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    Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

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    weikuang88

    Posts : 2
    Join date : 2014-03-23

    Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by weikuang88 on Tue Apr 15, 2014 10:35 am

    HI,
    I live in Taiwan. The AC voltage is about 110-115V in Taiwan. Recently I tried to upgrade to VTA driver board, and change the quad cap (80/40/30/20 uf) from Dynakitparts. A 2200ohm resistor should be connected from the lug of 20mfd to the lug of 30mfd. Because the voltage in Taiwan is lower than in US. Should I have to change to around 1200 ohm resistor ? Question Question 

    tubes4hifi
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    Join date : 2008-11-30

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by tubes4hifi on Tue Apr 15, 2014 10:46 am

    the important thing here is that you want around 380vdc on the driver board, and usually 2.2K is about the right amount to get that.
    Try it first, then adjust if necessary. You will need to have all tubes installed and biased before making the voltage check.

    weikuang88

    Posts : 2
    Join date : 2014-03-23

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by weikuang88 on Tue Apr 15, 2014 10:57 am

    tubes4hifi wrote:the important thing here is that you want around 380vdc on the driver board, and usually 2.2K is about the right amount to get that.
    Try it first, then adjust if necessary.   You will need to have all tubes installed and biased before making the voltage check.

    Thanks!

    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by sKiZo on Tue Apr 15, 2014 3:35 pm

    Assuming your B+ is in tolerance, I'd think you're probably better off with the lower line voltage. My line voltage averages around 124VAC, which is not what a tube amp likes to see. I added a DIY bucker to my system a while back that drops it down to around 117VAC on average, and my ST120  "breathes" much better now. Switch the bucker out, and there's a definite difference - it sounds a bit constrained, or pinched, without it. Here's a pic showing a GOOD day here, comparing the wall out and amp in power ...



    I used a 6.3v 6a transformer on mine. In your case, you might want to try something in the 4v range to BOOST the power, rather than cut it. All depends on how you wire it up. Before doing that, DO get an average on line power - a Kill-A-Watt meter is real handy for that. Worse case scenario - with 117vac out of the wall, a bucker like mine would be feeding 117-124vac to the amp, but with the smaller 4v transformer, your amp would see 114-119vac.

    One thing you probably DON'T want to do is use a solid state rectifier that includes a thermister for slow starts. Those never really go fully open and you'll see at least a 1vac drop from that. Browning out can be just as hard on an amp as over voltage situations. Shouldn't be a problem if your power averages on the high side of the range you mentioned, but worth noting if you're more around the low side on average.

    hifitracer

    Posts : 2
    Join date : 2014-05-05

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by hifitracer on Mon May 05, 2014 9:12 am

    Hi folks, sKiZo, I would like to learn how to DIY the recommended voltage drop bucker, grateful if brothers could show the wiring or schematic, thanks.

    Captain Coconut

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    Age : 64
    Location : Great White North

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by Captain Coconut on Mon May 05, 2014 1:05 pm



    OR


    sKiZo

    Posts : 1285
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Shall I change the 2200 ohm resistor on the quad cap (VTA drive board),

    Post by sKiZo on Mon May 05, 2014 9:15 pm

    Yup ... Basically, one of your "hot" AC wires is direct linked to the bucker's outlet AND one of the transformer's high side leads. The other "hot" AC wire AND the other high side of the transformer AND one low side wire are joined, and the other low side wire goes to the bucker's outlet. The ground passes straight through to the outlet's grounding terminal.

    Here's mine on the inside ... wiring is simple enough ...



    Most any power strip works well - having the power switch and circuit breaker built right in is real handy.

    Of course, you'll want to test everything with a meter before hooking it up live, and you'll want to use a quality transformer that can handle the peak load of whatever you're bucking.

    I believe to BOOST current, you just switch the low side leads, but don't quote me ... never tried it myself.

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