Mod to use Weber Mass 200 with a 2 ohm amp?

Discussion in 'Amp Tech Center' started by itsGiusto, Jun 14, 2021.

  1. itsGiusto

    itsGiusto Tele-Holic

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    I realize this is a long-shot, but figured I might as well ask. I just traced the circuit of my Weber Mass 200 attenuator:

    [​IMG]
    Note: this is the basics of the circuit, but I left out the line-out portion of the circuit for simplicity-sake.

    On the website, Weber says that this attenuator can work with 4 ohm, 8 ohm, and 16 ohm amps due to impedance swamping. My question is, is there some mod I can do to get it to work with 2 ohm amps?

    A bunch of other Weber attenuators that feature impedance switching seem to accomplish this sort of thing by putting a big resistor of 2 times the desired impedance going to ground right before the dummy speaker coil. This is the schematic of a different Weber attenuator, the Mass 100, which uses impedance switching, not swamping:
    [​IMG]

    So what if I put in a switch to switch in a 4R 100w resistor in parallel to ground right before the dummy coil in my Mass 200? Any chance of it working? Or would I blow out my amp's transformer?
     
    Last edited: Jun 15, 2021
  2. wabashslim

    wabashslim Friend of Leo's

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    Write or call Weber.
     
  3. itsGiusto

    itsGiusto Tele-Holic

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    Do you really think they'd tell me anything? I figured doing that would just be a waste of time and make me look stupid. Most gear companies I've dealt with aren't too keen on telling you how to mod their products, it could be a liability for them.
     
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  4. Jon Snell

    Jon Snell Tele-Holic

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    If you require a 2 Ohm load then it will be a solid state amplifier and to be frank, a solid state amplifier couldn't care less if the load impedance is higher than the minimum allowed.
    It is unusual for a valve amplifier not to have tappings for 8 Ohms so what are you trying to do and with what?
     
  5. itsGiusto

    itsGiusto Tele-Holic

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    Its for a vintage Fender silverface Super Reverb, I'd like the option of driving it harder at lower volumes by using an attenuator. Super Reverbs have a 2 ohm output.

    Worst comes to worst I can build a simple resistive load to act as an attenuator using 2 resistors (see here), but I'd like the ability to have variable volume output, which comes with the Weber attenuators.
     
    Last edited: Jun 15, 2021
  6. Jon Snell

    Jon Snell Tele-Holic

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    Replace R3 with a 2 Ohm 100W resistor or add another 4 Ohm across R3, that will give you what you need.
     
  7. NTC

    NTC Tele-Holic

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    4 x10" Bassmen, brown Concerts, and Super Reverbs are all 2 ohm output. Buying a tube ot for such amps is no problem, either.
     
  8. johnDH

    johnDH Tele-Meister

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    I have a design for an attenuator front end that creates 2, 4 and 8 ohm inputs. You could use part of it.

    Attenuator M2v 200420-3.gif

    Just look at L2, R13 and R12. You could wire up something like that in front of your Weber. The advantage of it is that it it is a reactive module, unlike the Weber which is virtually just a resistive attenuator once you switch it down to 4Ohms.

    Id suggest to use about 60% of the values in the middle line, so L2 = 0.2mH, R12 = 2.5 Ohm and R13 = 6 Ohm, and then don't switch the Weber to lower ohms.

    The new module with the Weber at 16 ohms combined with this module in parallel, will be close enough to 2ohms.

    You could wire up the three new parts and put them into another parallel amp jack, leaving the Weber unmodded. Or build them into the Weber.

    The new parts will absorb most of the amp power. It will be a lot quieter than usual with the Weber.
     
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  9. loopfinding

    loopfinding Friend of Leo's

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    side note, the irony of reverse engineering something, putting it on a libre website, and copyrighting your drawing...
     
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  10. itsGiusto

    itsGiusto Tele-Holic

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    I think you're referring to the second schematic. That's just there for comparison, that's not the attenuator I own. Also, that one already has the ability to work with 2 ohm amps.

    What did I copyright? If you're referring to the second schematic, I just found that one online. Looks like it's from 2000
     
  11. itsGiusto

    itsGiusto Tele-Holic

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    That's awesome, thank you! So in-effect, my amp would be primarily driving the choke and 2 resistors, and the Weber would just be sort of an extra add-on?

    L2 is a choke, right? What should its power rating be if I want it to work with a ~50 watt amp?

    The only think I'm uncertain about is, the weber isn't necessarily 16 ohms. I'm not sure what the impedance is. Remember, my attenuator is the top schematic, not the bottom. The Mass 200 uses "impedance swamping", not switching, so it's supposed to be able to handle 4, 8 and 16 ohms, without being switched. So how do I know what the impedance of it actually is?
     
  12. archetype

    archetype Fiend of Leo's

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    Weber has some very helpful folks, but there's the liability, as you mention. They've been burned by people misusing and undersizing attenuators.

    I'd email CJ Sutton [email protected]. They may have dealt with this issue before and have an answer. The worst they can do is decline.
     
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  13. loopfinding

    loopfinding Friend of Leo's

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    oh yeah, not a dig at you at all, just whoever the hell had the absolute nerve to slap their name on it, hahah. at least make it CC or something.
     
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  14. johnDH

    johnDH Tele-Meister

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    hi @itsGiusto

    No problem!

    The coil would be an air-core inductor wound with 16 gage wire. I usually suggest 18 gage but this is a bit thicker to deal with high currents in a 2 ohm system. If you're in the US, try Parts Express or Madisound.

    I was having a look at info on Weber Mass 200. As you note, there's no impedance switches. He has decided that it is safe for any amp ever, in tbe range 4-16 ohm! That's a big call.

    From what you traced out, the basis of it looks to be similar to other Webers, with an LPad pot and a speaker motor. A reference I saw suggests that it might be an 8 ohm motor, different from the smaller Webers which seem to use 16 Ohm.

    I think we'd like to know more, in order to best work out your added load components. Would you be up for doing some simple resistance measurements, across a cord plugged into the Weber input?

    So, dc resistance at full volume, mid-turn and minimum volume, all with and without the real speaker plugged in. And also measure the speaker separately. That's 7 readings, and deduct the resistance of the meter leads.

    From that, I can back-figure approximately what the amp sees at audio frequencies, and the components to add to make the amp see close to a real 2 Ohm speaker load overall.
     
    Last edited: Jun 15, 2021
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  15. wabashslim

    wabashslim Friend of Leo's

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    I've called them a few times, including some talk about mods, and always got good conversation even if the answer was No. I've sent them pictures of speakers "supposedly" reconed by them for verification, and got the yay or nay, with explanations.
     
  16. itsGiusto

    itsGiusto Tele-Holic

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    Thanks! So unfortunately, I don't actually have the Super Reverb with the 2 ohm load yet, it's an amp I'm considering buying and I'm going to go see this weekend. Whether I could attenuate it is sort of one of my considerations for whether I want to buy it.

    So since I don't have the amp yet, I could only take 3 of the measurements for the attenuator, which were the DC measurements of the Weber input, without the speaker attached. I took the measurement from the tip to the sleeve of the cable I plugged into the Weber input.

    The measurements are (with the DC measurement of the test leads subtracted):

    Full Volume Up: 48.2ohm
    Volume halfway up: 34.2ohm
    Volume all the way down: 14.2ohm

    From these measurements, it seems likely that the dummy speaker inside the Weber 200 is a 16-ohm speaker. Do you agree?

    If I decide to buy the amp, I'll plug in the speakers, and then I'll let you know what the remaining 4 DC measurements are.
     
    Last edited: Jun 16, 2021
  17. johnDH

    johnDH Tele-Meister

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    it looks like its set up as a 16Ohm or thereabouts

    Just thinking further. If the 2 ohm speakers from the Fender are plugged into this attenuator, then at max, it will be a 2ohm load and at min it will be that 14ohm value. That's a huge range and I'm not sure its all good.

    Thinking....
     
  18. itsGiusto

    itsGiusto Tele-Holic

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    Well, I ended up emailing Weber last night. They told me not to try modding the Mass 200 in any way, but that the Mass 200 should actually work already with a Fender Super Reverb's 2-ohm load. So that's good news!
     
  19. johnDH

    johnDH Tele-Meister

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    I guess he should know, and i don't know because i dont understand the risks involved!. If you want to be careful, maybe you could start with attenuator at max volume and amp at min building up slowly, and then just turn the attenuator down a tad.
     
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  20. itsGiusto

    itsGiusto Tele-Holic

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    I think the reason why the 16-ohm dummy speaker load doesn't make sense at a glance has to do with the impedance swamping aspect of the attenuator. I know very very little about this, except that I think that the attenuator works similar to a Zobel network (see Weber Mass "Impedance Swamping"? thread for some more info). It's supposed to smooth out the frequency response of the power amp, and provide something of a constant load. Maybe that means that it's able to handle a wider range of output impedances, without needing to exactly match them?

    I do think it's interesting that the Weber attenuators that feature impedance swamping (like the Mass 200 and the Mass 150) are the same ones that feature a treble knob instead of a switch... I think that might have something to do with the impedance swamping, since I know that Zobel networks are supposed to feature both capacitors and resistors, in order to even out the frequency response. On other Weber attenuators (like the MiniMass and the Mass 100), the bright capacitors are switchable in and out of the circuit. Perhaps having them not switchable, and thus in the circuit at all times, is enough to "swamp" the impedance and make the attenuator work across a wide range of output impedances.

    Like I said, I don't fully understand it (and I would like to understand it better)... but maybe that has something to do with it. I wonder if someone who knows more, like @FenderLover could comment.
     
    Last edited: Jun 18, 2021
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