Micro Bassman LTP Questions

Discussion in 'Shock Brother's DIY Amps' started by chutson89, Jan 30, 2020.

  1. chutson89

    chutson89 TDPRI Member

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    Hi all, I'm new here and this is my first amp build. I'm trying to keep this build pretty stock, but I'm a little confused by all the volume pots. On the layout I see a PI Master Volume, Preamp Master Volume, and a Bright Volume. Is the Bright Volume pot replacing the bright switch of the original Micro Bassman? If so, could I covert this to a bright switch? If I needed to put one if these pots in back of the chassis (kind of a set it and forget it), which one would you choose? Is the PI Master Volume, the final "master volume"?
     
  2. sds1

    sds1 Tele-Afflicted

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    Hi, welcome.

    See if this helps a little:

    [​IMG]

    The schematic doesn't quite match the layout (schematic omits the preamp send/return, shows a raw switch, etc?), but the answer to your question is better visualized there:

    - The Bright Volume is the normal gain/volume pot for the 5F6A Bassman Bright channel. It controls gain into V2A.

    - The Preamp Master Volume drives the Phase Inverter V3. If you implement the preamp send/return per the layout then it drives the preamp send/return (which in turn drives the PI).

    - The PI Master Volume controls gain into the 12AU7 output section


    The pre-PI aka Preamp Master Volume is probably the most extraneous of the 3 IMHO, although they all serve a unique purpose and should not be considered redundant.
     
  3. chutson89

    chutson89 TDPRI Member

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    Hi, thanks for the clarification. That makes sense to me. Since you brought up the schematic, it did remind me of a question regarding the "Feedback Switch" in the upper right. Is that a literal SPST switch? I didn't see that switch in the layout.
     
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  4. sds1

    sds1 Tele-Afflicted

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    It is, yes. It simply takes that 15K feedback resistor in or out of circuit. And I agree, it seems to be omitted from the layout.

    If you feel up to the challenge, I would highly recommend you first come up with a schematic and layout that match each other for the exact circuit you want to build. You can learn so much just from building/modding the amp on paper! Especially on your first build, not having accurate documentation could lead to build errors that you will then have to troubleshoot, and will definitely make it a bit harder to ask questions of the group. Everyone always asks for the schematic first.

    I see more and more differences between these 2 drawings the more I look. The schematic omits the line out jack...

    cc: @robrob
     
  5. chutson89

    chutson89 TDPRI Member

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    I appreciate your responses. I plan on sticking to the schematic (with the addition of the effects loop), and was using the layout to visualize how it should all go.

    If I understand correctly, I can simply omit the line out jack and the corresponding 1K and 22K resistors. At least that was my plan.
     
  6. Dannebrog53

    Dannebrog53 TDPRI Member

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  7. Dannebrog53

    Dannebrog53 TDPRI Member

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    My apologies for the previous reply, my Apple wireless mouse decided to make the page disappear stage right. I'm new to the forum and have just begun my build of @robrob's baseman micro LTP. I've got most of the parts together and begun the board, but before I get to far there's a few things I'd like clarification on. Like @chutson89 I'm wondering if 3 volume controls are necessary and personally would like to eliminate one volume control. All my amps are fairly simple control wise, no effects loops. For effects I use a Rangemaster clone treble booster or a Brian May, a Fender standalone tube reverb and occasionally a way pedal. I prefer natural distortion over pedals. I would obviously keep the gain/volume control just after V1A, would it then be best tone wise to have the master volume before the PI or PPI? I want to try and get that original tone/crunch of an overdriven power section. On his website @robrob points out that "the LTP has a large output swing resulting in a sweet overdrive tone" and that it acts as an additional gain stage. @robrob also points out that the small 12AU7 power tube can be overwhelmed by some pickups and effects pedals. With that in mind would the PPIMV be better to control the signal input to the 12AU7 and still maintain the tone?
     
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes.

    The pre-phase inverter master volume (in front of the phase inverter) would be the one to eliminate. You can simply delete the pot.
     
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  9. Dannebrog53

    Dannebrog53 TDPRI Member

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    Thanks for the quick response @robrob
     
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