Oversize Caps - Inrush Limiting an option

Discussion in 'Shock Brother's DIY Amps' started by SerpentRuss, May 11, 2021.

  1. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    I have a bunch of 820 µF, 400-volt caps. Other than the obvious downfall of a very large physical size, the inrush they'd generate when they're first energized would be pretty brutal. Would it be possible to use a device like I linked below to limit current?

    https://www.mouser.com/datasheet/2/600/SL1050001-276043.pdf


    Edit - Forgot to state this would go on the primary side of the PT and the cost is less than $1
     
    Last edited: May 11, 2021
  2. wabashslim

    wabashslim Friend of Leo's

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    Sure...it'll give your amp repairman a good story to share with his tech buddies while they're sitting around drinking and ranting about owner "innovations". Maybe he'll even post it here!
     
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  3. zeppelinofled69

    zeppelinofled69 Tele-Meister

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    I mean... I have seen people use them and even reccomend using them on the 6G15 circuit, as there is no real warm up time before the B+ gets to the tubes.

    I think the main thing with these is just making sure the voltage and current ratings are correct, however I can't speak to using such large caps in the power supply. It has always been my understanding that the in-rush current with huge caps is mostly an issue with tube rectifiers, but I have never seen anyone try to use such large caps in a tube amp for guitar before.

    Soldano and Hiwatt both used upwards of 120uF on the first filtering stage, but I don't think I have seen anything larger than that... and those are both solid state rectified circuits.

    EDIT: Your point about these going on the Primary... I have always seen them on the secondary side of the PT, since you would not want to limit current to the Heaters while they are trying to warm up.
     
  4. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    Thanks for responding. I would have never even considered using them on the secondary side, but they are rated to 480 volts.

    I'm just in the cost-saving mode where I hate to do nothing with these caps which I got for free. I could series them to reduce the capacity, but they're so darn big. I'm definitely talking about SS rectification. Maybe the inductance of the transformer would be enough to limit the current when the switch is thrown. I can build a test supply with a dummy load and see if a 1 amp fuse holds.
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I have them in my hifi amp. On the secondary. Unfortunately I'm no expert on them and I haven't measured they working yet.

    This is what I found in my preparation for that build:

    "The thermistors make a big difference. The power trafo does not clang at turn on and B+ rise is
    delayed a bit, which reduces the chances of cathode stripping.
    You can put the NTC devices on either the primary or secondary. The secondary connection is
    particularly useful when B+ and heater windings share the primary. The shared primary is not a
    concern in "El Cheapo". However, a proven track record is. Jim McShane uses a CL120 on each
    side of the rectifier winding in H/K Cit. 2s, with CONSIDERABLE success. Follow the leader!
    Jim: Eli's explanation of where I use the limiters is correct. I DO NOT want to delay heater current
    or (worse by far) bias circuit. So I use the limiters in the secondary supply between the power
    tranny and the voltage doubler. It works VERY well!"
     
  6. dougsta

    dougsta Tele-Meister

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    While putting a build spec together for a BF '64 Vibroverb Custom Re-Issue hand wired build, I noticed Fender used this one CL60-11




    Screenshot 2021-05-11 at 21.25.08.png
     
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  7. NTC

    NTC Tele-Holic

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    What are you going to do with caps on tge primary side? Have DC go into the transformer? Then it becomes a really low value resistor with no "transformation. "
     
  8. SerpentRuss

    SerpentRuss Tele-Meister Ad Free Member

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    Using the limiter because of the cap values, they're not used in direct conjunction with the caps. It would go, on the ac feed to the primary or on the PT secondary before the bridge.

    Edit, Sorry, I see you're asking about the diagram above. That's probably a noise shunt.
     
  9. AlfaNovember

    AlfaNovember TDPRI Member

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    They're popular with the DIY HiFi crowd. I've used NTC thermistors in several projects over the years, on the B+ secondaries where the goal is to give the heaters a head start before full HV hits the plates. However, I don't know about suitability for the inrush current for such large caps; I'm sure the amperage could be calculated, but I've always been a "solder it and see" cowboy.

    With regards to voltage ratings of resistive elements, I always took the rating to mean 'across the device' - and the relatively few ohms at play here aren't going to be much of a voltage drop. I just now measured one from my junkbox at 10 ohms cold.
     
  10. NSB_Chris

    NSB_Chris Tele-Holic

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    I bought them for one build and purchases several at the same time. Now I just put one in on the primary of every build. NTC 10ohm 5 amp.
     
  11. NTC

    NTC Tele-Holic

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    Actually, I misunderstood what you wrote. I thought you were putting the cap on the primary side, not a current limiter. Doh.
     
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