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Marshall 1960A @ 4 Ohms

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by III, Jan 6, 2008.

  1. III

    III Member

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    When running 4 Ohms into the 4 Ohm Mono input of my 1960A, only the two speakers on that side produce any sound.

    Is this normal? Or could there be a switch or wiring problem?

    16 Ohms Mono pushes all four speakers as does running two amps 8 Ohm Stereo.
     
  2. John Phillips

    John Phillips Member

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    There is a fault with either the slide switch or more likely one of the contacts in the jacks. This is a common problem with these cabs.

    That is why these switching panels are a bad idea - depending on what exactly breaks the circuit, you could be left with no load on the amp at all, instead of half the speakers. There's a chance you'll blow the output transformer if that happens.

    Do your amp a favor, select which impedance you want, and fit a pair of standard jacks (paralleled if you want mono operation, it provides a backup), hardwired for that impedance, and get rid of the PCB entirely.

    You may also notice a tone improvement as well, with the current not having to pass through eight little contacts.
     
  3. destro718

    destro718 Member

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    So you're saying just cut off the connector clips that attach to the PCB and just connect the the two positive wires and the negative wires and just solder + and - to one Jack?
     
  4. John Phillips

    John Phillips Member

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    For a 4-ohm cab, yes.

    For 16 ohms, connect one positive wire to the jack, the other negative wire to the jack, and the remaining positive and negative wires to each other, and insulate the connection (heatshrink is best). It doesn't matter which way round the pairs are.
     
  5. destro718

    destro718 Member

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    Ok doesn't sound so difficult. I would probably wire for 16. A regular jack is a lot cheaper then that Marshall switcher. I just have to decide if i want it stock or not.

    Thanks a lot !!
     
  6. destro718

    destro718 Member

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    Hey When you run that switch in "stereo" mode can you plug a left and and right out of one amp into the left and right of the jack?

    I have a Vavlestate 8100 i believe its 8 ohms. Would it be best to run that stereo mode and use two speaker cables one left to the left input on the cab and one out of the right into the right input? (considering the cab is in stock condition) I guess what im asking is how do you use the stereo mode.
     
  7. John Phillips

    John Phillips Member

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    The 8100 is a minimum 4-ohm output, so that's OK - you can run both sides with the switch set to stereo and two speaker cables. The total load is then 4 ohms (two 8-ohm cabs in parallel, effectively).
     
  8. destro718

    destro718 Member

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    Thank you man!! Oh and Happy New Year!
     
  9. III

    III Member

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    Thanks for clearing that up for me. I really like the switching options, and may get a replacement panel. Are there any switching panels more rugged than the Marshall PCB?

    But since that cab is also the one I play out with, hardwiring for reliabilty and amp safety is what I'll probably do.

    Hardwiring for 4 Ohms would probably suit me best so I could run my Blackface era Bassman, Fender Roc Pro or 1959 SLP RI. My only concern would be running the SLP RI at 4 Ohms just because I've heard that Marshalls are "happiest" at 16 Ohms. Any truth to that?
     
  10. John Phillips

    John Phillips Member

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    They actually produce most power at 4 ohms (assuming the RI transformer ratios are the same as the originals) so the whole output section is under a bit more stress than at the other impedances. I'm not convinced 8 ohms is any more stressful than 16, but you don't have that option anyway if you want all four speakers.

    Unfortunately the Marshal panel is inherently unreliable - it relies on the contacts in the jacks, the slide switch, lots of solder joints, the fairly thin PCB traces, and the push connectors to the speaker wiring. It's just not good design in such a critical application. If you don't need stereo, what I would do is to get rid of the switching panel, drill out the square hole to 1/2" round, and fit a heavy-duty (power type) DPDT switch to do the 4/16-ohm switching with standard jacks, which can be done.

    It's the switch contacts in the jacks which are more of a liability than the slide switch itself, not least because the jacks are under more constant stress and wear as the plugs get taken in and out and sometimes tugged sideways - so if you avoid using those, it's much better.
     
  11. Edmundo

    Edmundo Supporting Member

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    You could just wire the cab like this with a mono & a stereo switching jack. The circled number below the jack is the ohm setting for using that particular jack...

    [​IMG]
     
  12. AdmiralB

    AdmiralB Silver Supporting Member

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    How so?
     
  13. John Phillips

    John Phillips Member

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    You can do it with two mono switching jacks actually... I don't have a drawing but it can be done. I still don't like it because it relies on the jack contacts to do the switching.

    Because the OT ratio is slightly different for 4 ohms for some reason.

    Doug Roccaforte pointed this out a while ago, so I checked it and he's (not surprisingly :)) right.
     
  14. Edmundo

    Edmundo Supporting Member

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    Cool...that's a schemo I'd like to see!! :BOUNCE
     
  15. John Phillips

    John Phillips Member

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    I'll try to describe it and you can draw it out...

    On the 4-ohm jack, connect both switch contacts together.

    Connect the plug contacts of the 4-ohm jack to the switch contacts of the 16-ohm jack, tip to tip and sleeve to sleeve.

    Speaker pair 1 is connected with its positive connection to the tip plug contact of the 4-ohm jack and its negative connection to the sleeve switch contact of the 16-ohm jack.

    Speaker pair 2 is connected with its positive connection to the tip switch contact of the 16-ohm jack and its negative connection to the sleeve plug contact of the 4-ohm jack.

    Does that make sense?
     
  16. III

    III Member

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    This sounds like a good compromise if I don't just wire for 4 Ohms. I'll try to hunt down something heavy-duty, low profile and DPDT. Thanks John P.
     

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