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UL Bassman Mods

Discussion in 'Shock Brother's DIY Amps' started by Fred Mertz, Jun 17, 2016.

  1. Commodore 64

    Commodore 64 Friend of Leo's

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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That PPIMV is a complicated mess. I recommend the standard Lar-Mar which simply replaces the grid leak resistors with dual pots.
     
  3. Commodore 64

    Commodore 64 Friend of Leo's

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    Robrob, are you talking about the one in the schem above, or the one on my (Fred's) layout. Or both?

    Fred's seems to be this:
    [​IMG]
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    Can anyone comment on this voltage dropping scheme that Fred proposed? It's been suggested i use a pi resistor/filter resistor to drop volts instead.

    [​IMG]
     
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Commodore, that last MV layout is the Lar-Mar and what I believe is the best PPIMV.

    I'm not familiar with Fred's scheme but he knows his business so I wouldn't hesitate to follow his lead.
     
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  6. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, that's what I needed to hear on both accounts. It's the way I was feeling regarding Fred too. So I shall sally forth and get to work on the eyelet board for this.

    Thanks!
     
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  7. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, made some progress today. Got the eyelet board built. It's about an hours worth of work, start to finish. Took 2 hours to get the garage heated enough to work in it semi-comfortably. Ready for spring!
    [​IMG]

    And then got the mounting holes/standoffs reasonably close. Don't worry about the eyelet impingement on the upper left, I won't be using that eyelet.
    [​IMG]

    And here's a shot of the power supply. I'll ahve to futz with the dropping resistors, I'm sure. First stage is 2, 300V 220uF in series.
    [​IMG]

    And a gratuitous shot of the tools of the trade. Namely a bolt and steel rod ground to various point geometries. These can be chucked into my el cheapo Harbor Freight drill press. There are two bolt stops. One has a hole in it for doing turrets. Use the drill press to do the initial swage. Finally swaging done with a small ball peen hammer.
    [​IMG]

    Here's a pic of the drill press set up.
    [​IMG]
     
    Last edited: Mar 11, 2017
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  8. Commodore 64

    Commodore 64 Friend of Leo's

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    Wiring up my board. I think I have an error where the two channels tie together to go into the PI. I need the 100k plate and 220k grid stoppers to be in parallel, then through the.01 cap and that's not how I seem to have constructed the eyelet. Can someone review and give me ideas how to do this? (I can add eyelets easily enough.

    [​IMG]
     
  9. Commodore 64

    Commodore 64 Friend of Leo's

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    Think I got it sorted. These are mixing resistors, not grid stoppers.
    [​IMG]
     
  10. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, out with the old, in with the new. Here's a comparison shot of the the old '79 Bassman 10.
    [​IMG]

    next question, then is related to the bias circuit. The Fender 10k pot in series with other range resistors. Any reason I can't use one of the little 50k blue ones? I have a standard Fender 10k, but I think the little blue jobbie, will give me a little more adjustability. Do I still need a resistor in series (for minimum)? Like a 1.5k or something?
    [​IMG]
     
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  11. Wally

    Wally Telefied Ad Free Member

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    The 50K pot will give you a wider range. The 10k pot will give you a more precise adjustment......if both pots are one turn pots, that is. The 50k pot would have to have more turns in order to offer the same precision.
     
  12. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The resistor in series with the pot limits the max hot bias. Don't go too low.
     
  13. Commodore 64

    Commodore 64 Friend of Leo's

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    Andy B and Wally like this.
  14. Wally

    Wally Telefied Ad Free Member

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    Thanks for that info, commodore.
     
  15. Commodore 64

    Commodore 64 Friend of Leo's

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    Yeah, hopefully the mouser link is useful. Can be tough to find stuff on mouser. The first ones i ordered had the adjustment screw on the side, which is a real pain to access for adjustment when mounted normally.
     
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  16. Wally

    Wally Telefied Ad Free Member

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    That Mouser book is a big one isn't it?? Lots of options there....
     
  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Thanks Commodore, that is a really nice bias trim pot.
     
  18. Commodore 64

    Commodore 64 Friend of Leo's

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    I'm wiring this up, and I didn't feel right about my first FMV Channel. So I'm revising it to this.

    [​IMG]
     
    Last edited: Apr 15, 2017
  19. Commodore 64

    Commodore 64 Friend of Leo's

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    [​IMG]

    I'm confused regarding the OT wiring. I assume the green is the 4 ohm tap?

    I don't even know if I need the external or the line out jack, so I was thinking of simply putting a single jack in there. What exactly is the grn/yellow? According to that diagram it looks like a center tap for the secondaries. But I've never used an OT with that.

    Edit, found this: http://www.tubeampdoctor.com/en/sho...ansf_for_2x6L6GC_2_4_Ohms_70W_ultralinear_156

    grn/Yellow is apparently 2-ohm tap. So I'm just gonna take out the other ext jack and line out, and just use the 4-ohm tap.
     
    Last edited: Apr 15, 2017
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  20. Commodore 64

    Commodore 64 Friend of Leo's

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    Question on the Hoffman input jacks:

    [​IMG]

    I can isolate the jacks from the chassis. I have isolation washers. Does the diagram above assume isolated jacks? I assume non-iso jacks, the "G" is grounded to the chassis, even if a ground wire isn't installed.

    I want to use a ground bussbar, and isolated input jacks. Can I just use the diagram above, where the "G" is attached to the ground bussbar via a jumper?
     
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