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Proofread my Bassman Micro (EF80) layout, please...

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Jan 22, 2021.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I'm not ready to build this, but I'm thinking I may at least order a custom board from Doug Hoffman, which means I want to make sure I have the board right, which kind of implies a plan for the whole layout.

    NOTE: Mine is pretty much a straight copy of the awesome Bassman Micro design by @robrob for his newer pentode (EF80) version. I'll insert Rob's for reference here (see link for high-res version):

    Rob_Bassman_Micro_EF80_Layout copy.jpg

    It does have a few mods also from Rob's pages:
    • a bigger (audio taper) mid pot rather than a raw switch,
    • an NFB cut on a 15K resistor,
    • the option to run a sag / dropping resistor (value TBD) between rectifier and reservoir so I could use a slightly hotter PT if I wanted,
    • Isolated terminal posts to hang resistors off sockets
    • Rob's 1/8" headphone jack instead of aux speaker
    I mostly just moved the bits around to see how it'd fit in a Hammond 12 x 7 x 3 chassis.

    *Obviously, this is not a final layout in any way; no one should copy it or build from it.*

    HC Scale BMM_EF80_v4.png

    And below, a PDF if you need more resolution...
     

    Attached Files:

    Last edited: Jan 22, 2021
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The layout looks good to me.

    One option I'm adding to the Micro amp web pages is the $28 AnTek AS-05T200 toroidal power transformer. 200v @ 100ma and 6.3v at 4 amps. It needs a bridge rectifier which can be made with 4 1N4007 diodes.
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks, Rob. It’s more a tribute to yours than anything. And yes, I saw your latest update about the Antek. Thanks.

    I’m not thinking of too many sonic mods, but I guess the other thing is to look over my board to see if an extra eyelet or eyelets — eg bridge rectifier — would give me more options or ease little tweaks.
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You can use two 3-terminal tag strips to create a bridge rectifier. There will be room where the rectifier tube would have been.

    [​IMG]
     
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  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Cool. Thanks, Rob. I don't know from SS rectification, but Merlin also mentions commercial bridge rectifiers with screw holes for mounting. Something like this, maybe?

    upload_2021-1-24_8-39-3.jpeg

    On a different topic, Rob, somebody asked me if it'd be good to ground the screens filter with the reservoir cap instead of the preamp. I've never been very clear on where to split a ground. Thoughts?

    And on a related note, I guess you could just run a unified bus. Though I've built most of my amps with split bus and they've been very quiet.
     
  6. Nickfl

    Nickfl Friend of Leo's

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    I think people do split grounds just because they've seen a lot of people do split grounds... I'm not sure there's any good reason to do it, just run a unified ground bus and reference it to the chassis at the input jack.

    You might also consider adding a dedicated elevation point for your heaters like Rob has on his jcm 800 layout. You're only going to have about five volts or so on your cathode resistor with this amp, that may help a bit with the heaters but it would be nice to have more especially with a cathode follower. Plus it gives you a bleeder resistor as a bonus.
     
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  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Thanks, Nick, super thoughts. I like your bus thought, and I hadn't even thought about the amount or method of elevation.

    BTW, as you get it sorted, would you consider sharing here {EDIT: or, way better, in Rob's main EF80 thread) the changes you liked in your EF80 Bassman Micro? -- I know you've mentioned a few in your rebuild thread.
     
    Last edited: Jan 24, 2021
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, those bolt-on rectifiers use the chassis for cooling and are usually rated for 10 amps or more so they should be bullet proof.

    The reason the screens filter (usually B+2) is connected to the preamp instead of the power amp is because the first filter cap has a large ground pulse from the rectifier. It's thought that the separation helps cut 120Hz hum. I am a fan of the unified bus but I have seen instances where a unified bus was noisy and separating it into a preamp and power amp ground solved the issue.
     
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  9. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Can a bridge rectifier like this be used with a PT that has a center tap on the HT?
     
  10. dougsta

    dougsta Tele-Meister

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    The classic bridge rectifier using 4 diodes can be used on any AC source that's within the specs of the diodes.
    If you have a 300-0-300 and you rectify the 0-300, you'll get aprox 1.4x 300 as volts dc with a filter capacitor, so 420vcd, you'll need a capacitor rated high enough or use the Fender trick of using lower rated caps doubled up. You'll also only be using half the winding so only the current rating for that winding.
    Best to use a tube rectifier with HT CT transformers.
    You don't want to be rectifying the 300-300, 600 x 1.4 is a potential 840vdc!

    Doug
     
  11. dougsta

    dougsta Tele-Meister

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    I have a quite a few old radio transformers with 250-0-250. I should really use my EZ80 valves with them in a project.
     
  12. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Following up on the ideas from @Nickfl to provide more elevation to the artificial CT (via a divider off the HT) and also trying to steal a few ideas from the 'optimized' single-bus 5E3 drawn by @robrob , I came up with this inglorious buzzard.... (Ignore the moved dropping resistor to create B+3 -- I started out thinking I'd put the divider off the second filter).

    Don't trust me on this. It could be totally wrong. :D

    upload_2021-1-25_11-36-18.jpeg

    Here's the overview showing the ground bus.

    BMM experiment v2.png
     
    Last edited: Jan 25, 2021
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  13. robrob

    robrob Poster Extraordinaire Ad Free Member

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    It looks good to me.
     
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  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Let's not confuse the shock brothers. Diodes can replace the tube rectifier without a problem. The PT with Center Tap can be used with SS.
     
  15. dougsta

    dougsta Tele-Meister

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    Yes, sorry, you can of course replace the tube rectifier with 2 ss diodes, the question was about bridge rectifiers.
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You can use a tube rectifier and get the benefits of its voltage drop and sag with a transformer without a center tap by strapping on the current return side of a bridge rectifier. This also employs backup diodes:
    [​IMG]
     
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  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Just for completeness, a few more tweaks. I moved the B+3 dropping resistor back where it started, added a bright switch with, as Rob suggests, a bigger cap, and did a bit more to show some of my 'circuit' mods in purple. FWIW, as noted originally, this is drawn in a 12x7x3 chassis. Oh, and yeah, that's a shielded input cable -- picture the shield grounded at the input end.

    BMM experiment v3.png
     
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