Can I change the filter caps in my JTM45RI myself?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by retro, Jun 22, 2006.

  1. retro

    retro Member

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    I want to start modding my stock JTM45RI and do as much of the work myself.

    I want to start by changing the caps to 32X32uF and 16X16uF and then later add a Metro board....

    The only thing that is preventing me is a lack of experience draining caps.

    However, I recall a thread just before the crash where John Phillips mentioned that on Marshall's, one can drain the caps easily using the power switch.

    I think, if the amp is on, standby is off, and I turn the power switch off then the current will drain through the tubes?

    Can someone advise me if this is the correct procedure?

    Also I intend to get a decent Fluke. If someone would be kind enough to also let me know the proper way to measure DC in the caps that would be much appreciated.

    Thanks. I promise I will keep one hand in my pocket.
     
  2. TheAmpNerd

    TheAmpNerd Member

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    If you are new to amps and the lethal voltages inside them,
    I would NOT rely on power or stand by switches.

    What caps are stock in the reissue?
    What are you replacing them with?
    What are the coupling caps in the amp?
    Are you going to do anything with them?

    What do you want to accomplish with your
    parts changing?

    BEFORE YOU DO ANYTHING

    buy Tom Mitchell's book, How to Service Your Own Tube Amp.
    read it cover to cover first.

    Chapter 11 tools is good for stuff you'll need.
    P117 explains bleeder resistor and tools.

    It is a good primer on what you need
    to purchase. It also has soldering basics,
    flow charts etc.

    You can insert a 1K 10 watt resistor,
    in an aligator clip test lead wire
    the aligator clips are insulated too..

    Clip---wire---1K---wire---Clip

    Splice it in the center solder it
    and heat shrink or insulate it.

    This whole wire is insulated now,
    even the aligator clips.

    Attach one side of it to ground
    and attach the other side to the
    + side of the caps to bleed them down.

    Bleed every cap in the amp. Just to be sure.
    Then check the caps with your meter to verify
    no charge.

    Good luck and keep us informed.
     
  3. John Phillips

    John Phillips Member

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    With a JTM45 it's actually very simple, and you can rely on the tubes. I'm not being 'casual' about this, it's a fact.

    With a JTM45RI the standby switch is upstream of all the filter caps, so switching off the standby and leaving the amp powered up will discharge the caps. (The HT fuse is also upstream of the caps, so pulling this will have the same result.)

    This is not necessarily true for other amps though. If you're at all unsure, there is a more certain method with any tube-rectified amp.

    Pull just the rectifier tube and power the amp up - fully, including the standby switch. (No sound will come out, and you don't need the speakers connected.) Twenty or thirty seconds should be enough, but wait several minutes to be certain, if you're worried. Then power off again and you can work on the amp safely. With the rectifier out, the caps cannot charge up no matter what, and they will discharge via the tubes which are conducting when their filaments are hot.

    *The only tube-rectifier amps this doesn't always apply to are dual tube/SS rec amps such as the Mesa Dual Rectifier series (the Blue Angel in particular, which runs the preamp supply from the SS rec and remains on even with the rectifier tube pulled and the standby off).*


    To measure voltages on caps with a meter, simply clip the negative probe to the chassis (I just poke it through a convenient hole usually) and check the voltage on the cap terminals with the positive probe. It's not unusual to read a few volts on 'discharged' filter caps, they can 'charge up' again slightly due to the chemistry of the electrolyte inside.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    The racket scientist strikes again. : )
     
  5. retro

    retro Member

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    Awesome, thanks much John.

    Thanks for your concern and suggestions, TheAmpNerd.
     

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