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First Amp: Bassman Micro LTP Build/Mod

Discussion in 'Shock Brother's DIY Amps' started by JaiJaiBuilds, Sep 24, 2019.

  1. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    Hey guys, I'm new here, made an account here because you guys were recommended while reading robrob's Micro Bassman Build (https://robrobinette.com/Bassman_Micro.htm).

    I'm extremely early into the build (haven't even ordered parts yet) but I wanted to ask you guys about a possible mod I could do before I go ahead and order the parts.

    Since Rob starts by saying how easy it would be to convert the Bassman into a JTM45 (switch out valve in V1), I was wondering how feasible it would be to just have two valves in V1, a 12AY7 (Bassman) and a 12AX7 (JTM45) and place a switch in between them so that only one of them is selected at any one time...or am I oversimplifying things?

    [​IMG]
     
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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, it would be that simple. Just put the switch between the 33k input grid stop and the tube grids.
     
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  3. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    awesome! Thank you for the quick reply! And thank you for your website man it's really great, I've just been reading all of it for the last two days! I'm a university student so I don't see the amp getting built very quickly, but I'll post pictures as I go. :)
     
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  4. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    awesome! Thank you for the quick reply! And thank you for your website man it's really great, I've just been reading all of it for the last two days! I'm a university student so I don't see the amp getting built very quickly, but I'll post pictures as I go. :)
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    Wait 'till you get a job and family... :)
     
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  6. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    haha man if I do that imma be waiting a long time!
     
  7. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    The output transformer that the build calls for (Hammond 125B), has an impedance ratio of 22.5K ohms : 8 ohms,

    However, I have a donor chassis/cabinet that has 2x12 speakers with an impedance of 8 ohms each...so in series together, they will have 16 ohms of impedance.
    I could not find data in Hammond's website about the wiring for a 16 ohm speaker to produce the same primary impedance.

    However, I was able to find another transformer (OT5PP+) from musical power supplies that has wiring options for 4, 8, and 16 ohm speakers, all with a primary impedance of 22K ohms...

    by using the musical power supply OT, the primary impedance is off from the Hammond's by 0.5k ohms...will this resulting change impact tone/reliability of the amp?
    or should I just get different speakers?
     
  8. sds1

    sds1 Tele-Afflicted

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    I think you can use 125B and just fudge a little. For example, 1+4 (21.5k @ 15R) or 4+6 (24k @ 15R)

    Even more so with OT5PP+, a 0.5k discrepancy would be considered negligible.

    See what the other folks say.
     
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  9. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    Thank you!

    Just for general theoretical understanding...how much of variance in impedance can an amp circuit tolerate?
     
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  10. Paul-T

    Paul-T Tele-Meister

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    2-10% would be a negligible difference, the impedance varies far, far more than that for various other reasons (heat, frequency etc) - when manufacturers quote a figure, it's pretty notional in any case.

    Wire the speakers in parallel for 4 Ohms total - then with the transformer you mentioned, couldn't you use taps 5 and 6, 4 ohms secondary, 22k primary ? The other taps mentioned would be fine, too.
     
    Last edited: Sep 27, 2019
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  11. sds1

    sds1 Tele-Afflicted

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    I'm not exactly sure but I feel very comfortable with 10% variance based on what @Paul-T mentioned -- impedance is AC resistance which means it will vary by frequency, so datasheet ratings can only be ballpark by nature anyhow.
     
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  12. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    Yeah I did think about wiring for 4 ohms but at the time I didn't know if the 0.5K ohm difference would be significant...but yeah if that is negligible, I can use the Hammond and wire it so that the amp accepts 4, 8, and 16 ohm speakers - which is great! Thanks guys!
     
  13. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    Thank you, as well!
     
  14. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    Also, I noticed that the negative feedback loop resistor is configured for an 8 ohm load, would I have to reconfigure it to a different resistance for the 4/16 ohm loads? Or should I just leave the same value for all three loads?
     
  15. sds1

    sds1 Tele-Afflicted

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    The NFB resistor stays on the 8R secondary no matter which speaker is selected.

    Not sure what OT you're decided on but if you are without the 8R secondary you'll need to adjust the NFB resistor to compensate the chosen secondary.
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The Hammond 125 series don't work like a typical multi tap output transformer so I would adjust the NFB resistor and connect it to the actual tap wires used for the speaker.
     
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  17. JaiJaiBuilds

    JaiJaiBuilds TDPRI Member

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    I'm using an Epiphone Blues Custom 30 as the donor amp, and I found these transformers/chokes in it.

    There's a 5H Choke.

    A 350-0-350 power transformer.

    And a 22k primary impedance output transformer.

    Would it be possible to use them in the Bassman build? The Hammond inductor recommended is 5H as well, and the impedance of the output transformer is very close to the one recommended in the build.

    The power transformer is of a much high voltage rating than the recommended 240-0-240 secondary output, but would it be possible to get the output down to that level using resistors?
     

    Attached Files:

  18. gmdattilo

    gmdattilo TDPRI Member

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    I think there may be 3 errors in the Bassman Micro LTP schematic. The schematic and the layout don't agree. The power tube coupling caps are shown as 10nF. I think they are supposed to be 100nF. Same with the AC ground cap in the LTP. The schematic shows 10nF but the layout shows 100nF. In the image below the corrected (I think) values are in red. I used 100nF in my build and things seem right.

    And to address the first post... I've done the 2 preamp tube (AT7 & AX7) thing in my build. My switch is between the plates of the 2 preamp tubes and the 22n coupling cap. Each of my preamp tubes has its own 33k grid stopper at the tube socket and is permanently connected to the input jack.

    Bassman_Micro_LTP_Schematic_Markup.png
     
  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The choke and output transformer will work just fine. The power transformer, even when coupled with a 5Y3 rectifier tube, the voltage would be too high for the little power tube.
     
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  20. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You are correct. All three are errors on the schematic. It's embarrassing, I don't know how those errors existed for this long. Thanks for pointing them out. I'll correct the schematic.
     
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