Bassman Micro PI Master Volume

Discussion in 'Shock Brother's DIY Amps' started by Hyakuya, Sep 20, 2021.

  1. Hyakuya

    Hyakuya Tele-Meister

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    Playing around my new Bassman Micro build and I couldn't work out what the PI master volume was for as it doesn't seem to do anything when it's twisted... :confused:

    check, check, check, etc.... I think I've spotted something.

    I've built the LTP EF80 like this....

    upload_2021-9-20_15-46-42.png

    but the schematic looks like this... red arrow to the PI master vol labelled 1MA MV

    upload_2021-9-20_15-44-45.png

    It takes me a while before a schematic makes sense in my head but these aren't the same are they ?

    Shouldn't the PI master connect where the grid stop and grid leak resistors are tied together ?
     
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  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Somebody smarter than me could answer that from first principles. Me, I’ll just hope @robrob is watching the sky outside the Batcave…
     
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  3. 2L man

    2L man Tele-Holic

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    There is 1M and (56k + 56k) parallel so at first 1M pot has only very little effect when both 470k/56k limit signal, but 1M should eventually quiet it when turned fully to zero ohms. Perhaps some volume will still come although there come short over 56k resistors when signal can "change" this potential.

    Perhaps two 100k resistors and 220k or 250k potentiometer would work better?
     
    Last edited: Sep 20, 2021
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Annnd… you may be on to something (or maybe the schematic and layout got out of sync somewhere else) but here’s the layout for the 12AU7 version…

    5226F277-F920-41BC-B9A3-6A8E8AD339C7.jpeg
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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  6. Hyakuya

    Hyakuya Tele-Meister

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    I did a quick and dirty test.... Disconnected from the grid stoppers and clipped the wires to the other location.

    @2L man is right. The pot has some effect from zero to about 4 but from there to 10 it doesn't do much.Nasty buzzing noise at zero too.

    I'll wire it up according to the schematic.

    Edit: by zero I mean full resistance. Zero on the dial....
     
    Last edited: Sep 20, 2021
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  7. dan40

    dan40 Friend of Leo's

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    I have always wired them as they are shown on the layout pic. It's not my favorite MV because you get very little effect from it until the pot is turned almost all the way down. I will occasionally use this circuit with a capacitor across the pot to dull the high end a bit. When wired with a cap across it, it becomes a "cut control" and it is very effective at taming a brighter amp.
     
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  8. pelayostyle

    pelayostyle TDPRI Member

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    Weird. I just finished building this amp today and came across this post. I didn't catch this discrepancy in the schematic and built according to the layout. Thanks for the heads up. I'll go back to it and fix it.
     
    Last edited: Sep 21, 2021
  9. jtcnj

    jtcnj Tele-Holic

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    I built the JCM800 Micro later version with only the Pre-PI MV.
    I experimented with the Post PI MV by clip leading in the 1MA pot- at the voltage dividers as in the 800 layout.

    It seemed to work mostly smooth over the pot range; or I did not notice the audible change in volume bunched up at one end of the pot range.
    It may have been more effective at the lower range though, as stated above.

    I never really thought about it until now, but as you turn the pot "up" to ten - there is 1MΩ between the wiper and the leg - and 0Ω when you turn it to "down" to zero, correct?

    If so, as you turn it down, do the 2 inverted grid signals co-mingle and cancel each other out?
     
    Last edited: Sep 21, 2021
  10. dan40

    dan40 Friend of Leo's

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    Yep. It's just mixing the two out of phase signals from the PI. Same principle applies when two speakers or two pickups are out of phase with each other.
     
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