Can Capacitors Question

Discussion in 'Building the Classics' started by Something Else, Sep 25, 2018.

  1. Something Else

    Something Else New Member

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    In a JTM45 type build, can you use the same type can capacitor that is used for the mains and screens for the phase inverter as well? For example, I want my can capacitor brands to match so I am wondering if this will work. I have two of can capacitors rated at 32uF+32uF at 450V. Usually people use a 16uF 450V for the phase inverter and a 32uF 500V for the mains and screens.
     
    Last edited: Sep 26, 2018
  2. Timbo

    Timbo Member

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    Possibly not, As the voltage may exceed your 450v cap can.
    Do you know the voltage after rectification at the first filter cap. (main reservoir cap)
     
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  3. Something Else

    Something Else New Member

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    Thank you Timbo, no I do not know the voltage after rectification at the first filter capacitor. I am merely collecting parts for a build, and I wondered about this question. I apologize if it is a rookie type of question.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    "..can you use the same type can capacitor that is used for the mains and screens for the phase inverter as well?"

    The caps for mains and screens all should be rated 500 - 600 volts. The 450V caps may not be rated high enough. (probably not).

    On the stock JTM 45 B+ voltage is at least 450 volts or higher.
    The caps need to be rated higher than the voltage is.

    If you have 16 uF 450-500V you can use it for the PI.
    If you have 32 uF 450-500 V you can use it for the PI.

    But you cannot use 450V for mains and screens. Those must be 500 V at least or higher.

    Schematic
    https://drtube.com/schematics/marshall/jtm45tr.gif

    jtm45.png
     
    Last edited: Sep 26, 2018
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  5. neikeel

    neikeel Well-Known Member

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    The above is all correct. What you need in your amp will depend on your PT and resultant voltage on the anodes after rectification.
    My originals are around the 450v mark, the RIs are around 400v.
    I would use a dual 32 500v for the can up top for mains and screens and get a dual 16 500v or 450v for PI. You can then experiment with just 16 or 32 for pi or, as I prefer link the mains can with half the internal can for 32+16 =48 for the mains (max safe mains filtering for most GZ34s is <60uF).
     
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  6. Something Else

    Something Else New Member

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    Thank you guys, so since typically the schematics say to use a 16uF 450V for the phase inverter, would it change the tone if I utilized a 32uF+32uF 450V capacitor instead for the phase inverter? Have any of you folks tried this before? I have located a 32uF+32uF 500V for the mains and screens so that is taken care of now. These are both LCR branded ones, and I feel it would be odd to have mismatching brands for my build, and unfortunately, I haven't been able find a LCR 16uF+16uF 450V.
     
  7. neikeel

    neikeel Well-Known Member

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  8. ampmadscientist

    ampmadscientist Well-Known Member

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    The brand won't be a major factor.
    You can use 32, which is 1/2 of the can cap. You can use 32+32 if you want.
    Don't worry about using 16, you don't need to.

    I don't have much faith in "re-forming" capacitors.
    I just buy new ones.
     
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  9. Something Else

    Something Else New Member

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    Thank you ampmadscientist! For what I am going for the brand matters. I was going to buy the regular F&T’s, but I want the look of the old JTM45’s as well at least according to the Bluesbreaker book. Luckily, I have two of each kind, and so I guess if one fails I have a backup. I briefly read the thread on reforming capacitors, in your opinion neikeel, is that absolutely necessary? If these are quality NOS LCR capacitors shouldn’t they be ready once used? I don’t have the equipment to reform them.
     
  10. neikeel

    neikeel Well-Known Member

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    If you have LCRs they will not be new and will have been sitting on the shelf for a while. And yes you will need to reform them.
    You do not need any equipment (except a 2w flameproof 100k resistor and a multimeter) that is it (Are you sure you read that thread?).
    Once you have built the amp on a JTM45 you will notice that the HT switch comes after the rectifier, before the first filter cap - perfect for a simple cap reform.
    Simply clip (or solder across the standby/HT switch terminals (effectively shorting it out with the resistor). Switch the amp on without tubes (obviously you need the rectifier in!) and with your multimeter check the dc volts across the resistor. Initially the volts will be high then they will drop to 20-30v and after a couple of hours or so NOS caps will drop down to <4-5 volts across them. That way you know they are 'good' caps.
    If they do not then they are not good and you should not use them.
    Don' forget to drain the caps through V1 plate resistors to ground before unclipping/soldering the resistor and putting your tubes in to test and bias etc.
    The reason that this is important is because the JTM45 is relatively under filtered and you want the caps to be at top spec.
    The only reason I use a 16/16 inside is because after having had quite a few original/RI and clone JTM45s I like 48mains/32screens, 16PI and 16preamp as the best balance to low noise but nice feel to the amp 'in the hand' when playing. YMMV.
     
  11. Something Else

    Something Else New Member

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    Thank you! I am going to keep these LCRs and think about what I want to do with them, following your recommendations and the website, has me worried about them exploding! The process above makes it sound simple, I need to watch more YouTube videos on this.
     

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