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UL Bassman Mods

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

  1. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I believe that you have a standard bridge rectifier now. It is using the full AC swing of the PT. The changes would not alter the current demands on the PT. The only changes would be placing the dropping resistor between the rectifier and ground and eliminating the filtered B+ voltage feed going to the PT secondary center tap.

    I'm 99% sure that the bias supply is a separate winding with no connection to the secondary winding. You could confirm this by testing for continuity between the bias winding and the secondary winding. The F&T capacitors that you currently have are fine.

    This mod, as well as the others, is totally optional. If this is outside your comfort range, then don't do it. I was looking for a way to reduce the B+ from 500+ volts DC feeding the 6L6s down to an operating range that IMO is less stressful on the tubes. IMO 500 volts is way too high and requires a stone cold bias for them to operate at this voltage.
     
  2. Commodore 64

    Commodore 64 Friend of Leo's

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    Gotcha. This is well within my comfort level. We want to drop some volts then. I usually use 300-330V center-tapped transformers (with rectifier tubes) so I'm certainly more comfortable with <500 volt B+.

    I ordered 10, 10W 750 ohm resistors, I figure paralleling 2 of them would give me 20W @ ~375ohm?

    This is good, I can build a more traditional power supply.
     
  3. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Two 10 watt 750 ohm resistors will work just fine. Just to be safe, check for continuity between the bias winding and the secondary winding. I'm 99% certain that they're not, but the accountant in me would prefer the 100% confidence interval.:D
     
  4. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, so I have many 1n4007 (1A, 1KV) and 1n5408 (3A, 1KV) diodes as well as a 400V 25A, and a 400V 6A bridge rectifier. On a bridge rectifier, the 400V is that AC in or DC out?

    Any reason to use anything other than the 1N4007?
     
  5. Commodore 64

    Commodore 64 Friend of Leo's

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    Fred, No continuity between bias tap and secondary.
     
  6. Fred Mertz

    Fred Mertz Tele-Afflicted

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    What is wrong with the diodes that are on the board now? The 1N4007 diodes will work fine if you want to rebuild the board. I had a family emergency come Friday night. I haven't had any time to work on a layout and will have limited time for the next few days.
     
  7. Commodore 64

    Commodore 64 Friend of Leo's

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    Yeah, I'm just gonna rebuild the board, I have loads of diodes might as well do a clean sweep. DO I need those snubbing caps that Fender used on all of the diodes?

    I hope everything's OK with your family. I hate news like that :(.

    I ordered 4, Jensen Mod10 35. Built and covered a 24.5" square cab. Still need to build head cab, too. So I got plenty to do, and I also downloaded digitrace, so I've been playing with that, too.
     
  8. andyfromdenver

    andyfromdenver Friend of Leo's

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    S brother Fred, hope everything is ok.

    Commodore, I can draw up a board for you to take some load off FM.
    I haven't followed all the discussed changes, but if you'll compile all the stuff you guys want by showing posts or details, i'll draw up to the Phase inverter, if you'll take it from the power section and bias. Lemme know and i'd be glad to offer that; I'm still on vacay until Friday.
     
  9. Commodore 64

    Commodore 64 Friend of Leo's

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    This was FM's original mod summary for the Bassman10.
    [​IMG]
    [​IMG]

    I added a 12AX7. So I've got 3, 12AX7, and a 12AT7 now. The layout FM for the Bassman 10 just has the 2 12AX7, and the other difference from that layout is that I have a TMB on both channels. So with the added 12AX7, I'm hoping to have one channel AA864, and the other will be the Hiwatt FM is talking about.

    I think everything from the PI on will stay the same, and I can handle the diode bias and power section easily enough. So basically I just need the preamp modified for TMB on the one channel, and then use of the added triodes I now have.
     
  10. andyfromdenver

    andyfromdenver Friend of Leo's

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    ok so just like the above with an additional unbypassed gainstage after each, like what he posted for the bassman 70.
    Fred, I don't know if you're checking in, but what about the cool HIWATT phase inverter?
    lemme know this one thing and i'll work on it today.

    [​IMG]
     
  11. andyfromdenver

    andyfromdenver Friend of Leo's

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    on second thought that might convolute things.
    Fred essentially drew up the layout you need above, except keep the bass channel the same as fender (cept add a triode before mixing) and add a triode after his posted tmb channel, then mix and go to the phase inverter? if this sounds right, I'll draw that! :)
     
  12. andyfromdenver

    andyfromdenver Friend of Leo's

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    so I got started, but Fred and Commodore. The updated bassman 10 schematic and layout you guys posted aren't consistent with eachother. look at the tone stack connections on V2.
    you have a .001 off the plate before the treble cap, that isn't present in the layout.
    you guys let me know if the scheme is the ideal, and I can draw that up.
    also, Fred I noticed an error on the modded 70 scheme you posted, it's small, but the bass chnl tone stack needs to come off the plate not the B+ (for interested parties).

    here's a pic of the start:

    likey? :)
    [​IMG]

    also, Fred et al, in your ammended 70 (which I used parts of for this) do you guys feel funny about feeding 6 triodes from one B+ battery (cap). maybe the newly added post-stack-recovery-extra-zing-triodes need to come from a earlier B+ cap and resistor. make sense?

    edit: I just realized the triple plate resistor groups is impractical for the center ones, going to rework that cause I forgot about the coupling cap off that middle one. tbc...
     
    Last edited: Jun 28, 2016
  13. Commodore 64

    Commodore 64 Friend of Leo's

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    Yes, Andy. This is exactly what I need. I think Fred's Bassman 10 layout has the Lamarr post-PIMV, with the ganged pot, so I'd like to do that. As far as the two input channels, I'm guessing that with the extra 12AX7, I can pretty much do what he envisioned with the Bassman 70 (AA864 one channel, Hiwatt on the other).

    Also, I have 4 power nodes to work with in the current doghouse, first one being 300v, 220uF caps in series.

    [​IMG]
     
  14. andyfromdenver

    andyfromdenver Friend of Leo's

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    cool! maybe we can wait for el man to chime in, and maybe add another dropping resistor and cap in the B+ string? If anyone feels it's needed.

    So I can draw the top one in pic with another gain stage at the end and the interesting tone stack and bright switch off the cathode bypass. or the bottom one with the bright switch reducing the coupling cap, and add a middle pot (edit in: I would need help adding a midpot as this isn't the typ fend config). [​IMG]

    How do you like to drill the board? I use a template from the board supplies from Watts audio, but then go inbetween the designated holes to make things more Fender-y. [​IMG]

    also, on the V1 cathode caps and resistors, how would you guys feel about combining? so a single 820ohm resistor and (if wanting to be official) a 50-100uf/25v elytic for bass freq too.
     
    Last edited: Jun 29, 2016
  15. Commodore 64

    Commodore 64 Friend of Leo's

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    In the past, I've just printed a layout, overlaid it on the board, and drilled the holes.

    I'll let FM decide what we should do with the values. I do want to use the mid pot, so I don't have any unused holes in the front, if possible.
     
    andyfromdenver likes this.
  16. Commodore 64

    Commodore 64 Friend of Leo's

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    Since we are lowering the B+, I could easily just rewire that doghouse for an extra power node, since I won't need the first stage to be in series...

    Andy, I'm looking at the Bassman 10 schematic that FM modded, and V1B cathode appears to connect to Node A. Does that mean that voltage would be 520 V?

    Also, I don't see where node B is used at all, so I think we could repurpose that?
     
    Last edited: Jun 29, 2016
  17. EddieLocrian

    EddieLocrian Tele-Afflicted

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    no idea...
    Gonzo.gif
     
  18. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Good eyes Andy. I corrected the errors that you found.

    Modded Bassman 10 UL Layout.gif
    Modded Bassman 70.gif

    Commodore the two As that you see are to show the shared cathode connection at the 1K resistor. I carried this forward from the original Fender schematic.

    I think that additional filtering between the second and third gain stages would be a good idea. There is the node with the 30K resistor connected to ground that could be re-purposed. I would mod it to something like this.
    UL Power Supply Mod.gif
     
  19. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Andy, I did tweak the PI. There should be some negative feedback in the circuit to off-set imbalance attributable to mismatched triode sections. I also did some simulation with different plate resistor values to match the gain of the two triode sections as close as possible.
     
  20. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, so should I just have Andy make the Bassman 70 layout (since I have the additional triodes) and work with that? Remove both mid pots (that's OK with me)?
     
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