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Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-70 what is a safe input VAC

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    raudio44


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    Join date : 2014-09-12

    ST-70 what is a safe input VAC Empty ST-70 what is a safe input VAC

    Post by raudio44 Mon Sep 26, 2022 8:37 am

    Hi everyone, could use some idea of what range input VAC (high and low) is acceptable for my ST-70. Looking at a bucking transformer on eBay that was noted in another post here. What should be the lowest VAC that would be, OK? Thanks, and appreciate any responses, R
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    TubesGlo


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    Post by TubesGlo Tue Sep 27, 2022 9:08 am

    My ST-70 is stock but completely overhauled with fresh components and is tube rectified. I built a bucking transformer plugged into a line conditioning surge protector. The transformer knocks voltage down to 117-119VAC. I have a CL90 inrush limiter after the fuse which knocks voltage down to 115- 117 on average. The very lowest I would want to see would be 110. It was recommended to me by Dave Gillespie to aim for 115.

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    raudio44


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    Post by raudio44 Tue Sep 27, 2022 9:31 am

    TubesGlo wrote:My ST-70 is stock but completely overhauled with fresh components and is tube rectified. I built a bucking transformer plugged into a line conditioning surge protector. The transformer knocks voltage down to 117-119VAC. I have a CL90 inrush limiter after the fuse which knocks voltage down to 115- 117 on average. The very lowest I would want to see would be 110. It was recommended to me by Dave Gillespie to aim for 115.
    My 70 is about the same, updated, but pretty much stock. Wouldn't think going down to 110 would be ok but am not a tech so I wouldn't know. Good to know and what I wanted to find out. Thanks for the reply and info, much appreciated.

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    tubes4hifi
    tubes4hifi
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    Post by tubes4hifi Tue Sep 27, 2022 12:10 pm

    any AC input between 110-120vac should be fine, I like to aim for 117vac and use a variac

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    Brap
    Brap


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    ST-70 what is a safe input VAC Empty Safe Input VAC

    Post by Brap Wed Sep 28, 2022 12:00 pm

    Per Bob Latino, 117-118 using an inexpensive variac. I fluctuate wall VAC +/-8 at my plac and a variac works great. $65.00 well spent and can use it on the bench too.
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    raudio44


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    Post by raudio44 Wed Sep 28, 2022 12:29 pm

    Well after spot checking my VAC (haven't done it that much and seems to come in at about 124.5) it was at 126VAC! that did it. Had been thinking about this for a long time and finally bought the one off eBay which has an outlet for a 3% and one for a 7% drop. More than I wanted to spend and not up for building one. For me, at least, seemed like the way to go. My main question is what is too low, but I think under 115VAC. Any thoughts? Thanks everyone, your input is much appreciated!
    Brap
    Brap


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    Post by Brap Thu Sep 29, 2022 8:00 am

    115VAC should be fine. I am able to adjust the output on my variac to dial it in depending on what the wall is giving me. Have a Furman in my rack of gear which provides a readout of the wall voltage. My VTA is plugged into the wall at a separate outlet and I placed the variac in series there. I read the Furman voltage, then adjust the variac to 118VAC for the VTA.

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    TubesGlo


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    Post by TubesGlo Thu Sep 29, 2022 8:21 am

    Brap wrote:115VAC should be fine. I am able to adjust the output on my variac to dial it in depending on what the wall is giving me.  Have a Furman in my rack of gear which provides a readout of the wall voltage. My VTA is plugged into the wall at a separate outlet and I placed the variac in series there. I read the Furman voltage, then adjust the variac to 118VAC for the VTA.

    Good way to go. My Furman has 3 settings, wide , normal and narrow. Incoming is usually 122-124. I set to wide and my bucking transformer knocks it down to 117-119. I've measured after the CL-90 where it's brought down another 2 volts. With the stock PA060 I'm happy at 115-117.
    Brap
    Brap


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    Post by Brap Thu Sep 29, 2022 8:25 am

    Three settings -- havent seen that one. What model? I have the PL plus DMC. See my avatar and you may see it in the rack
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    TubesGlo


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    Post by TubesGlo Thu Sep 29, 2022 9:09 am

    My mistake, I have the Furman Elite DMi in theater rack and a APC H15 on the 2ch gear which has the 3 voltage trim settings.
    penguinpages
    penguinpages


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    Post by penguinpages Fri Sep 30, 2022 8:38 am


    Interesting topic. Can someone add a bit more meat on the bone on this topic that is more electrically trained (vs youtube self trained like myself Smile


    I have Dynaco ST120 amp kit from this company. And my input voltage ranges were 126-128 and some days I saw 124 so ugly. So I put in variac and set voltate to 110 as that reflected the Dynaco Tube design "sweet spot" based on it being built back pre-120 switch (from 110v)


    Question:
    1) What is the pros / cons of setting lower or higher input voltage range (What is the best voltage to set the unit at and if I deviate up / down what is the effect)

    2) What is the amp "input impedance"? . I have a set of Active Servo Subs which take of the bottom 1.5 octave of load from the amp, but posting about maybe moving from active (DSP) to passive filter may increase quality and so need to know this value https://www.youtube.com/watch?v=65eFr2rKy3M

    Brap
    Brap


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    Post by Brap Fri Sep 30, 2022 8:56 am

    Cant speak to the servo subs since I don't use one and if I did it would be active out of the pre-amp not connected to my VTA. According to Bob, 117-118VAC for a VTA ST-70 is preferred.

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