover - for me that is a necessity when running the tube stuff, what with two active cats, two very active dogs and four almost equally active grandkids rotating through on a regular basis.

    mkcarnut

    Posts : 10
    Join date : 2016-11-13

    Re: MKCarnut Custom M125 Build

    Post by mkcarnut on Mon Nov 21, 2016 12:14 pm

    Wayne - thanks for the lead on Front Panel Express. That could be a very good option.

    Peter - good point on the heat. There will be at least 5 mm gap between the top of the case and the bottom of the glass. And, worst case I could make the glass easily removable so I can lift it off when I play it but put it back in place when not in use. I have looked into some decorative metal panels, but I am really surprised at how "proud" the sellers of them are. Very pricey for anything other than cheap looking stuff from Home Depot/Lowes.

    Mark
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    Peter W.

    Posts : 411
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: MKCarnut Custom M125 Build

    Post by Peter W. on Mon Nov 21, 2016 12:35 pm

    but I am really surprised at how "proud" the sellers of them are.  Very pricey for anything other than cheap looking stuff from Home Depot/Lowes.


    If you have the capacity to bend sheet metal (are you friends with any roofer or anyone else with a Cornice Brake?), an excellent source for stock is the McNichols Company:

    http://www.mcnichols.com/gallery/product-photos/expanded/decorative-metal

    You would get enough for multiple covers for under US$200 delivered.

    wanders

    Posts : 10
    Join date : 2016-03-16

    Re: MKCarnut Custom M125 Build

    Post by wanders on Mon Nov 21, 2016 2:24 pm

    wgallupe wrote:Hi Mark,

    3) Take a look at Front Panel Express for designing your top and bottom plates. They are available in aluminum. I found it to be very difficult to drill holes in the VTA/ST120 stainless chassis when I customized it.

    Good luck and have fun!

    Wayne

    Plus 1 for Front Panel Express. I used them for my recent sp-14 preamp build; my first attempt at CAD design and custom faceplates. The staff is very helpful: willing to answer as many questions as you care to ask. No strings.

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