Big Red Amp - custom rack power amp build

Discussion in 'Amps and Cabs' started by dohmar, Aug 12, 2014.


  1. dohmar

    dohmar Member

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    Hiyas
    I decided a while ago to get Neale Amplification (www.amplifiers.com.au) to build me a custom rack amp, after he fixed my Mesa 20/20's blown output transformer. We've just started to progress with the design and fabrication. I'm in charge of the housing, so here's a few pics of current progress.
    The housing is a 14 year old, 19" 4RU high ATX PC case. Its sat in my wardrobe for over 10 years, so I figured it'd make a nice housing. I disassembled the front plate and removed parts of the chassis that weren't required (backplate, fan bracing, front panel). While the case was at Neale Amps (for measuring dimensions required), I sandblasted and powdercoated the front plate. Last weekend I delivered the front plate back to Neale so he can fit controls, and I took back the chassis so I can cut the end off.

    First pic is of the front chassis pre-cutting. The place where I have the fan is actually where the dust filter (removable) used to sit. It just happens to be perfect for a 120mm fan.


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    Next pic is the front panel - unhinges for access to the previous 5.25" drive bays - this will be the panel that hides the tubes behind a clear plastic window. I should be able to tube roll to my hearts content without removing the amp from the rack.

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    This is the front plate after I had my mechanic do me a favour and cut it down a bit
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    And then it got powdercoated 'racing red'
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    This is roughly where the chassis is gonna get cut...
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    So thats where I'm at. The amp itself is meant to be a compliment to the mesa 20/20. The mesa sounds very pretty but it lacks a certain grunt. As I run a stereo rack setup, it makes sense to build a custom power amp.

    Specs are, a cathode biased KT66 based amp which will be able to run EL34's, 6L6's and possibly one or two other types of tube. 35 watts per channel - the design could go to 50w but I feel I'd rather run it at a lower power as I prefer that kind of sound (plus the tubes would wear out less in theory). 2x 35 watt should be loud enough for an amateur like me. The power transformer and OT's will be over engineered for the purpose. These are baseline specs and are still in the drawing phase, but if anyone has any comments/suggestions/personal experiences, i would be intrigued to hear it.

    More pics to come!

    -D
     
    Last edited: Mar 20, 2017
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  2. dohmar

    dohmar Member

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    [​IMG]

    Got the chassis back from the powdercoater now. Was lazy and didnt get it down to the guy till last week. Chassis and front plate goes off to Neale amplification this weekend. It finally starts...

    -D
     
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  3. JMMP1

    JMMP1 Supporting Member

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    That's a really cool idea to change tubes. I am looking into building my own power amp, and the idea of a 4u computer case has popped up, but seemed like extra metal working/ fabrication. Curious to see your journey and end result!
     
  4. dohmar

    dohmar Member

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    So I dropped this box off to Neale amplification and we did some mockups of placement. Heres pics below. OTs are 50 watt Weber JTM45 replacements but will be running each one at 35 watt. Test fit KT66 and EL34 for width and height - both have ample clearance.

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    More photos to come as the build progresses!

    -D
     
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  5. yuzerneigm

    yuzerneigm Member

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  6. dohmar

    dohmar Member

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    OK So some progress. I went over today to Neale Amplification to see the progress of my big red amp. Its approx 50% done so far. Here are some photos;

    Firstly, OT's and Power tranny and caps are mounted in the case
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    The 4x power tube sockets are mounted to the chassis segment that used to house 5 1/4" devices;
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    And heres the underside of it...
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    Probably will be ready in a couple of weeks if everything runs to plan!

    -D
     
    Last edited: Jan 16, 2015
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  7. big mike

    big mike Fixed Bias Moderator Staff Member

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  8. dohmar

    dohmar Member

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    OK So its completed! Tests OK on the scope, and Neale Amps have sent me some photos. Will need to bring my preamp and cabinet over to do a real test on it but its functionally complete. I still need to weld some dust covers on the rear open panel but thats something I can do later. I'm pretty pleased with how it looks. Looking forward to some tube rolling to see what it sounds best with.

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    -D
     
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  9. M@tt C

    M@tt C Supporting Member

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  10. dohmar

    dohmar Member

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    Picked it up today, mounted it in the rack, connected it and belted out a few chords. Sounds great, but I'm going to let it run in for a couple of days so right now its just sitting with the volume at 100% but no source input. Its a very quiet amp electronically speaking, no EM leakage and the fan runs at a low enough rpm that it's inaudible. Going to have the house to myself tomorrow for a few hours, so it'll be earplug time at my joint
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    It also looks neat when the lights are off ;)
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    -D
     
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  11. dohmar

    dohmar Member

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    So I got the switch replaced the other day. Didnt like the feel of a push on/off so changed to a single throw lever style switch. Also added a backplate so noobs won't electrocute themselves on the 300+ volts
    Also chucked the rack, pre and effects in a single case for portability. Stupid idea. The rack weighs like 35 kilos/70lb (at a guess) now... Will eventually get motivated to align that grille a little better

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    Has over 200 hours of playing time on it now and has broken in quite nicely!

    -D
     
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  12. michael.e

    michael.e Silver Supporting Member

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    Love it! Great job!
     
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  13. nickcoumbe

    nickcoumbe Member

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    Like it. Nice to see some quality work done in Adelaide.

    Any clips?
     
  14. dohmar

    dohmar Member

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    I've got the soundcard, the leads, the stands but not the mics, and I'd really love to get some bass traps before I record - that said, I'm keen to give some clips eventually - and with a caveat - its just a power section - which means if I run the JMP1 or the Triaxis, they'll both sound different through the same amp. Effectively, its a dual JTM45 power secion with a cathode bias.

    When I get the Overdrive Deluxe, I'll definately make some clips because it will show off the preamp circuits that Neale makes

    -D
     
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  15. dohmar

    dohmar Member

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    Last mod required was completed just a while ago. Since the airflow around the front wasnt great with the acrylic, I've cut down some mesh and spray painted it black. It acts like a heatsink and allows the air from the rear to blow through.

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    Project Completed.

    -D
     
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  16. PeteB

    PeteB Member

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    Hi, Pete from runeaudio here, followed your links... That is a great project and case. Love the tube amp and the powder coated finish, and making use of old computer chassis and hardware! Now, the question that's been nagging all of us who live on the wrong side of the Pacific.... Do tubes really work installed upside down? :D

    Cheers!
     
  17. dohmar

    dohmar Member

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    Yep, and they sound magnificent to boot! ;)
    -D
     

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