Marshall Ohm's selector

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TimD, Dec 26, 2004.


  1. TimD

    TimD Member

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    OK, I've lost the ohm's selector on my Marshall for the last time. If a man would want to hardwire the amp to a specific impedance, how would he go about doing this?
    I've pulled the chassis and see the various wires on the selector.
    Anyone have any idea as to what to do next?

    This has to be the silliest design I have ever seen.
    This is the second one of these that I have lost.
    Surely there has to be a better way.
    Thanks!

    Tim
     
  2. John Phillips

    John Phillips Member

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    Work out which is the correct pin for the impedance you want - it's the one at right angles to where the number is, if you have the 'square window' type selector. If in doubt, put a selector cap in and wiggle it - you'll see the tag move.

    Then simply unsolder the black wire from the center tag and solder it to the pin you want. Don't 'bridge' the two tags, since if someone refits a selector in a different position it will short the OT.

    Do the supply-voltage one while you're at it.

    I like to put stickers over the selectors with "Hardwired for ..." written on them. Saves confusion later.


    BTW, this is the single most important reliability upgrade you can do to a pull-out/fall-out selector Marshall, and may save the OT. Those selectors have a nasty habit of becoming intermittent, even if they don't fall out completely. They are crap and should never have been used in the first place IMO. (Actually the earlier 'bean' type used on the Plexis are far less bad.)

    If you need to retain use of the selectors, the right thing to do is to sacrifice a small amount of originality and replace the selectors with the rotary switches used on the reissues. Better than losing the OT.
     
  3. mofinco

    mofinco Member

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    That's not as easy as it sounds. I tried that on my early-76 SL. The later rotary switches are not a direct fit. They're larger in diameter and the mounting holes are more widely spaced. Installing one would have required some irreversible chassis modifications.

    What I did was to just buy two of the selectors from MarshallParts.com - look at the bottom of this page:

    http://www.marshallparts.com/content/estore_list.asp?page=6&category=4

    They're cheap enough to keep one new one in the amp and one new spare in the gig bag.
     
  4. TimD

    TimD Member

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    John, thanks for the help. I have a couple of issues though, I do not have the selector now, and can't put it in to determine the impedance, Also, there aren't any black wires on the selector?? There's purple, green, yellow and pink.

    Any ideas?

    BTW, I just tried to order a couple of replacement switches from Parts is Parts and they appear to be out.

    Tim
     
  5. John Phillips

    John Phillips Member

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    That will keep you in spare selectors, but losing it isn't the real problem. It's the poor quality of the part in the amp that is the issue. The small metal tubes in that part make poor contact with the pins, especially when they get old, well-used (and stretched) or corroded.

    If you get a bad connection in one - all too common - you have an open circuit. This is VERY bad news in any circumstances - it's just the same as running with no speaker connected. Worse, if it happens when the amp is cranked up, you're breaking the circuit under power, which is the worst of all possible ways to operate a tube amp. Count on major damage to the tubes, sockets and/or OT.

    Keeping a spare selector is NOT the answer... sorry. Getting rid of the damn unreliable crappy things is. If that means slightly opening out the panel to fit a good-quality rotary switch which won't fail and take out your original OT, do it. If you don't need more than one impedance setting, just hardwire it - that way you keep the original part too.

    FWIW, I never leave the original selectors in and operating unless the amp is strictly a perfect collector's piece that will likely not be used. I will clean and retension the earlier 'bean' type since they can be made to work properly (and are a lot easier to get into anyway).

    The wire you need to move goes to the center tag. It's fairly easy to see which this is. The colors do vary... although the one to the center tag is usually black. In any case, it's the one that goes to the speaker jack.

    Do you still have the voltage selector? It's the same plug.

    Normally, the 8-ohm tag is in the middle, closest to the edge of the chassis, the 16-ohm is to the left of this and the 4-ohm to the right as you look at the inside of the chassis with the knobs facing you.

    If the print is still on the outside of the selector, the number is at 90ΒΌ to the hole which it corresponds to.
     
  6. TimD

    TimD Member

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    Ok, just to make sure I don't mess this up.
    My amp does not have a voltage selector. Crazy huh.

    Looking at the amp, with the knobs facing me, there are
    4 tags. Three on the outer perimeter and one in the center.
    The wire coming off of the center tag leads to the spkr jacks.
    Looking at the selector: the remaining wires are viewed as if looking at a clock. I hope this helps.
    At noon, there is a single tag with two wires. At 4:00 a tag with a single wire and then @ 8:00 a tag with a single wire.
    So now we have 12:00 w/two wires, 4:00 w/one and 8:00 w/one wire. The center tag again goes straight to the spkr jacks.

    With this visual, which wire would be 8 ohm?
    And am I correct in stating that the 8 ohm wire simply needs to be soldered to the center tag?

    Thanks for your help and sorry I'm being a little dense.
    The last time I lost this plug you were a great help.

    Thanks,
    Tim
     
  7. BWilliamson

    BWilliamson Guest

    Another option would be to use a selector switch from Hoffman Amps. One a couple of 100w's I've had with 4 speaker jacks I replaced one of the jacks with Hoffman's switch.

    If you don't have that option, you could pull the current ohm selector and go to Menards or something like that and get a small piece of aluminum and drill a hole and mount it in the original hole.
     
  8. mofinco

    mofinco Member

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    Easy for you to say - it's not a 'slight opening of the panel'. You're talking about more than a 'slight' enlargement of one hole and drilling a new hole for another - in both the back panel and the chassis itself.

    I don't advise anyone to cut any holes or enlarge any holes in these older amps which are getting increasingly harder to find in original condition. You may do that to your own amps, but I wouldn't suggest anyone else cut up theirs.

    Protecting the OT is definitely a priority, but not at the expense of chopping up a chassis. I do agree with removing one of the output jacks and installing another brand rotary switch there - that's reversible without chopping a chassis. Or hard-wire as described here.
     
  9. John Phillips

    John Phillips Member

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    The one with two wires is the 8-ohm - the other wire is the feedback supply. Remove the wire from the center tag and solder it to this one if you need 8 ohms. Make sure all three wires are connected to the same tag.

    :)

    Yes, you have to take a file to it and open out the holes, unless you have the correct size sheet-metal punch.

    I'll do it to any amp where use of the selector is necessary - actually it rarely is, most owners only have one cab they use - and where the amp is a 'working' amp. Amps are made to be played IMO. Their value lies in their tone, which is to a large part dependent on the OT. So I'll happily do a non-reversible alteration to the chassis (provided it's done discretely and to a professional standard) which protects the original OT and therefore the value of the amp. If you're putting strict chassis originality above the OT, that's backwards IMO.

    Fitting a rotary switch in one of the speaker jack holes is usually not an option - the whole reason most people need to keep the impedance selector is because they use two cabinets sometimes. Anyway, IMO it's less of a 'correct' solution.


    No, I will not drill holes in the front panels for any reason, and I'm very wary of drilling them in the back too (certainly on any amp with is in decent original condition). But I will also open out the Bulgin socket to take an IEC mains lead, if requested. I don't consider that an important loss of originality either.

    Neither do Marshall BTW - if you send an old amp back to them for an overhaul, it comes back with an IEC and new selectors. It's a legal requirement in the UK.


    This is a just a difference of priority - I consider these things as musical instruments, not museum pieces, and I'll always take reliable working condition over strict originality. How do you feel about replacing old filter caps?
     
  10. mofinco

    mofinco Member

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    A tad defensive, eh? :p Don't like people challenging your opinions? ;)

    Don't get me wrong - I modifiy the hell out of almost everything I own. Some of the things I've done have raised a few eyebrows in certain places...

    But I pick my modifications carefully. I've done many things to my own equipment that I wouldn't necessarily recommend to anyone else, such as routing out the inside of the control cavity of two of my Norlin LPs so that I can use conventional-length pots in the neck control positions, or completely reconfiguring a Mesa rack power amp from a dual-channel to a single-channel, multiple-mode amp, or gutting a pre-CBS Bassman down to the tube sockets and transformers in order to build something else into the chassis. Been there, done all that, and more. The only thing I regret, looking back, was the Bassman. Fortunately, that's being restored.

    How do I feel about changing filter caps? I do it without blinking. For the record, my early-'76 100watt SL is modified. I reverse-engineered it from U.S. MkII specs back to the earlier, pre-MkII metal-panel specs, and then did a couple of common mods on top of that. And changed the filter caps while I was at it. None of that required a single irreversible mod to the chassis.

    Again, gotta be smart about what you cut up. Modifying a relatively-common, not-particularly-collectible Mesa rack power amp, which there is no particular demand for... no problem here. I did it for my use in my band. Never a thought of selling it. Same with my guitars. There isn't a single guitar I own that I bought with the thought of ever selling it. So, looking at the big picture, there are some things I don't think twice about cutting up.

    But in other cases - such as older Marshalls - I won't cut up (or enlarge a hole, or drill new holes front or back) for the sake functionality. On pieces like that, I will always go the extra mile to find a non-invasive way to get a particular task done. All the mods on my SL are 100% reversible, should anyone ever want an original-spec MkII SL again (not that anyone would! :p ).

    Like it or not, the market for early-mid '70s Marshall heads is going up. It may never reach the level of the late-'60s amps, but they're going up nonetheless. And like it or not, examples with no extra holes are going for more money than drilled and filed examples are, functionality be damned. It's a strange market, but no stranger than the market for '50s sunburst Les Pauls and old Stratocasters. Smart owners of those guitars wouldn't dream of making some mod 'for the sake of functionality'.

    Since, for me, my Marshall is primarly for my own entertainment and has been on stage with me exactly once, it's not inconceivable that I could end up selling again some day. Not likely, but more likely than me parting with my main stage rig. And the mods I've done to mine are 100% reversible. And to some people, like it or not, that's a valuable asset. It would be stupid to ignore that in today's market.

    But, hey, that's just me, eh?

    I'm putting chassis originality on an equal level with OT originality, for the record.

    Fitting an aftermarket rotary switch in a speaker jack location is definitely an option on a U.S. amp. European-market amps I've seen have only two speaker jacks, but U.S.-market amps have four. No problem to forfeit one for the purpose of a rotary switch and retain the ability to plug in multiple cabinets.
     
  11. John Phillips

    John Phillips Member

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    Now that is interesting... yes, all the Marshalls I've ever seen or worked on over here have only two speaker jacks - except the SLP reissues, which have four, and I always wondered why they did that...

    That is good information, and yes, then you can easily use one of the jack holes.

    I do disagree about putting chassis originality on a par with the OT though - one affects the tone (and the true value of these old amps IMO), the other doesn't.

    There's also a world of difference IMO between drilling a hole in the panel where none should be, and carefully opening an existing one out a little so the correct, modern part used by the same company can be fitted.

    No, I wouldn't necessarily do it to a 100%-original, perfect-condition amp - even a metal-panel, which are really very common over here - especially if it was going to be a 'collector' piece (which frankly, nearly anything in that kind of condition needs to be treated as these days). But on anything else I would consider decent selectors and a modern, safe power socket to be an upgrade.


    I'm not defensive at all about my opinions BTW :) - I simply strongly believe that strict originality is over-rated, especially on things like amps, where some parts must eventually be changed to keep them working. I don't believe in hacking stuff up for no really good reason - remember I advised someone here not to modify his AA165 Bassman - but I don't believe in putting originality over reliability either. Filter caps and crappy impedance selectors are good examples... especially as they could potentially destroy something far more valuable and irreplaceable if they fail.

    Anyway, if you're going to talk about '100% reversible' mods, remember that it won't be long before collectors are as nutty about original solder joints inside amps as they are already about guitars ;).
     
  12. TimD

    TimD Member

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    John, thanks for your help!
    Your instructions are super clear.

    I can handle this now.

    Tim
     
  13. TimD

    TimD Member

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    Got it!.

    John thanks for all of your help.
    Got the wires resoldered and the amp is up and running.


    Thanks!

    Tim
     
  14. clay_finley

    clay_finley Member

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    US Super Leads had 4 speaker jacks, but the 50w models always had 2 (US or otherwise).

    Hi Rick!
     
  15. TimD

    TimD Member

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    I need to bump this up to the top. I just aquired a 16 ohm 4x12 and now need to hardwire the amp to 16 ohm.
    John or anyone happen to know which tag is the 16 ohm tap?

    The upper center tag was 8ohm and there are two remaining tags, one at approximately 8 o'clock and one @ 4 o'clock

    Thanks!
    Tim
     
  16. trdlasvegas

    trdlasvegas Member

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    I just installed one of these new selectors on a old amp and I agree you have "open up" the hole a liitle to fit it. But I just saw this on another BB, and I can tell you this is slick. The guy (sorry I didn't get his name) made a small spacer to mount it with no change to the vintage amp.
    [​IMG]

    -Tony
     
  17. TimD

    TimD Member

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    Bump....still looking to see if anyone has any info on which lug is the 16 ohm tap.

    Thanks!
    Tim
     
  18. V846

    V846 Member

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    What modle amp is it?

    David
     
  19. V846

    V846 Member

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    Doh,pose to be model.. anyhoo if it's a 100 with 2268 OT

    Green=16
    Yellow=8
    Black=4

    50 with 789-139OT
    Grey=16
    Green=8
    Yellow=4
     
  20. TimD

    TimD Member

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    the 50 watt model is it.

    Thanks for the info, I figured the grey would be the 16 ohm tap.

    Thanks for the help.

    Tim
     

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