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    No variac... use a light bulb


    j beede

    Posts : 350
    Join date : 2011-02-07
    Location : California

    No variac... use a light bulb

    Post by j beede on Thu Nov 07, 2013 12:59 am

    I am not sure who it was on this forum... but thank you for suggesting the series incandescent light bulb in the absence of a variac.

    Last edited by j beede on Thu Nov 07, 2013 1:23 pm; edited 1 time in total


    Posts : 1337
    Join date : 2013-04-01
    Location : Michigan USA

    Re: No variac... use a light bulb

    Post by sKiZo on Thu Nov 07, 2013 11:54 am

    Dim bulb tester is what you're talking about, right?

    That doesn't limit or reduce current ... it just gives another path for it to go if something goes horribly wrong. If the bulb lights, kill the power immediately or there's a good chance you're close to letting some of the magic smoke out of the amp. Track down the problem and try it again. Good to have an assortment of bulbs on hand too ... start high, then work your way down to maybe 25 watt before you call it a pass.

    Variac is still a good idea, as that DOES bring the voltage up slow. Start around 80 just to tickle the circuits and make sure you've got good connectivity, then raise it in 10vac increments to operating voltage over the course of an hour or two. Lets the electrolytics reform properly as those can go weird over prolonged storage periods. In a lot of cases, a variac is a good thing to leave in line with the amp if the house current is normally high. Mine runs around 122+vac consistently, and these tubers prefer to see something around 117vac. Only concern there is it does take a while for a variac to stabilize, so you'd more or less have to leave it on all the time or plan waiting an hour anyway before turning on the amp. I can handle waiting five minutes for the tubes, but an hour is a bit of a stretch ... an ti ci pa shuunnnnnn! tongue 

    Good you got it going though ... these old classics deserve a second chance! Just keep in mine you should at the very least be planning to replace any and all electrolytics AND any carbon resistors ... the clock is ticking on those if they're original.


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    Post by deepee99 on Fri Nov 08, 2013 12:55 pm

    Is there a sensible alternative to the $100 Chinese-made variacs on Amazon? I got a whole month out of one before it developed a ground fault and its replacement lasted about half that long. Would love to run my tubes at 110 volts 'stead of the 120 + or - we get out of the wall here, but need something I can count on. A Fuhrman box is great at overvolt protection but how can you dial things down a bit and leave running (safely) 24/7?


    Posts : 47
    Join date : 2012-04-15

    Re: No variac... use a light bulb

    Post by turbotoy on Fri Nov 08, 2013 1:27 pm

    I've been using a Superior Electric Powerstat for well over a year now. I had to wait for a while, but I found one on eBay for around $45 shipped that just needed a new cord but otherwise cleaned up like new. Made in the USA and I'm sure much better built than any new cheap imported model.


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    Join date : 2012-11-30
    Location : SF Bay CA

    Re: No variac... use a light bulb

    Post by arledgsc on Sat Nov 09, 2013 12:51 pm

    Yes, the light bulb is a poor man's current limiter.  Built several Heathkits that employed this method for initial start up.  The 100W bulb is connected temporarily in series with the AC line.  If the bulb glows brightly then you have a probable short circuit.   No glow and an open circuit.  You wouldn't want to operate normally with the bulb in place but just insurance for bad wiring or component.  Works for old school power supplies but would not try it with a modern switching supply.


    Posts : 1337
    Join date : 2013-04-01
    Location : Michigan USA

    Re: No variac... use a light bulb

    Post by sKiZo on Sat Nov 09, 2013 1:34 pm

    My dim bulb tester is prettier than your dim bulb tester!

    Thinking about building a custom case for it ... tongue 

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    Re: No variac... use a light bulb

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