Princeton Filter Cap Job

Discussion in 'Amp Tech Center' started by cyborg_stew, Feb 28, 2020.

  1. cyborg_stew

    cyborg_stew TDPRI Member

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    I'm going to replace the original Mallory cap can on my '78, and I have a question before I order a new one.

    There's a 22uF soldered from one of the leads of the can to ground. Is this wired in parallel in order to change the 1st filter cap to 42uF? Or is this added capacitor there to replace the original 20uF? I love how the amp sounds, so I want to keep the values the same as they are now.


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  2. Paul G.

    Paul G. Friend of Leo's

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    When you're disconnected, see if you can measure the capacitance of that lug to the common lug. If you're seeing 20uf there (more or less), this is more filtering. If you don't get a reading, then someone replaced one bad cap in the can.

    You can always hook everything up per the schematic, test your amp and see if you hear a difference. If you think you want the extra filtering, have at it, it shouldn't hurt anything. I put higher value caps in the first stage of filtering on a few of my amps.
     
  3. Paul G.

    Paul G. Friend of Leo's

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    You're going to need a big, high-power soldering iron with a large chisel tip to remove the tabs soldered to the chassis.
     
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  4. schmee

    schmee Poster Extraordinaire Silver Supporter

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    My 40 w will do it with flux. The 75w does fine otherwise.
     
  5. muscmp

    muscmp Tele-Afflicted

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    looks like it replaces the square icon's connection of the cap can. as long as that is what the cap can requires, it should be good. if you want to replace the others from the cap can, try to replace them as close as possible to what the cap can requires. you can use F&t or some other brand name to replace it. you don't necessarily need to replace the cap can with another and it will be cheaper.

    note that the icons on the bottom of the cap can are also listed on the side with the values.

    play music!
     
  6. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Yeah, that's intersting, It appears in paralell so they are doubling up the filter in that node. Not sure you need it but if you like do it. Even Vibrolux and Deluxe amps use 20uf.
     
  7. Whatizitman

    Whatizitman Friend of Leo's

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    That cap was likely added later. It should have bypassed the tab on the can if it replaced it. You might be able to find a multi can with 40uf if you like how it sounds. But keep in mind new filter caps in general are gonna make it sound different, even if same rating. That's just the nature of the beast. I would first try to spec. BTW, it's cheaper to use discreet caps and bypass the old can, if you don't have a nuclear level iron. Plus it would be easier to swap different ratings to try out. Just sayin'.
     
    Last edited: Feb 28, 2020
  8. elpico

    elpico Tele-Holic

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    It's definitely connected in parallel to a cap that's in the can. (There would be no connections to that terminal of the can if it wasn't in circuit anymore). But that doesn't necessarily mean it's 42uF now. If the cap in the can wasn't doing much anymore, or if it had failed open, then adding 22 in parallel to bupkiss still adds up to 22. Since you want to replicate the sound you could do a capacitance measurement of that set of parallel caps and see what it really is, but I'd want to lift the connection to the next cap temporarily to make sure the rest of them don't throw off the measurement.
     
  9. Whatizitman

    Whatizitman Friend of Leo's

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    You may want to take out the death cap and put in a jumper while yer in there. Kinda weird it's still there, considering all the recent work (bias cap/resistor, etc...). You never know what might find in a 40+year old amp. I would start from spec, and eliminate any weirdness.
     
  10. cyborg_stew

    cyborg_stew TDPRI Member

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    I didn't know what that blue blob cap was doing there. lol. It's not connected to anything anymore. The previous owner had the electrolytics replaced and put a trim pot in the bias circuit. So I'm guessing he just disconnected the death cap and left it in there.
     
  11. cyborg_stew

    cyborg_stew TDPRI Member

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    Thanks to all for the replies. I suspected that it was wired in parallel to the original. But since I don't have any tools to measure capacitance, I'm kinda outta luck. All I have is a multimeter, and it doesn't have a capacitance setting.

    Either I'll just get a can with the same values as the original, and solder it up as it is now; or I'll take it to my local tech. I was looking to do it myself to spend less time without amp and to save a little money.
     
  12. Jon Snell

    Jon Snell Tele-Meister

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    As my rule, I always disconnect/remove/replace the old faulty capacitor. I usually remove the insides of the old capacitor and place new, much smaller, capacitors inside the old can. It avoids the faulty one going short circuit and causing issues at a later date and the amp still looks original.
     
  13. cyborg_stew

    cyborg_stew TDPRI Member

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    One more question. The only cans I'm finding are 40/20/20/20 or 20/20/20/20 CE Manufacturing cans rated at 55C or 65C degrees. The original Mallory is rated for 85C. Would it be safe to use a 55C can?
     
  14. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    55C is only 131 Fahrenheit. It will get hotter than that in your amp. It means a shorter life for the caps.

    I think @Jon Snell gave great advice. Stuffing the old can with new caps. I would think radial caps would fit the best and they are usually cheaper. I wouldn't hesitate to put Nichicons in there. YMMV.
     
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  15. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    I’ve installed that ^ CE cap in a PR that has been cranked all the way up 10 hrs a month for 4 years, in 2 hr sessions. Zero problems. Have no fear.
     
  16. slider313

    slider313 Tele-Afflicted

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    That's why the can is hanging below the chassis. It will never see the heat single caps mounted inside the chassis will.
     
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  17. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    True, they won't get as hot as caps inside the chassis but, the inside of my car gets hotter than that in the summer.

    Consider a Princeton black amp cabinet in the sun at the summer gig, or even in a hot club. Rectifier and power tubes in proximity to the cap can. It may even be tilted back keeping more of the heat inside. It is a safe bet it will be hotter than 131 Fahrenheit.

    Will the caps survive 55C or more? I would think so, at least most of them. Some amps have made it over fifty years before having the caps replaced. (Think bell curve). The other side of the bell curve has caps being replacing under warranty.

    Would "I" suggest a cap rated at 55C when one rated at 85C to 105C is available at little extra cost? No.
     
  18. slider313

    slider313 Tele-Afflicted

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    The internal operating temperature is what we're talking about, not external temperature(s). I highly doubt the caps inside of the can will ever see 131 degrees.
     
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  19. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Mojotone sells a JJ 40/20/20/20uF 500v cap can that's perfect for the Princeton Reverb.
     
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  20. Commodore 64

    Commodore 64 Friend of Leo's

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    Last edited: Mar 3, 2020
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