Please help with star ground

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Fifthstone, Oct 6, 2006.

  1. Fifthstone

    Fifthstone Member

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    I'm building a 5F1 and would like to go with a star ground. I've done lots of reading (including the excellent article on Aiken's site) but am still a bit fuzzy. Can those with some experience here please chime in and let me know if the strategy below sounds right?

    1. Star ground point would be a few soldering lugs trapped under a PT mounting bolt.
    2. Ground from fuse -> .05uF cap -> star ground
    3. Green and yellow PT leads -> star ground
    4. First filter cap (16uF) -> star ground
    5. Pin 2 of 6v6 -> star ground
    6. Third filter cap -> star ground

    Should the grounds from the input jack and preamp tube and 1500 resisitor on the right end of the board also go directly to this same star ground?

    Is a shoulder washer and plastic washer inside the chassis enough isolate the input and speaker jacks?

    Thanks!
     
  2. Thames

    Thames Member

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    Star grounding isnt ALWAYS the solution. I now perform multiple star grounds, or at least 2 : one for power section, one for preamp.
     
  3. wilder

    wilder Member

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    I use two as well. One at each end of the chassis.
     
  4. Fifthstone

    Fifthstone Member

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    Interesting. So have you found this dual star ground to give quieter operation?
     
  5. donnyjaguar

    donnyjaguar Member

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    Its a lot easier to do star grounding in a power amplifier than it is in a musical instrument one by virtue of the number of wires you have to solder or otherwise affix to one place. :) This should be the CT of the secondary of the power transformer (or paired anodes in a bridge rectifier - voltage doublers a little different but I think you get the idea).

    The output power amplifier section (typ output tubes & phase invertor, or perhaps driver-phase invertor-output, if so equipped) draws appreciably more current than the balance of the MI amp so I'm of the opinion you should take care with this/these grounds ensuring its routed directly back to the star point of your power supplly section. The pre-amplifier sections are less critical so if you run a ground from the star point to another buss-bar or what have you, and use this for the small signal sections, I tink you'll find that it works perfectly fine. What you are trying to avoid is any voltage drop that would happen if you say grounded the cathode resistor of V1 betwixt the cathodes of the power tubes and the star ground point. I think you can see that there will be a voltage differential (potentially - no pun intended) between what I'll call the system ground of the PT secondary center tap and that of the V1 input section. Although very small in amplitude remember the cathode of V1 is also an input so any noise or variance in voltage here will be amplified - and the input signal from the amplifer itself is also very low. Given that most every guitar amplifier uses class A amplifiers in the small-signal area, the change in current is largely non-existent hence they aren't really going to create noise for themselves.

    Now, I just re-read my posting and it makes perfect sense to me, but if someone wants me to explain it a different way I'll understand. :)

    Cheers!
    DJ
     
  6. teleamp

    teleamp Senior Member

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    2. Ground from fuse -> .05uF cap -> star ground

    Omit that step, if the cap fails, there is a potential shock hazard.

    5. Pin 2 of 6v6 -> star ground

    That was Fenders way of saving wire. Here is my suggestion for the filament wiring; twist the green 6.3v leads and attach them to the pilot lamp assembly, Editted per hasserl's post below: at the pilot lamp assembly connect 1 100 ohm resistor to each connection, connect the other end of the 100 ohm resistors to pin 8 of the power tube, if your transformer has a green/yellow 6.3v center tap, cap it off and do not connect it to anything, the resistors will act as protection for the 6.3 filament winding should you have a catostrophic tube failure. Now take some more twisted wire and cnnnect to the pilot lamp assembly and connect to pins 2 and 7 of the 6v6, then take some more twisted wire and connect to pin 2 and 7 of the 6v6 and run to the 12ax7 connect one of those to pins 4 & 5 and the other one to pin 9.

    Now for the rest of the grounds; at the power transformer, connect the ground from the power cord, the power transformer high voltage center tap, the first two power supply caps, isolate the speaker jack and run a ground wire from it to this point as well, and I also ground the 220K resistor from pin 5 of the 6v6 at this point as well. This is groinding point one.

    The next place I ground is at the input jack (do not isolate, use a star washer between the jack and the chassis, make sure the surface is clean, use steel wool or emery cloth to be certain) at the input jack ground the last power supply filter cap, the cathode bypass resistors/capacitors, the volume pot ground, and the 1meg resistor connected to the tip of the input jack.

    If you follow the above suggestions you should end up with a quiet operating 5F1 circuit.

    If you need the insulating washers for the speaker jack, pm your address to me and I will send you a couple.
     
  7. hasserl

    hasserl Member

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    Do yourself a favor on the 5F1, don't bother with the star ground on that amp. Use the plain ol brass plate method Leo used and you will be fine. That is a low gain, low power amp, build it and ground it the way Leo did and you will not have any noise issues. Get fancy with a star ground and you may just introduce noise into the system. Just do it the old fashioned way. If you have a problem then and you have to get rid of some noise, then you may want to think about doing something different with the grounds. But in my experience that will not be the case. If you want to do something fancy that mght actually help, float the filament circuit ground at the power tube cathode.

    Remember the KISS Principle! After safety this is the most important thing to keep in mind. KISS
     
  8. teleamp

    teleamp Senior Member

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    +1, I totally spaced that, I do it on my amps as well. It does work. I will edit my post.
     

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