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Replacing the filter cap on a 66 bassman head

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ToneGrail, Jan 9, 2006.

  1. ToneGrail

    ToneGrail Member

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    My fender bassman is developing a loud hum which I am told is the filter cap. Does anyone know the best way to ground out the cap so I don't get shocked? Can I just put a screwdriver across the leads?
     
  2. AL30

    AL30 Member

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    Yes, that will work but I wouldn't recommend it. Here is an article from Schematic Heaven about discharging caps


    http://www.schematicheaven.com/

    AL
     
  3. Chiba

    Chiba Gold Supporting Member

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    A tube-amp tech friend of mine showed me a simpler (and faster) way.

    Make a probe/discharger with a 1 or 2W resister in the 100-200k range. Both ends should have alligator clips on them, or one alligator and one probe end. Solder the resistor to the wires and encase that section in heatshrink or electrical tape. I went one step further and taped one end to a simple wooden chopstick.

    1. Plug a guitar into the amp while it's powered up.
    2. With the power & standby switches still on, pull the plug from the amp (or wall).
    3. Play the guitar til you can't hear any more sound.

    That gets rid of most of the electricity inside. Then you:

    4. Clip one end of your probe/discharger to the chassis.
    5. Touch the other end to each lead of the filter caps.

    Doing it this way you're much less likely to hear any bangs or see any flashes. Playing the guitar while the amp loses power gets most of the voltages down past where they could kill you, but without finishing it off you could still get a decent jolt.

    --chiba
     
  4. Shea

    Shea Member

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    This amp should have resistors in it that start bleeding down the first filter stage the moment you turn off the amp. If you close the standby switch, they'll bleed down all the filters.

    So I would unplug the chassis, remove it from the head, flip it over, take off the cover pan, and just measure the voltages across the filters with VOM. It's probably less than a volt on all of them.

    Marshalls are a different story. Those caps will hold enough of a charge to make a huge spark for a week or longer.

    Shea
     
  5. John Phillips

    John Phillips Member

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    On a '66 Bassman it's extremely simple and you don't need any special procedures or tools. Simply turn off the power switch while leaving the standby on (up) and wait a couple of minutes. All the caps will discharge through the two voltage-divider resistors on the first filter cap stage, which is 'stacked'.

    Important - this is not true for all amps, so unless you know for sure that yours does have a stacked filter cap stage with resistors, don't rely on it. Deluxe Reverbs and 50W Marshalls are two common examples which do not have them.


    (edit - Shea beat me to it! :))
     
  6. fullerplast

    fullerplast Senior Member

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    Just on general principle, I'd avoid removing the doghouse cover on any amp you are not sure is discharged.....there's only about a half an inch between the metal cover and potentially 400V on the cap leads! I'd suggest measuring voltage inside the amp before opening the cover. IOW, treat the amp as though it has 400V present until you are sure it doesn't......
     
  7. bob-i

    bob-i Member

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    The fastest way.... Lick the + side of the cap. When you get back from the hospital it'll be fine.

    I like to use a larger resistor than 150ohms. Discharging slower is easier on the caps, plus you can do it with a 1/2 watt resistor.
     
  8. VintageJon

    VintageJon Member

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    1K to 2K resistor here...

    USE ONE HAND! KEEP OTHER IN ABCK POCKET and you'll lokely survive it.
    (Have taken 700VDC on one hand and I'm still here. Makes you jump about and heart goes a bit wierd, but you'll likely survive it. (IF YOU HAVE OTHER HAND ON CHASSIS YOU WILL LIKELY DIE A HORRIBLE YET QUICK DEATH. ONE HAND METHOD IS MANDATORY!!!)

    -Jon
     
  9. AL30

    AL30 Member

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    700 Volts !!!!!:eek: That was a bit of a tingle I suppose?

    AL
     
  10. ToneGrail

    ToneGrail Member

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    Thanks for all the info guys! I just wanted to be sure that I didn't fry myself like I did a few years back when I neglected to discharge the caps when replacing a fuse holder on my JCM800. That was a lesson I will never forget. The shock threw me halfway across the room. I was ok, but badly shaken.
     
  11. scottosan

    scottosan Supporting Member

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    On that amp, you could also short pin 1 of the 1st gain stage to ground. Then take a lead with insulated aligator clips and keep pin 1 shorted to the chassis. Insure the caps ar drained by using a multimeter on the positive terminal of the filter caps. If this doesn't work, use the larger resistor and drain directly from the positive treminal of the cap and use the lead to keep it shorted to ground.
     
  12. billdurham

    billdurham Member

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    the Series filter caps..the "stacked" scenario started in 64 transitional blonde Bassman amps. I have one of these..its a black faced "blonde"! Fender didn't change the schematic until the Blackface amp..the AA-864 model in 65, but they started making circuit changes during the year in 64. On the Blackface and beyond, the easiest thing to do is just unplug the amp, and turn on both the power and standby switches and this will bleed the filters. True 6G6 blonde bassman do not have the bleeder resistors so bleeding will take a little longer.

    BD
     

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