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For the techies...ground switch

Discussion in 'Amps and Cabs [BG]' started by samurai_24, Jul 14, 2005.

  1. Ok guys, for starters I have an old Fender PA100 amp. I read somewhere a while ago that it is safer to completely bypass the ground switch and switch to a 3-wire plug. However, this amp already has a 3-prong plug.

    Here's the schematic:

    When I opened this amp up, the grounding switch has already been disconnected, but the ground pin on the power cord itself has been broken off.

    My question: should I re-hook up the ground switch, or leave it out of the picture? I already plan on replacing the power cord.
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    First fix the cord. As to the ground switch what it actually does is to send the A/C 'hot' through a bypass capacitor to the chassis ground for noise filtering. The problem is that which of the A/C leads is actually hot depends on whether the A/C supply being plugged into was properly wired. The switch chooses which of the A/C leads gets the filtering. This was de riguer in 1972, not any more. Bag the switch. Instead solder three 250v .047mfd capacitors across all the A/C leads (hot to neutral, hot to ground, neutral to ground) so that no matter how the outlet you plug into is wired there will be noise filtering in place. While you're at it parallel onto the caps three MOV surge suppressors.
  3. Le Basseur

    Le Basseur

    Mar 26, 2002
    If your actual (3-wired) power cord looks damaged or you need a longer one,replace it,but remember the wires' color code (the grounding wire should be green-yellow if you use a standardised mains cable).
    About the ground pin,yes,the AC ground MUST be hooked up on the amp's chassis.The most common grounding point is one of the power transformer's bolts.Just make sure you have a strong mechanical contact (or use a big soldering iron for a massive soldering point) and you're OK.
  4. Thanks to both of your for your responses...

    Bill- I plan on installing an IEC receptacle... like this:
    Can I just solder those caps directly on the back of it? Also, what exactly do you mean by "parallel onto the caps"? I'm sorry I'm not completely in the know about electronics. (my brother is an engineer, I'll be having him do the work)

    Le Basseur - are you saying that the green wire should be directly connected to the chassis?

    Thanks guys.
  5. Le Basseur

    Le Basseur

    Mar 26, 2002
    Not trying to argue...but are you sure this was the right purpose of that cap originally?
    I thought it creates a "virtual ground" and I'd never imagine it has something to do with noise filtering.
    And,one more question,if you say that that cap performed the noise filtering of the hot AC leg,why it was actual 'till 1972 but is obsolete today?What changed? (excuse my ignorance,I don't live in the US).
  6. Le Basseur

    Le Basseur

    Mar 26, 2002
    Yes,see the pic
  7. So I can indeed completely bypass the grounding switch. Is noise filtering the only benefit of installing caps? I can understand the MOVs as they will basically provide built in surge-protection right?
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    With the original Fender chassis with a two wire cord that's exactly what you had, the ground switch alternating between the A/C legs as you never knew which might be the neutral. The quiter position was the right one, discovered by trial and error, especially if you kissed a mic and got a 110v smack in return.

    With a three wire system the chassis is always grounded and the switch connects the bypass cap from ground to hot, with hot again found via trial and error, to filter A/C line noise. But this configuration won't do anything to shunt off noise on the neutral line. Caps across all three legs will shunt noise sources of all permutations, including a disconnected ground lug at the recepticle.
    Yes, the caps connect each of those lugs, as do the MOVs. Usually the ground lug is bolted to the chassis quite close to where it enters, though some amps use what's referred to a 'star ground' master grounding point, something you won't find on a vintage Fender. If you have room you can get an IEC with a built-in twin T filter that's even more effective than caps alone; make sure if you get one it's rated at least 5 amperes.

    Yes. You end up with what's typically found in filtered surge protected power strips of the under $100 variety.
  9. Thanks for the info guys. Much appreciation.
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The 3 wire grounded plug is the way to go... and the capacitors are not a bad idea.

    BUT, if you decide to use them, I would not use the 0.047 uF. That is a bit large, with more leakage current than usually allowed these days.

    Also, you should use a part which is intended and rated for use on the AC mains. A plain 250V part isn't good enough.

    Approved parts for across-the-line and line-to-ground usage are easily available.

    What you want are ones of at least classification "Y2/X1". I would suggest a value of 0.0022uF or 0.0033 uF. The parts should carry at least UL and CSA recognition, possibly VDE also.

    Suitable parts are available from Digi-Key electronics ( www.digikey.com )

    You should have your brother the engineer (or other qualified individual) check the part numbers and specs out before you obtain and install them.

    As always, if you are in doubt about your skills (or your brother's, I know about engineers...), have a technician do the work. Line voltage is capable of killing you.

    MOVs IMO tend to cause as much trouble as they cure....or more.
    If you insist on using them, make sure you get ones rated well above the line voltage. A lot of trouble has been cauised by people using under-rated (like 130VAC) MOVs on the US power. An MOV slowly degrades as it gets "hit" by transients. Eventually if it degrades below line voltage, it turns "on" all the time and is a short on the line.

    BTW, yes, the ground switch was to create a "virtual ground", cancelling out leakage current. That reduced hum and stopped the shokcks.
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    +1 on Jerrold's advice about the caps; in addition to the leakage, caps that are too large in value may give you a bit of a bite if you touch the prongs after unplugging the amp. I believe there is a UL and CSA (safety agencies) limit on the capacitance allowed across the AC line on electrical devices.

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