Could someone with a Weber MASS and a multimeter...

Discussion in 'Effects, Pedals, Strings & Things' started by Ventor, May 5, 2008.

  1. Ventor

    Ventor Member

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    ... check the input jack impedance rating at full attenuation / half attenuation & no attenuation (levels at 0, 5 & 10) respectively?
    I have a problem with my MASS 150w that when I put the levels lower than 5 the impedance on the input jack increases from 12 (halfway) to 25 ohms (full attenuation). I use an 8 ohm cab / head so the head is suffering from a 3-1 impedance mismatch, and this is quite bad for the power amp section of my tube amp.
    I want to be sure this is only a problem of my Weber and not some faulty design.

    I also found that the lower switch (high/low switch) of my model is in the way of the MASS motor causing a dent in the speaker. So if anyone could check that too that would be great. :AOK

    Edit: I have a Weber MASS 150W btw.
     
  2. paraedolia

    paraedolia Member

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    I'd email Ted Weber if I were you -- he's pretty good about supporting his work.
     
  3. Ventor

    Ventor Member

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    I did and he says it's not uncommon to have that high impendance. WTF? I don't plan to blow out my expensive amp because he failed to mention that you should never use the MASS with over 50% attenuation.
    He also mentioned that many people have bought the unit and never reported any problems with it. Perhaps they didn't notice they had to buy new power tubes twice as often... :mob
     
  4. vibroverbus

    vibroverbus Member

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    When you say "Impedance" I imagine you are measuring "DC Resistance"?

    I don't know exactly what is in those things aside from a driver motor, but I'm pretty sure they are not a dead 'dummy' load.

    If there's any cross-over / tone-filtering network of caps in it, 25ohms DCR is going to have nothing whatsoever to do with what AC impedance is. Could be .1ohms for all you know. You need an AC bridge type instrument like an impedance bridge or ESR meter to confirm what the AC impedance looks like.
     
  5. StompBoxBlues

    StompBoxBlues Member

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    Hold on a second. It's hard to be sure I am understanding what you are writing here...are you saying Ted Weber said you shouldn't use a Weber Mass set to more than 50% attenuation? OR is this your interpretation?

    I've used mine with a Germino 45 (JTM-45 clone) with quite a lot of attenuation with no problems. But I have a hard time believing that they designed an attenuator that would damage the amp when used in the useable range. In fact, I use the Mass as dummy load (100% attenuation) when working on an amp at times.
     
  6. Ventor

    Ventor Member

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    No the fact is if you attenuate over 50% you're mismatching your head impedance by over 2:1, as if you were using a 16 ohm cab on a 8 ohm head.
    At 100% attenuation it's closer to a 3:1 mismatch.
    Ted told me that if my amp can't handle a 2-2.5 to 1 mismatch I should get a better amp...
    At lower amp volumes there's not really a problem I think because you're not pushing your OT or tubes that much. But when I cranked my uberschall the tubes began to crackle.

    @ vibroverbus: I didn't measure it, it was an amp tech with over 15 years of amp/effects building experience who did. He used a multimeter to measure it. When the attenuator was on bypass the impedance was 6.7-7 ohms which was what he expected from a 8 ohm speaker, at low attenuation it was around 7.5-8.5 ohms.
    What else could cause a crackling sound in the power tubes?
     
  7. vibroverbus

    vibroverbus Member

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    Well I found a weber mass schematic, and assuming it's set on 8ohms and using an 8ohm speaker... after a quick breakdown...

    With the external speaker plugged in, and set to 0 attenuation, DCR should be just slightly less than just the speaker itself (because there should be a 25ohm rheostat in parallel). So 6.something is fine. Max attentuation should be pretty much the same, perhaps a slight difference since the motors impedance and/or DCR is bound to be slightly different than the real speaker.

    The only time it should be more IS if you have no external speaker plugged in and turn it to something less than max attenuation. At an extreme (zero attenuation with no speaker plugged in) the 25ohm resistor IS the load indeed. And I would agree completely with you - that would be a no-go setting for any of my amps certainly! But I'd also assume it would be part of the instructions - something like "should never be operated at less than max setting if no speaker is connected"...

    If your 25 ohm measurement is, on the other hand, with a speaker cab plugged in, then you've got a wiring or connection or other (fried resistor?) problem with the unit I think.

    And to re-iterate, your dood was measuring DC resistance, not AC impedance. This is not BS terminology stuff - it's like talking about an engine and using horsepower and torque interchangeably - sorts similar scale of numbers, but not the same at all. When he correctly told you re: "6.xohms is fine for an 8ohm speaker" I'm sure he knows and means "6.x ohms of DC Resistance is fine for an 8ohm nominal impedance speaker". The unhelpful part is that both are measured in ohms, and, you can make certain rules of thumb to equate them, but they are different things entirely.
     
  8. Ventor

    Ventor Member

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    Thanks for the info vibro.
    My particular MASS does not have an impendance selector switch however.
    Tonight I'll take some pictures of the unit (it doesn't seem quite complex).

    I saw 2 cap's next to the rheostats, and 2x 150w resistors (I think they are, 2 times 4 white blocks next to each other) and that's about the only electronics used in it (besides the mass motor and 2 resistors between input & line out).
    What schematic did you use, the one for the 200W kit?
     
  9. vibroverbus

    vibroverbus Member

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    Yeah, eggsactly. Impedance selector shouldn't matter, principle is the same, assuming he hasn't re-engineered the basic concept. And it seems too coincidental as it explains your measurements pretty precisely so I have to guess it's the same basic design.

    Do you know if your measurements taken with speaker plugged in or not? If it was just on the bench with no cab then it'd be the correct readings.

    I have to say if I was selling it I'd probably screen-print that warning on the front ... although I probably worded it improperly, because I think the "0" setting is max attenuation, right? So warning should be something like "Unit should only be operated at '0' setting if no speaker is connected"
     
  10. StompBoxBlues

    StompBoxBlues Member

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    I think that wording would give the wrong impression. I think better is more what you wrote before, that "if no speaker connected, you must set attenuator to maximum attenuation ONLY" .

    The way you wrote here, makes it seem like you should never set it to zero when there IS a speaker connected, to me.
     
  11. Ventor

    Ventor Member

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    He did all the measurements with a speaker attached yes.
    So you're saying it's normal if you use max attenuation ('0' setting, no sound coming out of the speaker) and a speaker cab connected you get 25 ohm?
     
  12. StompBoxBlues

    StompBoxBlues Member

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    If you read again, he was claiming it was right "with no cab" attached, meaning no speaker...so it seems NO it apparently is not normal.

    I have a Weber Mass at home, am at work now and have band practive tonight but maybe tomorrow can see about measuring mine speaker attached, and not attached at "dummy load" setting.
     
  13. Ventor

    Ventor Member

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    That would be great, thanks. :BEER
     
  14. vibroverbus

    vibroverbus Member

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    good call... :jo
     
  15. daddyo

    daddyo Guest

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    I've been using a Weber MASS 50 for 5 years and have tried all the attenuation settings with no problems.
     
  16. vibroverbus

    vibroverbus Member

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    :NUTS:crazy

    Again, perhaps my wording was confusing, although I didn't think it was that confusing.

    The unit should be perfectly safe WITH a speaker plugged in at any setting.

    WITHOUT a speaker plugged in, it is best to only operate at the '0' setting.

    This is all predicated on the 200W kit schematic and assumes the commercial models are basically structurally the same... If some of the commercial models had enhancements this may not apply at all. For instance, if I had one of these I'd put a shorting jack on the speaker output and wire the shorting-tab up to an 8ohm power resistor, so that if there's no speaker plugged in it provides a decent resistance at any setting.
     
  17. StompBoxBlues

    StompBoxBlues Member

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    I wonder if we have the same unit. Mine has the tone controls (for the line out) on the left side, etc.

    I used a parallel 16 ohm (8 ohm) speaker cab.

    In any case, I measured the input (the speaker cable going into the Weber from the amp) and am really surprised about the results.

    At the speaker cable into the speakers I get 7.2 ohms. Just about right.

    BUT, with bypass (and as expected it is the same whether I am on high or low range) I get 18.1 ohms.

    When not in bypass (mine has a knob that goes from 0-10, 10 being no attenuation)

    "LOW range"
    ==========
    10 = 28.5 ohm
    5 = 27.0 ohm
    0 = 26.5 ohm

    "HIGH range"
    ===========
    10 = 13.6 ohm
    5 = 13.2 Ohm
    0 (Dummy load with speaker attached) = 12.9 ohm


    I forgot to measure simply dummy load with no speaker attached, can do that tomorrow
     

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