Best Grounding Scheme Method Aluminum Amp Chassis?

Discussion in 'Building the Classics' started by RickyLee, May 30, 2016.

  1. RickyLee

    RickyLee Well-Known Member VIP Member

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    I have an aluminum chassis which is the only one I have that will fit in this slightly smaller JMP style headbox I want to use. Figured I might as well do something with it.

    :D

    I am thinking I should try some type of star grounding scheme or maybe even an above ground reference scheme like you see in the JCM900 SL-X.

    This chassis has no welded studs to go off of anywhere, as in for the wall plug Earth Ground, so bummer there.
     
  2. jack daniels

    jack daniels Well-Known Member

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    Ask AMS, he has all the answers...
     
  3. RickyLee

    RickyLee Well-Known Member VIP Member

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    I was just reading that it is possible to bond solder to aluminum. I have an old high wattage iron to try and the trick besides heat is to scratch the surface of the aluminum with the solder tip while soldering/heating to remove the oxide film from the aluminum. If you sand or scratch the aluminum first, it will just come back instantly as it has air contact.

    I will try a combination of a stud with a wire all bolted to the chassis as well as trying to bond it with solder. That will be my power cord Earth Ground. Then will try looking at the SL-X above Ground schem and star Ground everything to that point with an aprox 800 ohms above Ground. I am going to look at the SL-X schem here in a bit . . . .

    ADD: Crap, I am even thinking might as well go for DC heaters while I am at it. This build will probably go in the high gain direction. At least 4 gain stages if not 5 or the 5th can be an added Lead stage.

    I have a set of 30W transformers, so this could get very interesting for sure.
     
  4. neikeel

    neikeel Well-Known Member

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    There is this:[​IMG]

    Or this:[​IMG]
    Both work ;-)
     
  5. RickyLee

    RickyLee Well-Known Member VIP Member

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    @neikeel: Thanks for the schems. That lower 1959 looks very close to what I did on my PlexiRod build. And it is a very quiet amp with lots of gain on tap.

    The top one looks like an aluminum chassis. Is that what they actually are, aluminum chassis schemes? Would that 1959 be what the first Marshall aluminum chassis did or is that an upgrade to quiet those old aluminum chassis Marshalls?
     
  6. RickyLee

    RickyLee Well-Known Member VIP Member

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  7. john l

    john l Well-Known Member

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    I too like the Larry method, its much quieter then the buss on the back of the pots and interestingly enough it seamed to make little tricks like DC elevating heaters more effective. My 4 stage monster is set up this way and its disturbingly quiet for being such a chugger.
     
  8. danfrank

    danfrank Well-Known Member

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    Aluminum oxide DOES NOT come back instantly; especially where you live where it's as dry as a bone. It will happen rather quickly in humid places like the gulf coast or the east coast, but not where you are. Just use a steel screw and a couple of star washers, you know, the kind that digs into the chassis when you tighten down on the screw. Unless you're in the habit of pouring beer or soft drinks over your amp regularly, you won't have a problem.
    As for soldering onto aluminum, it can be done with "acid flux" like ruby fluid, but acid flux is generally a no-no with electronics. Acid flux WILL dissolve any aluminum oxide that may have formed at your connection. It will also disolve any copper, so this is why this is done before the amp building starts. If you plan on going this route, drill all holes that your chassis needs, then solder the studs onto the aluminum chassis using acid flux. After this wash the hell out of your chassis with a solution of baking soda and water and use a stiff nylon brush; this will neutralize the flux residue. After that, wash thoroughly with plenty of water and dry.

    The key to good grounding and a quiet amp is to have each stage have it's own ground path to it's respective filter cap. What you don't want is for the ground returns of the different stages to criss cross each other. This is an invitation for ground loops which is what the ground "hum" noise typically is. Using the chassis is a TERRIBLE way to implement a ground system in an amp. Early amps were made this way because the manufacturers didn't know any better.
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Thanks for the great info.

    Using the scheme you mentioned of tying each gain stage Ground to its respective filter cap Ground is what I did on my PlexiRod. Very quiet amp indeed as it turned out.

    But on this aluminum chassis, would I not be better using a star Ground layout. BUT, run each stages Ground direct shortest route to its respective filter cap Ground. Then try to minimize Ground runs to the star point, by consolidating wires and avoiding criss crossing of course?

    My method above would probably have to use filter caps mounted on the turret board vs the chassis mounted filter cans.
     
  10. danfrank

    danfrank Well-Known Member

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    Hi,
    Yes, star grounding is good also. What you want to avoid, for example, is attaching the grounds from the 1st 12AX7 tube to the 1st filter cap ground closest to the PT.
    One more thing I forgot to mention which is important is to attach the chassis ground to circuit ground at only one place. This helps a lot in avoiding ground loops. Some people like to tie the 2 grounds together at the input to the amp, but I find this way to be finicky, sometimes it works good for me, other times, not so much. What I like to do is attach both grounds at the 1st filter cap coming from the rectifier; this becomes my "star" point and I attach a lug to one of the screws that holds this cap to the chassis which then goes to the (-) lead of that capacitor.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    You actually have 3 grounds to the chassis:
    1. Safety Ground (from power lead)
    2. Power supply grounds
    3. Audio circuit grounds

    PART 1: Safety Ground Wire
    The safety ground connection to the chassis (from power receptacle) is attached w/ half steel half aluminum stud. (one method)
    Aluminum side of the stud is welded to chassis or bolted using aluminum hardware (nuts and washers).
    Sure, welding is always better because oxygen cannot get between the 2 aluminum parts.
    In aluminum to aluminum hardware contact, grease such as NO-OX-ID prevents oxide between aluminum parts from interrupting the contact.

    The steel side of the stud w/ steel hardware is for connecting your copper saftey ground wire usually w/ a ring terminal. The ground wire is crimped and soldered to the ring. The steel hardware and your ring should have similar plating to prevent oxide from forming between the parts, thereby losing contact.
    After you get your ring tightened down to the steel side of the stud w/ steel hardware, you can also solder the steel / plated parts together. (actually the best way is to solder steel / copper / plated parts together)

    No, you cannot solder copper wire to aluminum...in case you were wondering about this.

    Using that method, you are not dependent on steel or copper (plated parts) screwed or bolted directly to aluminum. This is the worst possible method.
    A. Dissimilar metals contact causes oxide between the parts to form.
    B. The oxide is a perfect insulator.
    C. This is the main concern - that dissimilar metals will lose electrical conductivity, and there will be no ground to the chassis.
    D. No matter how tight the hardware is, the oxide can (for sure) still form between the dissimilar metals.

    NO-OX-ID grease: It's actually intended for aluminum to aluminum parts contact only.

    HOW COME it has to be like this?

    Because people DIED! This is why the electrical grounding rules (for aluminum) were created.

    Yes it is a law.

    E. Electricians / Manufacturers are not allowed to ground aluminum using dissimilar metals that are screwed to each-other.

    F. You cannot use the aluminum chassis as a Current path. This is illegal.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Actually, it's NEC that has the answers. (National Electrical Code)

    You probably should go earn yourself an NEC certificate.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    Star grounding is the noisiest method.
    Because power supply noise is introduced into the audio ground.

    *because the noise is AC, and AC travels both directions. It's not a one way path to the chassis.

    The quietest method: Audio and power supply ground is separated, the resistance of the chassis is in between the two.
    (or other methods)

    *In many newer amps, you will see a resistor between the power supply ground and the preamp audio ground.

    * In older amps, the chassis acts as this resistor. The resistance of the chassis between power supply and audio ground might be 10 ohms or more....and this is how it was done in the old days.

    The power supply is grounded on the PT side of the chassis.
    The audio is grounded on the opposite side of the chassis.
     
    Last edited: Oct 5, 2016
  14. ampmadscientist

    ampmadscientist Well-Known Member

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    PART TWO:
    The aluminum chassis cannot be used as a current path. (it's a safety code violation)
    Which basically implies that filter cap grounds + other power supply grounds and power tube cathodes are hard wired back to the center tap of the power transformer...
    instead of using the aluminum chassis as the current path.

    And now you start to understand why Marshall, Boogie, Silvertone (actually Danelectro), etc...stopped using aluminum and changed over to a steel chassis.
     
  15. Thiez

    Thiez Well-Known Member

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    My marshall build has an 3mm aluminium chassis. I basicly used the 'larry grounding' but wired it as some kind of buss instead of using the chassis.
     
  16. ampmadscientist

    ampmadscientist Well-Known Member

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    As long as the aluminum is not a current path...
    As long as there are no dissimilar metals used to attach the ground to aluminum...
    Then you have done a good job.

    You remind us that it's all possible to do it the right way. I mean, it's more effort but still possible.
     
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  17. GIBSON67

    GIBSON67 Well-Known Member

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    Good thread, thanks!
     
  18. coldengray

    coldengray Active Member VIP Member

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    I have done the vintage ground bus soldered to the back of the pots, and I have done Larry. When comparing the two directly using a Metro Headmaster through the same cabinet I heard no difference in the noise floor. YMMV.
     
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Does your avatar provoke beastiality?
    According to the internet, yes.
    [​IMG]


    That is funny right there!


    [​IMG]
     
    Last edited: Oct 8, 2016
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  20. ampmadscientist

    ampmadscientist Well-Known Member

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    When you have the bias too cold, it works like a noise gate.
    It's not a fair contest unless the bias eliminates crossover distortion before clipping.
    Then, you will really hear what noise is there, instead of masking it by shutting the output off with cold bias.

    With a scope, you can really see the differences in noise levels.
    But w/ aluminum chassis, the safety issues change with age....it's not just the noise you need to worry about.

    READ THIS
    It's not just a noise floor issue. It's a safety issue.
    Learn this: aluminum chassis needs special grounding precautions.
     
    Last edited: Oct 10, 2016

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