Wrapping Up the Micro Bassman LTP Duo

Discussion in 'Shock Brother's DIY Amps' started by Todd Stock, Jun 20, 2019.

  1. Todd Stock

    Todd Stock TDPRI Member

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    My guitar build/repair partner and I just wrapped up a pair of Rob's Micro Bassman LTP amps...all that's left to do is the badging and the control plate install. Awesome little amps!

    A few comments for those looking at the build:

    - We both opted for the 12BH7 power tube - plenty of volume, although I have not measured output yet...more than enough to drive students out of the shop when we were playing with power tubes, and way louder than the Micro Deluxe over on the #2 repair bench

    - Rob's LTP layout illustration does not show the NFB switch, but easy to figure out where it goes based on the schematic

    - It's easy to miss the back-side jumper between the 15k ohm NFB resistor and the 0.1uF AC Ground cap and 68k ohm LTP tail resistor...both of us missed it the first time around due to failure of my color print to really show it and the brown-on-gray of the unzoomed image being tough to see on some monitors and devices

    - Shielded wire was used from the input jack to the 33K ohm input, and the 22g woven ground sleeve (with shrink-wrap cover) used to carry the preamp grounding bus to the input jack...the amps are both uncannily quiet in terms of hum, hiss, etc.

    - As mentioned, we added the NFB switch to the bottom of the chassis, and it really makes a difference in behavior...we may do the multi-resistor mod at some later date once we finish playing in the speakers (10" Cannabis Rex at present, but def could have levered in a 12" with the added cab depth), but for a shop amp, likely perfect as is.

    - We deleted the Phase Inverter MV - overkill for what what we needed and keeps the topography closer to Rob's inspiration.

    - Cabs were made of big box store 1x pine with dovetailed corners and Tolex as close to a Flat Dark Earth color as I could find, with one amp trimmed in bright metal and the other with oxide black fittings and black-dyed leather handle (was out shooting a Sig 1911 Fastback Emperor Scorpion Carry over the holidays and thought that an amp with that look would be kinda cool).

    Since switching the shop demo amp over from a highly modified HR Deluxe to the Micro Bassman LTP, I've had a half dozen offers to either buy the #1 Micro Bassman LTP amp or build one up (nope - never gonna happen...gonna stay a hobby).

    In summary, very nice job on the design, Rob, and the LTP option was definitely the way to go.

    Todd

    mBassmanCheesecake.JPG mBassmanFirstPlay.JPG BenchFull.JPG BassmanCabs01.jpg BassmanChassis.jpg
     
    Last edited: Jun 20, 2019
  2. thunderbyrd

    thunderbyrd Poster Extraordinaire Silver Supporter

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    I have wanted a "minature bassman" forever. how much does a completed one cost?
     
  3. Deebers

    Deebers TDPRI Member

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    Those are beautiful builds, @Todd Stock! I have been putting together a mental outline to build one of these in to a 4x8 cabinet. Did you use a hammond blank chassis? I like the look of your spacing and fit in to that chassis. I may have some questions for you in the future!
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The chassis and cabs look fantastic. Nice work. I'll edit the layout to make that underboard jumper more prominent.
     
  5. Todd Stock

    Todd Stock TDPRI Member

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    Thanks! Fun project and again, Rob, awesome job on design and documentation. These micro amps are fantastic, and really do capture a lot of that tweed Bassman mojo.

    BOM with everything (as in EVERYTHING) was about $440...but that includes some extra Tolex, lots of glue, a ton of extra resistors, and a fair amount of common hardware.

    We used a Hammond 13" x 7" x 2" blank aluminum chassis - plenty of room...I can post the spacing for stuff if needed. If going for the PI MV version or some other switches up top, a 14" chassis would be needed, and keep in mind that 2" depth is really the minimum depth..a little extra would have been nice for getting more wire-to-wire separation (but neither amp made ANY noise as packaged in that 2" deep chassis...amazingly quiet and a testament to the good design work).
     
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  6. Whoa Tele

    Whoa Tele Friend of Leo's

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    I'd love to hear clips of those.
     
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  7. Pierre67

    Pierre67 TDPRI Member

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    Nice builds !!! Did you use the suggested Hammond 270AX PT ?
     
  8. Todd Stock

    Todd Stock TDPRI Member

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    Yes - we used the Hammond 270AX PT and 125B OT for both amps. Worth noting that Rob has revised the Bassman Micro web page to make the LTP build the recommended version, and added an LTP BOM. Also worth noting that Rob revised the PNG version of the LTP layout to make it easier to read and use - thanks! The layout as Rob has drawn it results in a very, very quiet amp, and our addition of the NFB switch (per the schematic and Rob's notes) to the bottom of the chassis between the jacks and the power tube does not change that (Carling SPST installed in the signal connection between the speaker jack tip and 15Kohm NFB resistor).

    The more we play these little guys, the better we like them. A couple of the local pros and one of the touring guys we support have asked to borrow these for studio work, so hope to get some decent recordings of the amp in use.
     
    Last edited: Jun 26, 2019
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  9. Todd Stock

    Todd Stock TDPRI Member

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    A quick clip of Jonathan Sloane, Tele, and one of the Bassman Micro LTPs...no pedals, the terrible acoustics to be expected in a basement shop, and the usual not-so-great iPhone X mic. Jonathan was over picking up the guitar after a nut change and a health and welfare check, so was just noodling.

    We'll get something better whipped up in the studio once things slow down a bit.

     
    Last edited: Jul 12, 2019
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