Light bulb limiters -- why so much confusion?

Discussion in 'Shock Brother's DIY Amps' started by King Fan, Dec 19, 2019.

  1. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Actually I've made plenty of mistakes. In fact I think it's the norm for me!
    But many people talk of using a limiter to "form the caps" which is where I was coming from. However, I guess I AM lucky, my mistakes haven't resulted in any thing burning up! Usually a fuse blows, something squeals, tubes red plate or it sounds garbly!

    But it does beg the question: If using a limiter at low voltage protects from failure, then how do you know until you test at full power? :rolleyes: "my car doesn't vibrate at 30 mph so my wheels must be balanced fine"
     
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  2. jhundt

    jhundt Doctor of Teleocity

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    oh, I see where you're coming from. But I never worried about forming caps with the thing. I just use it to allow full-power testing without blowing fuses. I think you might not realize that the light-bulb limiter does NOT reduce voltage. You are NOT testing at lower voltages; you are able to test at more-or-less full voltage. The bulb limiter allows full voltage; only when the current is too great does the bulb light up. And the current is usually too great because there is a short to ground somewhere.

    That's why it is so very useful when repairing amps . If the amp is blowing fuses every time you turn it on, it is very difficult to locate the fault. When you use the light-bulb limiter, the fuse will not blow. All the excess current will be used to light the bulb, and you can go ahead testing the different sections of the amp for problems without worrying about blowing up or burning out some critical part.

    It is true that in final testing and use, the limiter will produce strange voltages. You can't accurately set your bias with the amp plugged into the limiter. I have forgotten that a few times and spent an hour or two wondering why the voltage readings are all off. But they are not drastically lower, just different. So you are not testing the amp in a low-voltage situation; you are testing at somewhere near full voltage, it is only the current that is being 're-directed' in the event of a potential failure.
     
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  3. King Fan

    King Fan Friend of Leo's

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    You raise good questions, schmee. I mentioned variac confusion in the OP. Stuff like:
    1. Does a limiter form caps like a variac?
    2. How important is it to form *new* caps?
    3. What other advantages does a variac have? When?
    4. What does a limiter add to a variac on startup?
    As @jhundt says, people sometimes think a limiter will form caps *and* test for all kinds of failures and *and* remove horrid age spots. As he says, it mostly just tests for shorts. But even before we get to using it in short-hunting, short-testing is a nice addition to a logical startup sequence where we test each section of the amp in sequence -- and any step can just possibly reveal a short.
     
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  4. sds1

    sds1 Tele-Afflicted

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    I have never heard this.

    Forming caps is typically done with a variac.

    Although it's true voltage ends up sagging on a limiter under load , it seems like you are conflating the two things.

    The limiter isn't intended to be part of any stress testing, it's for first-time startup to quickly identify and protect from shorts, basically. Also great for when an amp of unknown condition comes in for repair. And as an indicator of current flowing it's super handy.

    People that are confident with their builds, or are doing repeat builds (ie mass production as you mentioned above) have much less of a need for the limiter.
     
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  5. King Fan

    King Fan Friend of Leo's

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    How about limiters in more, shall we say, "enlightened" countries? :D Can you still find incandescent bulbs in your parts? What wattage is appropriate for 220-240V power?
     
  6. muscmp

    muscmp Tele-Afflicted

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    didn't know there was confusion. i have a 150 and a 300 watt incandescent i purchased at walmart. i use the limiter with a variac. confusius says no confusion, ha! interesting reads tho.

    play music!
     
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  7. jhundt

    jhundt Doctor of Teleocity

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    I am in Holland. I bought several 100-watt bulbs before they began to phase them out. I also have a specialty lamp shop on the corner, he assures me that he will be able to provide incandescent lamps in the 100-150 watt range for a long time.

    I have never really understood fully the difference in using various lamp values. For me the bulb-limiter is a very rough-edged tool; it's bright or it's not. I would like to understand the difference, and maybe I will learn more from the responses to this very useful thread.
     
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  8. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I've been using a 75W bulb, and it seems to serve the purpose. It saved me fuses when troubleshooting a SS amp a while back.

    BL2.jpg
     
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  9. Tube Life

    Tube Life TDPRI Member

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    Built this from Robs site. For me it’s piece of mind and just part of the start up process when first firing up an amp, which I’ve done twice now. In the pic is a 150w bulb but also have a couple of 25w and 60w bulbs in the stash.
    I can see it now, NOS 75w GE bulbs in original boxes! :rolleyes:

    D0EE0D00-39BF-47EC-891A-E96FCA4C0805.jpeg 895CA930-7D79-415B-8AB8-0113B8D98E46.jpeg
     
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  10. D'tar

    D'tar Friend of Leo's

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    OOOH! With milspec tungsten filament???? :lol:
     
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  11. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    Now that bench is waay too organized!
     
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  12. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Here is mine: I added the line meter last year. The limiter is plugged into a variac so I can maintain a steady voltage while I'm working.

    IMG_2686aa.JPG
     
  13. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    Andy, I've debated adding another switch to go from limited to wall power!
     
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  14. Lavochkin

    Lavochkin TDPRI Member

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    Here's mine, unused as of yet since I haven't finished the Micro Champ I started. If I ever make another amp, I'll need to either use a large chassis or find a diet that will gift me with drastically skinnier fingers. Anyway, I'm sure that the antique paint job will give just the right mojo! :)

    20191219_192918.jpg
     
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  15. Andy B

    Andy B Friend of Leo's Silver Supporter

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    I built a few of these limiters 5 years ago in my last shop before I retired. I have the one in the picture on my bench at home. You can’t beat the convenience of being able to switch the limiter in and out of circuit. The main switch is dpdt so when it is off both hot and neutral are switched out of the circuit.
     
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  16. shortcircuit

    shortcircuit Tele-Meister

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    I always use one on initial power-up. It has saved my behind twice. Especially important in my quasi-PTP builds.

    As has been mentioned previously, there has been many times where I forgot I was plugged into the limiter and freaked out over the voltages. What's worse was the few times when I tried to set the bias of the output tubes when plugged in, and thinking that the tubes were bad because they won't bias up.
     
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  17. King Fan

    King Fan Friend of Leo's

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    Cool stuff, you guys. Now I want to cross the streams a sec...

    It's not actually related, but I don't own a variac and I don't work with old caps.

    So *is* there a benefit to 'forming' brand new caps? Some folks do...
     
  18. mabinogeon

    mabinogeon TDPRI Member Silver Supporter

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    I have heard yes and I have heard no. I've also heard old caps can be "reformed" with a variac.

    Don't have one myself, so can't say one way or the other. No amp I've ever built or repaired has had a cap failure though.
     
  19. King Fan

    King Fan Friend of Leo's

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    Yeah, I always assumed 'forming' the caps applied mostly or entirely to older caps that had sat unused and got all confused about their mission in life... :) but maybe new caps need some reminding?

    EDIT: I found a 2008 thread on TGP where some respected dudes chime in on variacs, lightbulbs, and 'forming' e-caps old and new. Guess what? The wise dudes are guided in part by their own beliefs and opinions. But I read through at least the first page with some interest.
     
    Last edited: Dec 20, 2019
  20. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I have a variac that the power company "gave" me when I was working there, (there was a storage room of all sorts of stuff, much of it obsolete). I used one when I built a step up transformer pad to check the conductance of the line trucks insulation. Stepping up 120V to 69KV required the line voltage to be precise, so I installed a meter with an etched mark, and a variac to set it!
    Mine was panel mounted at one time, and I have yet to build an enclosure for it, but it does have 2 prong bakelite male and female connectors, and thick brown cloth wire!
     
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