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Thread: 1971 Fender Bassman Ground Switch

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    Member mikeyphys's Avatar
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    Default 1971 Fender Bassman Ground Switch

    Just a quick question regarding the ground switch on my bassman. I know there's a lot of variety in the wiring on these amps but can anyone explain the ground switch?

    3 positions, what does each position do?

    I'll hopefully get the amp modified to disable it but in the meantime I need to know what position the switch should be in so it's safe, from what i've read it's often described as the 'death switch' hah, slightly alarming.

    Anyway, the switch position has been changed during carrying it or whatever and I cant remember what position it was in to begin with.

    Anyone got any info on this?

    Cheers.
    Beaten Epi Les Paul. '71 Bassman.

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    Default Re: 1971 Fender Bassman Ground Switch

    i think that switch is to reverse the ground incase you get some hum... If you are not getting any ground noise just leave it where it is

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    Default Re: 1971 Fender Bassman Ground Switch

    Right well the switch may not be in the same position it was the last time I used it so is it safe to change the switch position during use, or is this a turn it off, change it, turn it back on situation?

    Did some reading, seems the switch is for reversing the polarity of the AC coming into the amp. I could understand that more with a 2-way switch, so why 3 way, what's the 3rd option?

    Does 1 - N - 2 make any sense to anyone cos I read that somewhere aswell in reference to the ground switch on vintage amps.

    Cheers again!!
    Beaten Epi Les Paul. '71 Bassman.

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    Default Re: 1971 Fender Bassman Ground Switch

    It sounds like somebody changed out the original switch to one that has a "middle" position. That leads me to believe your "death cap" is still in the amp, but that may also have been removed. If your amp has been fitted with a 3 wire cord, that whole polarity switch/mess is a non-issue.

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    Default Re: 1971 Fender Bassman Ground Switch

    If I open the plug and the ground terminal is wired up, will this make the position of the switch irrelevant?
    It's been working totally fine, i'm just concerned (mostly cos guitar wiring i'm fine with but high voltage is well beyond me) incase the switch is put in a position that could potentially become a hazard. Do you reckon the middle position is probably the position to avoid then just to be safe?

    I probably will take it to a tech once I have cash, it's got one of those red voltage selector switches, though it sounds like a good idea, I don't really want something like that so easily accessed incase it gets changed when the amp is running.
    Beaten Epi Les Paul. '71 Bassman.

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    Default Re: 1971 Fender Bassman Ground Switch

    The ground switch on old Fenders was simply reversing the ground. back then most electronic devices had a 2 plug setup. The ground switch was sort of the same thing as taking the plug out of the wall and flipping it over. As someone else said, if you have a 3 plug grounded plug that was added this will have no real effect. Just leave it in the position where you get the least hum.
    "So you will never have to listen to Surf music again" James Marshall Hendrix
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    Mojo's Minions J Moose's Avatar
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    Default Re: 1971 Fender Bassman Ground Switch

    Quote Originally Posted by mikeyphys View Post
    3 positions, what does each position do?

    I'll hopefully get the amp modified to disable it but in the meantime I need to know what position the switch should be in so it's safe, from what i've read it's often described as the 'death switch' hah, slightly alarming.
    Does the amp have a 3-prong plug?

    If so, the switch won't do anything... or at least shouldn't because it SHOULD be disconnected!!!

    Otherwise the switch is telling electrical ground which way to go.

    In the old days there was no common ground circuit... and really, no "correct" position for the switch other then the one that doesn't electrocute you.

    If the amp doesn't have a 3-prong plug, you should get it to a tech ASAP or do it yourself. All you need is an extension cord, cut one end... and a screwdriver.

    I've posted the instructions before... not very hard and yes... it could very well kill you. That's not a joke.
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    Default Re: 1971 Fender Bassman Ground Switch

    Just took the back plate off the plug and there are 3 wires. I take it this means the amp has been rewired in order to use the common ground through the mains socket?

    Does this mean the switch will literally have no effect or will it still be a case of see which position has less hum? Sorry if I'm repeating myself or causing you guys to repeat yourself here.

    btw, i'm in the UK and the actual plug is a british plug, the amp has a voltage selector on the back aswell, i dunno if this is stock or an obvious indication that the amp has been on a tech's desk at some stage.

    Anyway, thanks again to your time and help!
    Beaten Epi Les Paul. '71 Bassman.

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    Default Re: 1971 Fender Bassman Ground Switch

    Quote Originally Posted by mikeyphys View Post
    Just took the back plate off the plug and there are 3 wires. I take it this means the amp has been rewired in order to use the common ground through the mains socket?
    Huh?

    A picture would surely help quite a bit...

    Do you mean that the receptacle... the plug that goes into the wall outlet, has 3 prongs?
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    Default Re: 1971 Fender Bassman Ground Switch

    In the uk all wall outlets are 3 prong, and the ground prong on the plug opens up the neutral and live holes on the wall outlet (in most cases) even if the appliance itself doesn't require the ground connection so you need a 3 prong plug.

    But anyway that aside I opened the amp's wall outlet plug and there's 3 wires attached to the plugs 3 terminals. So would it be safe to assume the switch on the amp has been bypassed and the amp's ground is connected through the 3 wire cord?

    Sorry I can't provide pictures, I don't have immediate access to a camera.
    Beaten Epi Les Paul. '71 Bassman.

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    Electron Herder glassman's Avatar
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    Default Re: 1971 Fender Bassman Ground Switch

    Quote Originally Posted by mikeyphys View Post
    In the uk all wall outlets are 3 prong, and the ground prong on the plug opens up the neutral and live holes on the wall outlet (in most cases) even if the appliance itself doesn't require the ground connection so you need a 3 prong plug.

    But anyway that aside I opened the amp's wall outlet plug and there's 3 wires attached to the plugs 3 terminals. So would it be safe to assume the switch on the amp has been bypassed and the amp's ground is connected through the 3 wire cord?

    Sorry I can't provide pictures, I don't have immediate access to a camera.
    If you have access to a multimeter, measure the resistance between the ground terminal on the plug and the chassis. If the resulting reading is (or is very close to) 0 ohms, set the switch to the center detent and enjoy playing it! If not, as previous posters has said, find the switch position that provides the lowest amount of hum and leave it there.

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    Default Re: 1971 Fender Bassman Ground Switch

    UK plugs are quite different than 120V USA and use 220V.

    The picture below illustrates USA style wiring; however, it is different than UK style wiring.

    Take it to a repair shop - before you fry yourself!


    --------------------------

    The two flat pin used in UK carry 220V electricity and the flat bars on either side are suppose to be earth (Ground).

    UK plug wiring colours when wiring a plug. They are:

    Earth - Green and Yellow (Previously Green)
    Neutral - Blue (Previously Black)
    Live - Brown (Previously Red)



    ----------------------

    It should also be noted that the reissue Bassman amps have a non-functional ground switch located on the front panel. Fender's manuals states the following:

    GROUND SWITCH —Non connected. Installed topreserve the appearance of the original ‘59 Bassman. http://media.musicalplanet.com/pdf/FEN304.PDF
    Last edited by Stratman; 09-24-2010 at 01:32 AM.
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    Default Re: 1971 Fender Bassman Ground Switch

    Right, just to point out the amp is and has been working fine, i'm just curious for future use, should the switch position be altered during transport as it most likely will.

    I have a multimeter, did what Glassman suggested, wasn't sure which resistance value to use, tried all, got zero, this was with black wire attached to ground prong on plug and red wire attached to the chassis.

    I also tried a continuity test aswell. Tried touching all exposed metal areas with the red wire while the black was croc clipped to the groung prong on the plug. It buzzed on everything. Regardless of the position of the ground switch.

    Surely this suggests that the chassis is grounded.

    I'm not an expert, like I said before I can rewire my guitar but I realise out of my depth with an amp.

    On the back of the amp there's a red voltage selector. Says on it Pre-Set Voltage Warning. Was this a stock thing for all fender amps or could this be something that was part of the stock build for ones that were built for export to the U.K etc?
    The selector goes 117, 200, 210, 220, 230, 240. I keep it on 230 cos that's the current u.k voltage.

    Stratman - UK plugs have 3 rectangular pins arranged in a triangle and we're on 230V:

    1
    2 3

    Where 1 is ground, 2 is live and 3 is Neutral. This is if you are facing the pins.
    Beaten Epi Les Paul. '71 Bassman.

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    Electron Herder glassman's Avatar
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    Default Re: 1971 Fender Bassman Ground Switch

    Only export models had voltage selection options.

    It sounds to me that you are good to go...have fun playing!

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    Default Re: 1971 Fender Bassman Ground Switch

    Quote Originally Posted by mikeyphys View Post
    Right, just to point out the amp is and has been working fine, i'm just curious for future use, should the switch position be altered during transport as it most likely will.

    I have a multimeter, did what Glassman suggested, wasn't sure which resistance value to use, tried all, got zero, this was with black wire attached to ground prong on plug and red wire attached to the chassis.

    I also tried a continuity test aswell. Tried touching all exposed metal areas with the red wire while the black was croc clipped to the groung prong on the plug. It buzzed on everything. Regardless of the position of the ground switch.

    Surely this suggests that the chassis is grounded.

    I'm not an expert, like I said before I can rewire my guitar but I realise out of my depth with an amp.

    On the back of the amp there's a red voltage selector. Says on it Pre-Set Voltage Warning. Was this a stock thing for all fender amps or could this be something that was part of the stock build for ones that were built for export to the U.K etc?
    The selector goes 117, 200, 210, 220, 230, 240. I keep it on 230 cos that's the current u.k voltage.

    Stratman - UK plugs have 3 rectangular pins arranged in a triangle and we're on 230V:

    1
    2 3

    Where 1 is ground, 2 is live and 3 is Neutral. This is if you are facing the pins.
    Flat (Rectangle) - got distracted by my wife "get off the computer and eat"....

    Same plugs as in Hong Kong and Singapore...
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    Default Re: 1971 Fender Bassman Ground Switch

    Any of you guys familiar with these "export" models? I have never been inside of one. I am just wondering, if it is wired for a country with a 3 wire main why would the amp have that polarity switch?
    Anybody?

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    Default Re: 1971 Fender Bassman Ground Switch

    Quote Originally Posted by zzmoore View Post
    Any of you guys familiar with these "export" models? I have never been inside of one. I am just wondering, if it is wired for a country with a 3 wire main why would the amp have that polarity switch?
    Anybody?
    I've been wondering the same thing since people got back to me with their answers.
    I think someone else mentioned the reissue bassman's having a fake ground switch so the amp still looked like the original model.
    Beaten Epi Les Paul. '71 Bassman.

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