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    Proper use of the variac for tube equipment on startup?

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    eickmewg

    Posts : 41
    Join date : 2014-08-29

    Proper use of the variac for tube equipment on startup?

    Post by eickmewg on Mon Oct 06, 2014 7:40 pm

    I am using a variac to reduce my line voltage down to about 115 volts for my VTA ST-120.   I leave the variac powered on and at the approximate 115-volt adjustment point.  My question: is there any advantage in dropping the voltage down to say 90 volts to power up the tube amp and then bring the voltage back up to 115 volts in several steps over 30-seconds or so?  Or is that just a waste of time and overkill?  If it is useful, I can add this to my startup ritual.  I am currently using a GZ34 rectifier and I don't have the slow-start board.

    Bill

    deepee99

    Posts : 1333
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: Proper use of the variac for tube equipment on startup?

    Post by deepee99 on Mon Oct 06, 2014 8:05 pm

    eickmewg wrote:I am using a variac to reduce my line voltage down to about 115 volts for my VTA ST-120.   I leave the variac powered on and at the approximate 115-volt adjustment point.  My question: is there any advantage in dropping the voltage down to say 90 volts to power up the tube amp and then bring the voltage back up to 115 volts in several steps over 30-seconds or so?  Or is that just a waste of time and overkill?  If it is useful, I can add this to my startup ritual.  I am currently using a GZ34 rectifier and I don't have the slow-start board.

    Bill

    Just a knee-jerk response, but dropping the volts too low will put strain on your heaters and their xformer windings. Unless you're got some major powerline issues, I would recommend 117VAC and hope things don't get above 122 volts or so.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: Proper use of the variac for tube equipment on startup?

    Post by PeterCapo on Mon Oct 06, 2014 8:49 pm

    IMO, not really necessary as long as you have an indirectly heated rectifier like the GZ34. But, if you go to a directly heated rectifier or the Weber copper cap, you might benefit from a thermistor in the PT primary, or install the time delay board. Plus, I wouldn't think you need to drop the primary to 115VAC. I would think that 117VAC or 120VAC out of the variac should be perfectly safe - unless your AC line periodically surges too high or something.

    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Proper use of the variac for tube equipment on startup?

    Post by sKiZo on Tue Oct 07, 2014 12:19 pm

    May be some confusion between using a variac to reduce average high VAC or using one to bring up old gear that's been sitting a while.

    For line voltage reduction, just set it and forget it, but remember that the variac will cut voltage the exact same amount no matter what's coming in. Set to provide 115vac, if the average incoming level drops a couple points, you're now down to brown out conditions that can do damage over time. In my case, I was running an average of 124vac, but that's down to around 120vac after some recent changes by the power company. Good to double check every now and then to see what's happening.

    For bringing up vintage equipment, the 80-90vac setting would be a great place to start, as it's just tickling the components and giving them a chance to wake up. Especially critical for reforming some types of older capacitors. Leave it go a bit at that setting, and bring it up to speed in stages over an hour or so, ready to pull the plug at the first sign of any of the magic smoke attempting to escape.

    PS ... I built a bucker that I used here when the line in measured 124vac consistently. That drops the power supplied by 7vac, which gave me 117vac into the amp. Kewl! With an average of 120vac in, that does put me into brown out territory, so I took the bucker out of line. I just checked with the Kill-A-Watt, and I'm seeing 122vac - if that starts to look consistent, I might have to consider putting the bucker back.

    Or plugging the system into my variac .. jocolor

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