1960's Harmony H400A

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by korby, Dec 26, 2008.

  1. korby

    korby Supporting Member

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    I want to put a three prong cord on this but read somewhere that if you ground this it will short out . What do you think ?
    This is what I read .

    "Ooops! I just looked at that schematic. It's set up almost like an AC/DC amp, with one conductor of the mains line hooked to the chassis. You CAN'T ground that. You'd get an intense blue flash and pop when you plugged it in, then your circuit breaker would trip and your lights would go out. The only way to run that completely without risk of shock is to plug it into an isolation transformer. However, you can make it a lot safer by replacing the cord with a polarized two-prong one arranged so that the neutral wire of the socket is always connected to the chassis. As long as somebody didn't wire a socket backwards, you're OK with that.


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  2. jawjatek

    jawjatek Member

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  3. korby

    korby Supporting Member

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  4. jawjatek

    jawjatek Member

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    Electrocution. Duh!

    Sorry, look at the schemo - see the lower AC plug blade goes thru switch "SW" and straight to the earth symbol on the guitar inputs? Plug in a guitar and the strings are connected to the same blade. If the plug is rotated in the socket, or is wired incorrectly, that same side of the plug (and your guitar) gets connected to the hot AC wire. If you touch a ground while holding the guitar, like a PA mic, other piece of grounded equipment, water pipe, building metal, etc, etc., you will get the ****, and possibly the LIFE, shocked out of you.

    If you want to use this amp get an isolation transformer. That is what they are for. A harp-player buddy bought an amp like yours on Ebay (Danelectro Cadet), but he got a isotap on Ebay like the BK for around $60. Cheap insurance, IMO, and worth every penny.
     
  5. korby

    korby Supporting Member

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    Ok I got ya' . My cheap amp is getting less cheap . Will a Ebtech Humx fix the shock problem or just the hum ? I paid $150 for the amp I'm looking for the cheapest way to make it safe . Or should I sell it and let it be someone elses problem ? It does sound great . Has a Stones vibe . I'll check into the isotap .
     
  6. hasserl

    hasserl Member

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    Install an isolation transformer, for an example go to tubesandmore.com and do a search for [FONT=Arial, Helvetica, sans-serif][SIZE=-1]P-TN-51X[/SIZE][/FONT] That will provide isolation from the mains. Then you can connect a three wire cord with the safety connected to the chassis, just as you would with any other amp using a power transformer. Mount the trans in the cabinet and connect the hot lead to the switch to the primary side of the trans.
     
  7. Structo

    Structo Member

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    Yes the isolation xfmr is the way to go.

    It has a 1:1 ratio so it doesn't change the voltage, it just provides a barrier to the voltage on the secondary so you don't end up with a hot chassis.

    Install the three prong cord so the black goes to the switch then to fuse, then to the primary of the xfmr.
    The white (neutral) wire so connect to the other primary lead to the xfmr.

    On the secondary side of the xfmr connect the 100R/plate wire of the rectifier and the other wire to wire the switch was before.
    Ground the green wire to a transformer bolt.

    Search for P-T169SS http://www.tubesandmore.com/ $32.72
     
  8. korby

    korby Supporting Member

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    I've got a 12.6 volt AC 3 - amp power transformer sitting around , could I use that ? I have a friend who could do all the connections I'll let him see this thread and let him have at it .
     
  9. Trout

    Trout Member

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    Nope, you need one of the ones listed above.

    115-125 Primary, 115-125 secondary.
     
  10. korby

    korby Supporting Member

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    Ok , it's in my amp tech's hands now and I took a shot of the guts before leaving .
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  11. Jaybird03

    Jaybird03 Member

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    Nice pics. You will be glad you put in the isolation tran . Show us a pic of the finished job when you get it back from your tech.
     
  12. hasserl

    hasserl Member

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    I've got one of these, it's on my "to do" list.
     
  13. korby

    korby Supporting Member

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    I got the pricier one .
    [FONT=Arial, Helvetica, sans-serif]Click on underlined Part # for more information[/FONT] [FONT=Arial, Helvetica, sans-serif]P-T169VS[/FONT]
    [FONT=Arial, Helvetica, sans-serif]2007 Catalog[/FONT]
    [FONT=Arial, Helvetica, sans-serif]page 34 [/FONT]
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    [FONT=Arial, Helvetica, sans-serif]TRANSFORMER - LINE ISOLATION, HAMMOND, 175 VA [/FONT]


    [FONT=Arial, Helvetica, sans-serif]Hammond 169 Series Line Isolation Transformer
    • 175 VA capacity, 1300 secondary current ma.
    • Primary 115 VAC, 60 Hz.
    • Secondary 115 VAC
    • Electrostatic shield between primary & secondary
    • Connection by flexible leads, 8" minimum length
    • Perfect for circuit isolation or "Classic" bias power supply
    • CSA certified (# LR3902)
    [/FONT]
    [FONT=Arial, Helvetica, sans-serif][FONT=Arial, Helvetica, sans-serif]RoHS Compliant[/FONT][/FONT]
    [FONT=Arial, Helvetica, sans-serif]41.70[/FONT]
    [FONT=Arial, Helvetica, sans-serif]$37.53[/FONT]
    [FONT=Arial, Helvetica, sans-serif]Qty [/FONT]
     
  14. JJman

    JJman Member

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    Anyone know anything about those brown couplers? I've got a bunch I pulled from my old Harmony.
     
  15. korby

    korby Supporting Member

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    Gotter' done .
    Caps changed killed the hum , new old tubes made it twice as loud ,installed a fuse ,isolation transformer and grounded cord keeping us from getting killed and replaced puke green ,water stained grill cloth .
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  16. korby

    korby Supporting Member

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    Will using a pedal with a boost damage these old amps ?
     
  17. fisheric

    fisheric Member

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    hell no. it'll bring out rock & roll.

    i have a harmony 440, same as your except it also has tremolo.

    my harmony brand el84 that came with the amp is a Mullard :D

    its really a great amp. its hard to believe so much distortion comes from so little parts
     
  18. fisheric

    fisheric Member

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    Do you know that if you plug into the Microphone input you will get more distortion?

    to be honest about the boost. if you have a humbucker equiped guitar you dont really need a boost of any kind.
     
  19. fisheric

    fisheric Member

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    hmm... i thought mine was a similar circuit but maybe not. i have a 6au6, 12ax7 and el84.

    please disregard my previous statements.

    should i delete them? what is proper etiquette?
     
  20. JACKSONVP1P3R92

    JACKSONVP1P3R92 Member

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    i am looking for someone who could sell me a tube for a 68' harmony H400A guitar amp this please help
     

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