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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    No, I don't have one to play with. I've been looking, but can't find one cheap enough. I would prefer to have the extra tube for the third gain stage. But I don't know if it would be worth the effort to add an additional tube to the Bassman 10. I believe that some sort of clean boost pedal on the front end would be the low hanging fruit solution. It comes back to personal preference.
     
  2. FenderLover

    FenderLover Poster Extraordinaire

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    Your example of the Route 66 speaks volumes Fred - some people insist on global NFB like it's supposed to be there. Personal choice or mindset, doesn't really matter. The good doctor got that one rockin' with UL. One could do much worse.
     
  3. Commodore 64

    Commodore 64 Friend of Leo's

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    OK. I need to figure out how to go from schematic to layout. Is the best practice to hand-draw?

    When there's a line through a pot, does that mean the third lug is soldered to ground (or pot case?)

    This page is helping me a lot to visualize: http://amps.zugster.net/articles/tone-stacks
     
    Last edited: Jun 22, 2016
  4. Fred Mertz

    Fred Mertz Tele-Afflicted

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    I uploaded a layout in post #16 of this thread.:D
     
  5. Commodore 64

    Commodore 64 Friend of Leo's

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    LOL! SO you did. Awesomeness. Thanks a lot!

    (I ordered the pots and some other supplies from AES this morning).

    And right now the plan is to make the mods, as you have indicated (without adding another 12AX7), and build a pine cab to house it. I gig separate head and cabs right now and have a 1x15, 1x12, 2x12, and 2x10 already so that makes the most sense.

    As a 4x10, it really is a backbreaker.

    Also: is there anything to be gained (or lost) by jumpering the channels with a pigtail? The two times I've gigged it (with guitar), that's what I've done. It does seem to have a little more fullness and slightly more volume that way. But I might just be imagining it.

    One other minor thing. You specified 2M2 which I assume is 2.2M. I have 2M resistors, do I need to tack another 200k in series to each?

    Edit: Just realized I forgot the 10k screw pot for bias. Can I just use a standard linear 24mm 10k or a small 10k trimmer?
     
    Last edited: Jun 22, 2016
  6. Fred Mertz

    Fred Mertz Tele-Afflicted

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    The 2 meg resistors are fine. A standard 10K screw pot will work just fine. Just depending upon where the bias voltage range comes in and where the amp biases, the 15K resistor may need to be adjusted. You can jumper the channels if you want. The channels are in phase with one another. I would use an ABY pedal with this amp as the channels are voiced differently. This would allow you to switch between channels or jumper the channels on the fly. Guitar ---> Clean Boost Pedal ---> ABY Pedal ---> amp. Something like a Keely Katana would work well in this application IMO.

    The 4 10s in a separate pine cab would be nice.
     
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    That means the pot is being used as a variable resistor so the input and wiper are jumpered together.
     
  8. Commodore 64

    Commodore 64 Friend of Leo's

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    [​IMG]
    Yes, My Bassman is different that the one in your layout. But it's close. There's a TMB on both channels, a line out jack, and a humdinger pot for the 6.3v heaters.

    I'm not a huge fan of TMB, especially when I don't have gain to spare, like it seems in this circuit. All the lead dress needs addressing, especially the heater twists. I wonder if it makes sense to just replace that board and start over? Tube sockets need retentioning, and I'm inclined to replace them as long as I'm in here, I might as well do it right. I like the new Belton 8-pin socket springs better. Whaddyall think?

    [​IMG]

    I am thinking of building a pine head cab, and separate 4x10 cab, pine, using light speakers. Candidates due to weight (and price) are:

    Emi Legend 1028K (3 lbs)
    Celestion 10 30 (3.5 lbs)
    Jensen Mod 10 30 (2.5 lbs)
    WGS Vet 10 (3 lbs) - only 20 watts a piece though, is that enough leeway?
     
    Last edited: Jun 23, 2016
  9. Commodore 64

    Commodore 64 Friend of Leo's

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    Fred: Now that I'm considering a full rewire/rebuild, I'm leaning towards adding the 4th 12AX7 and doing your full on Hiwatt/AA864 mod. You wouldn't happen to have a layout of that would you?

    I don't know much about diodes and reliability. I was planning on leaving the power section as is. Any reason to replace the diode bridge?

    And should I go with a ground bus to star ground scheme?
     
    Last edited: Jun 23, 2016
  10. Fred Mertz

    Fred Mertz Tele-Afflicted

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    No I don't have one. But, if I get some time over the week-end I'll try to cobble one together. The added tube should really make a big difference in the amp. I don't have a problem with bridge rectifiers. I am curious as to the voltage readings for the DC B+ at the OT center tap and the AC feed from the power transformer to the bridge rectifier. I really don't care for voltage multiplier circuits.
     
  11. Commodore 64

    Commodore 64 Friend of Leo's

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    Is this a voltage multiplying circuit? Schematic says 520 at node A, the schematic that someone hand annotated says its 370-0-370 which would give us that with a bridge rectifier (~1.4)?
     
  12. Commodore 64

    Commodore 64 Friend of Leo's

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    OK. Gutted it. New sockets in. New eyelet board lacquered.
    [​IMG]

    Started working on the cabs. I'll make an eyelet board once you have a layout. No hurry on that, Fred. It's something I need to learn to do (go from schematic to layout), so after I've got the cabs knocked up, I'll start looking into that, too.

    The angle of the chassis front causes the case of dual pot to touch the chassis. Will that create a ground loop or anything?
     
  13. Fred Mertz

    Fred Mertz Tele-Afflicted

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    Yes, it is a voltage multiplier circuit. Notice in the schematic the PT center tap connection to the power supply filters. That is why I was interested in PT AC. The reading should be to the PT center tap.


    Not a problem. The control pot cases are grounded to the chassis. But this does bring up a good reason not to use the pot cases for circuit ground connections. Because each pot is grounded to the chassis at a different location, using the pots for circuit ground connections, results in multiple circuit ground connections which increases the risk for ground loops.
     
  14. Commodore 64

    Commodore 64 Friend of Leo's

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    Hmm. I see. Any reason I couldn't ground the center tap and just use two diodes and a traditional power rail setup? I suppose your answer depends on what the voltage measured there is now, right?

    Excuse my ignorance here, but could I

    1. Ground the center tap.
    2. Unhook the AC feeds from the rectifier board.
    3. Power it up and measure the AC feed, and determine what the step up is? A B+ of 400-450v would be plenty, no? If it's a 325-0-325 or so that'd work wouldn't it?

    I have a Variac, I could use low voltage for above testing...
    Also have a heathkit regulated low voltage power supply...
     
    Last edited: Jun 23, 2016
  15. Commodore 64

    Commodore 64 Friend of Leo's

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    OK, feeding 12v AC with a variac, and the CT grounded, I get 18V on each AC leg of the transformer (red wires). Does that mean I've got a 180-0-180 power transformer?
     
    Last edited: Jun 23, 2016
  16. Fred Mertz

    Fred Mertz Tele-Afflicted

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    That sounds about right and it correlates with the DC voltage supply. Working backwards. 520 / 2 = 260, 260 / 1.414 = 184.
     
  17. Commodore 64

    Commodore 64 Friend of Leo's

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    Alright, so that answers that question. Guess I gotta use the voltage multiplier topology.

    I wonder why they chose to use a lower voltage transformer. The iron itself is easily much larger than the Hammond 273BX I have (350-0-350 @201mA). Can't have been cheaper to make?

    Quick googling indicates there's higher ripple with voltage doubler. I've got a 90mA choke here. Orange used a choke in their voltage multiplier power: http://www.chasingtone.com/schematics/PPO25/OR120_72power.jpg

    Is 90 mA beefy enough?
     
    Last edited: Jun 23, 2016
  18. Fred Mertz

    Fred Mertz Tele-Afflicted

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    It actually was probably cheaper than a comparable transformer providing 360 volts to center tap (fewer windings). I don't believe that a choke would help much in this application.

    I believe that we may be able to change up the power supply circuit by taping off the PT center-tap and implementing a voltage dropping resistor between the bridge rectifier and ground to drop the B+ voltage.

    UL Bassman Power Supply Mod.gif

    I estimate that the circuit should draw around 95 ma. at idle. The current across the 370 R resistor should provide a negative bias of about 35 volts feeding into the bridge rectifier. The rectified DC voltage should equal about 460 volts DC, 1.414 x (360 - 35) = 460 volts. At clipping the DC B+ should sag down to around 430 volts.
     
    Last edited: Jun 24, 2016
  19. Commodore 64

    Commodore 64 Friend of Leo's

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    Any reason we just wouldn't use a plain old bridge rectifier? If we are confident the current capacity of the PT can hack a straight up bridge rectifier/capacitor input load (.62 of sec I A.C. according to Hammond), we'd be at ~ 500 B+ at node A. Right now, my first input stage is a pair of 300V, 220uF in series. They are F&T, would I need to procure some 100uF or would the 220uF be ok?

    And does taping off the center tap affect the Bias taps at all?
     
    Last edited: Jun 24, 2016
  20. SamClemons

    SamClemons Poster Extraordinaire

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    Don't know about the UL and ya'lls technical knowledge is beyond mine, but I know with a regular 50 watt bassman, just jumpering the channels together opens up all kinds of sonic possibilities with no mods. Craziest thing I had was one of the old 100 Fender tube PA's , essentially a 4 channel Twin, with reverb, you could jump all 4 channels together........
     
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