Marshall Class 5 impedance missmatch ?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 900z1, Feb 3, 2012.

  1. 900z1

    900z1 Member

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    has anyone tried an 8ohm cab with the marshall class 5 ?

    I know some tube amps can take a mismatch up to 100%, it is labeled 16ohms, and the internal speaker disconnects when you use the ext jack - does anyone know it this amp is too wimpy or if it should be able to handle an 8ohm cab ( for a total of 8ohms ) ? how about dimed ?
     
  2. Stu Blue

    Stu Blue Member

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    It is not a good idea to run any English amp on the wrong impedance, Marshalls in particular.... people who think it improves the tone are "mutton" (deaf).
     
  3. slagg

    slagg Member

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    Class 5 designer says it's fine to run at 8ohm,RE:Marshall amp forums..
     
  4. chervokas

    chervokas Member

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    That was the conventional wisdom based on the old transformers and possibly the potential for intermitent connectivity due to the old impedance selector switch used in Marshall amps in the 1960s and 1970s. A lot of these adages persist generations later even while companies have moved on to completely different designs and completely different parts.

    Most of the potential for damage from running a mismatched load comes from running a higher load when flyback voltages get stepped up by the OT, and so it depends on the specifc OT, its turns ratio, its insulation etc. I don't think you can just say anymore as a rule that you can mismatch Fenders safely but not Marshalls. That might have been generally true in 1968 but its irrelevant these days.

    The only way to be sure these days is to check with the designer/manual, though it is generally true that a mismatch of the sort the OP is talking about is usually safe, though, of course, it will result in a lower power transfer from amp to speaker and lower headroom from the amp.

    But I don't have any specific knowledge about the Class 5.
     
  5. pgissi

    pgissi Member

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    Yes the class 5 designer who I believe is named Steve has stated a mismatch with the class 5 is fine.

    IMO mismatch within reason meaning limiting to 100% so the stock 16ohm Class 5 output into an 8 ohm load is fine

    Of course any mismatch means less than optimal efficiency/power reduction and that means the possibility of an amp running hotter as you turn up to compensate. Heat is just as much an enemy as flyback and some amps are more tolerant than others.

    with a mismatch there is also a change in tone which varies from amp to amp but generally bandwidth is narrowed a bit, some low end and top end is gone you may notice more mids, depends

    But the Class 5 is an exception in some ways

    http://marshallvintagemodern.com/viewtopic.php?f=31&t=5539&hilit=8+ohms



    I read another post or maybe it was Steve, not sure, who stated a single ended class a amp is somewhat self limiting, the harder you drive it, the more it compresses with some sag, full dissipation occurs at idle so by nature of design there is a slight power reduction beyond idling and this contributes not only to the tone but the way the amp feels, part of the magic of single ended class a

    that is what I dig most about the amp, just the right amount of compression and sag and of course resulting in its tone

    I use 2 of them in tandem as a loud little amp platform, the biggest little tone that can be had! I can testify to the build quality and toughness, I have been using my mk1 since late 2009 and the mk2 since last summer and run them nearly dimed with the band, just to the edge of no more headroom which is just past 2:00 on the volume, they take a beating

    personally I want the max power and inherent tone so I stay with a 16 ohm load if I use an external cab
     
  6. thosk

    thosk Silver Supporting Member

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    Yep.
    Here's a link to the C5 forum. Lots of mods and info over there. The amp's designer(SteveD) is a regular poster.
    http://marshallvintagemodern.com/viewforum.php?f=31
     
  7. 900z1

    900z1 Member

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    Thanks for the replies.
    I'll check out the other forum.
     
  8. pdf64

    pdf64 Member

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    1/ I really can't think of any examples of completely new designs and parts? Tube power stages don't seem to have changed significantly in 6 decades, excepting the introduction of Si diode rectification in the 60s and a short lived trend for UL in the 70s. So perhaps the adage persists because the amps do?
    2/ The old models are still out there and are often highly valued.
    3/ Due to the above, the old models are often re-issued.
    4/ The new models seem to be the old models with a modded pre-amp.

    See http://www.aikenamps.com/Why70percent.html
    That shows how a 'low' load on stressed tubes (prime example being cranked EL34 @ B+ ~500V) can send plate dissipation to crazy high levels, which may greatly reduce tube life and increase the potential for catastrophic tube failure.

    Well, there's a profusion of other amp companies out there, but it's still broadly applicable, due to manufacturer affinity to tube type.
    My take is that, as general guideline, beam tetrode (6L6, 6V6) based amps are probably ok with a 1 step mismatch, whereas pentode amps are far less tolerant and so mismatch should be avoided.
    The basis for this is both due to the tubes-

    Pentode screen grids run hotter than beam tetrode screens and an 'up' mismatch (when cranked) exaserbates the screen dissipation;
    Pentodes tend to have lower plate dissipation than equivilant beam tetrodes;
    Beam tetrodes benefited from ruggedisation improvements for military applications - I'm not aware that EL34 / 84 went through that process?

    and the design, in that -

    EL34 tend to be pushed hard, with a high B+, and lower OT primary impedance than an equivilant 6L6GC amp;
    EL84 tend to be used with a B+ above their plate limit of 300V.

    Pete.
     
  9. chervokas

    chervokas Member

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    Well the circuits may be all based on old Rca or WE circuits, but the parts the parts aren't identical, and there's no historical version of the Class 5 or the Pro Junior or something, so I don't think you can say one's a Marshall, one's a Fender and therefore one can handle a mismatch well and one can't.
     
  10. CA_Dan

    CA_Dan Member

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    It's safe, but the tone is definetly different. It attenuates the highs somewhat due to insertion loss (reflective and standing waves I would guess) in the impedance mismatch.
     

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