2 prong to 3 prong ground wire conversion question...

Discussion in 'Ampage Area' started by erlomd, Apr 12, 2021.

  1. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    Quick question regarding 2 prong to 3 prong ground wire conversion.
    I did the swap on my 1963 Bassman 6G6-B but I bypassed the ground switch.
    however, I see that in this drawing, you can use the ground switch.
    Is it necessary to continue using the ground switch with the 3 prong upgrade? or can it continue to be bypassed?
    Thanks guys!

    Screen Shot 2021-04-11 at 6.22.09 PM.png
     
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  2. thunder58

    thunder58 Super Moderator Staff Member

    Age:
    62
    Dec 23, 2010
    tappan ny
    Admin Post
    Welcome to the forum @erlomd . Our friend @hcsterg is the go to guy to answer this
     
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  3. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    @erlomd ,

    I restored a Domestic (US / non-export) Bassman 6G6-B :

    [​IMG]

    I converted it to export 240V 50Hz by replacing the power transformer (Hammond 290EEX 125P7D 022814).

    I removed the GND switch polarity and its stuff, since it is useless here in France, and used the hole to install a bias adj. pot.

    The original cord was in perfect condition and was 3-conductors, so I replaced the plug for a French one, and compulsorily connected the earth conductor, as required by French standards.

    Personally, I would opt for the 3-prong with GND connection on your amp, but as you're in USA, the electrical regulations may be different ? o_O

    Edit : in a 3-prong connection, the polarity switch is useless, so you can bypass it - but again, this works in France / Germany...

    A+!
     
    Last edited: Apr 12, 2021
  4. thunder58

    thunder58 Super Moderator Staff Member

    Age:
    62
    Dec 23, 2010
    tappan ny
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  5. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    You wrote while I was typing @thunder58... ;) But I'm not sure to have answered as expected...:oops:

    A+!
     
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  6. thunder58

    thunder58 Super Moderator Staff Member

    Age:
    62
    Dec 23, 2010
    tappan ny
    Admin Post
    Well ...... after all ....your the " go to guy " . I'm sure you've helped our new friend :)
     
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  7. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL

    Thanks so much guys for the fast reply! So yes, it is ok to just bypass it.
    I did just that! I installed the new 3 prong cable a few months ago but I never upgraded the power transformer. Should I upgrade it?

    Also, I love that you used the GND switch whole to install a bias pot! I was wondering how the heck I was going to bias the new tubes?!!

    BTW, Your amp looks great!
     
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  8. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    Here's the old girl BTW:

    LRG_DSC05296.JPG
     
  9. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    Glad to come aboard!
     
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  10. audept

    audept Senior Gretsch-Talker

    Age:
    73
    Dec 1, 2010
    Sydney, Australia
    Welcome to the forum, erlomd !
    Welcome mat 50.jpg
     
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  11. 6124Bassman

    6124Bassman Synchromatic

    585
    May 20, 2018
    Phoenix, Arizona
    Welcome to GT! To add on to what hcsterg said above, the ground switch does absolutely nothing if the 3rd tab is left empty. The switch connects that 3rd tab to the other two tabs (separately, not at the same time of course). So when the capacitor is disconnected from the switch, even if the main leads are still connected, it does absolutely nothing.

    I generally leave mine in just for originality sake, however you just need to make sure that if you don’t bypass the switch you need to disconnect the cap.
     
  12. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    You welcome @erlomd, :)

    It's really not very difficult to do such a bias mod - that is easily removable if you want to return your amp to stock condition, in order to preserve its value.

    Let me prepare the informations about that mod, and I'll be back ! ;)

    A+!
     
  13. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    OK - here it is...

    That said, @erlomd : I made the supposition that you have the basic knowledge in electronics and also the basic tools (a decent DMM and soldering iron, ...) with enough personal confidence on your skills, otherwise it's better to refer to a qualified tech friend, considering the value of the amp, OK ? :);)

    Schematic of the 6G6-B bias mod :

    [​IMG]

    Comparative layout :

    [​IMG]

    Disposition in chassis :

    [​IMG]

    Now about biasing the tubes (Tung-Sol 5881 duet) at idle, the power transformer, etc...

    Setup of the output stage :

    -Vg = -47.5VDC
    Vp = 440VDC
    Vs = 439VDC
    Ik par tube = 35mADC


    This offers the following performances :

    [​IMG]

    You notice (above) that the HV capacitors are new (2x100µ/350V, 4x22µ/500V).

    Another interesting mod for lowering the hum :cool: :

    [​IMG]

    The 2x100R resistors accross the pilot light are indeed removed and replaced by the ones above. The center GND point is taken on one of the 1st preamp socket screws

    Hope this helps ! :)

    A+!
     
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  14. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    WOW! Thank you so much for that info! :)
    I’m definitely not an electrician, let alone a tech, but I do know some soldering basics, I’ve wired up plenty of circuit boards in the past and I have done the capacitor update and 3 prong wire update on this amp.

    I just usually need to know what needs to be changed, the part numbers involved and it’s downhill from there.

    I will definitely be upgrading those diodes, resistors and the Transformer. As well as that last mod! hopefully that will take care of it.
    By the way, are those 150 ohm resistors 1w?

    I already have upgraded my capacitors before since all the original cardboard ones where leaky. And I also installed new Russian 5881 tubes and 12ax7 pre-amp tubes. IMG_2610.jpg IMG_7304.JPG IMG_7099.JPG IMG_6934.jpeg
     
    Last edited: Apr 13, 2021
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  15. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    Fine @erlomd ! :)

    Reading and Seeing all this, no issue for you to work on your amp, then... :cool:

    Your 6G6-B is an older one that the one I had for restoration, since the input filter cap are 2x20µ/600V in // instread of 2x100µ/350V in series, balanced by 2x220K.

    Do you really need to change the power transformer ? I don't think so, if it works as expected. Remember that the reason why I did it was because the amp was 117V and needed an external stepdown Xfo to accommodate the French 230/240V.

    For the heater balance resistors, I used 1/2W, but if you have 1W, that's even better... The goal is to balance the heater line "at the feets" of the 1st preamp tube, where the sensivity to hum is maximum.

    A+!
     
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  16. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    thanks again! Will do that!
     
  17. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    Another interesting mod for lowering the hum :cool: :

    [​IMG]

    The 2x100R resistors accross the pilot light are indeed removed and replaced by the ones above. The center GND point is taken on one of the 1st preamp socket screws

    Hope this helps ! :)

    A+![/QUOTE]






    I just got the resistors to do this today but which prong do I solder it to?

    here's what mine looks like:
     
  18. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    @hcsterg

    here it is on mine. where do you suggest I solder the resistor poles?

    Screen Shot 2021-04-18 at 6.30.59 PM.png
     
  19. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    France
    @erlomd : the concerned pins are connected to the green wires (heater line).

    Here's the pinout for the 12AX7 tube :

    [​IMG]
    The pins 4+5 and 9 are concerned.
    4+5 are one end of the heater line (4+5 means soldered together), 9 is the other end.
    The 2 sections filaments are // in order to be heat on 6.3V (hence 4+5)
    So you connect one R at 4+5, the other R at 9
    You return the two R to GND on a lug screwed on one of the bolts of the socket.

    Here is a picture from one of my DIY amps. You can see here the 2 100R 1W green resistors going to pin 9 (at right) and pins 4+5 (soldered together) and finally both going to GND lug (center back) :

    upload_2021-4-19_0-58-31.png

    Hope this helps - let me know if you're still in trouble... ;)

    A+!
     
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  20. erlomd

    erlomd Electromatic

    11
    Apr 12, 2021
    Miami, FL
    Thank You!!! I'm on it!
     
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