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The Dynaco Tube Audio Forum

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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DavidR
Tom Pickett
Rbertalotto
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    Voltage Buck for ST120

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    Rbertalotto


    Posts : 98
    Join date : 2021-02-25

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    Post by Rbertalotto Fri Mar 12, 2021 8:49 pm

    Here's the deal. In my travel trailer I have a couple TVs that operate on 12V. But when the battery charger is working to charge the AGM batteries, the voltage will sometimes go as high as 14.5 volts. This is normal for the "bulk" charge of AGM batteries. But the TVs hate it and they will shut down. For under $20 I bought a 12V buck regulator. Put one behind each TV and no mater what the input voltage is, the output is always EXACTLY 12V.

    I have an ST120 that wants to see no AC voltage above 117v +/-. I'm using a variac as the voltage in my house is usually around 126v. Every time I turn the variac on I need to reset it. Guess I could just leave it on all the time, but I chose to shut it off when not using it.

    I've searched but I'm not finding buck regulators for these AC voltages and 5amp current capability.

    Anyone know of such a device?
    Tom Pickett
    Tom Pickett


    Posts : 203
    Join date : 2020-11-22
    Age : 58
    Location : Texas

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    Post by Tom Pickett Fri Mar 12, 2021 9:07 pm

    Rbertalotto wrote:Here's the deal. In my travel trailer I have a couple TVs that operate on 12V. But when the battery charger is working to charge the AGM batteries, the voltage will sometimes go as high as 14.5 volts. This is normal for the "bulk" charge of AGM batteries. But the TVs hate it and they will shut down. For under $20 I bought a 12V buck regulator. Put one behind each TV and no mater what the input voltage is, the output is always EXACTLY 12V.

    I have an ST120 that wants to see no AC voltage above 117v +/-. I'm using a variac as the voltage in my house is usually around 126v. Every time I turn the variac on I need to reset it. Guess I could just leave it on all the time, but I chose to shut it off when not using it.

    I've searched but I'm not finding buck regulators for these AC voltages and 5amp current capability.

    Anyone know of such a device?

    This may get you close or below 117V AC

    Dynaco Heathkit Fisher Scott Vintage Radio Bucking Transformer Voltage Reducer

    https://www.ebay.com/itm/Dynaco-Heathkit-Fisher-Scott-Vintage-Radio-Bucking-Transformer-Voltage-Reducer/193936058041?hash=item2d277d66b9:g:648AAOSwWY5ajym3


    Last edited by Tom Pickett on Fri Mar 12, 2021 9:24 pm; edited 1 time in total
    DavidR
    DavidR


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    Join date : 2017-08-10
    Location : MetroWest, MA

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    Post by DavidR Fri Mar 12, 2021 9:11 pm

    I believe this unit will give you a sine wave and keep the voltage setting better than a variac.

    https://www.amazon.com/dp/B000FBF08Q/?coliid=IU9VDYAMCXA9A&colid=292PN48OYLYA8&psc=1&ref_=lv_ov_lig_dp_it
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    Rbertalotto


    Posts : 98
    Join date : 2021-02-25

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    Post by Rbertalotto Sat Mar 13, 2021 10:17 am

    Looks like I'll just stay with the $55 variac. Most of these "Voltage Regulators" are "120V +/-5v".....Not good for what we want. And they start in the hundreds of dollars for not programable. A programable one is $2500!!!

    Many of them simply limit the voltage to 140V and then shut down......Yikes!

    https://www.circuitspecialists.com/variac-variable-ac-ps05kva_with_digital_display.html

    Circuit Specialists 5 Amp Variable Output Autotransformer with Digital Display...$54

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    Tom Pickett
    Tom Pickett


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    Post by Tom Pickett Sat Mar 13, 2021 10:38 am

    Rbertalotto wrote:Looks like I'll just stay with the $55 variac. Most of these "Voltage Regulators" are "120V +/-5v".....Not good for what we want. And they start in the hundreds of dollars for not programable. A programable one is $2500!!!

    Many of them simply limit the voltage to 140V and then shut down......Yikes!

    https://www.circuitspecialists.com/variac-variable-ac-ps05kva_with_digital_display.html

    Circuit Specialists 5 Amp Variable Output Autotransformer with Digital Display...$54


    Thats a very nice little variac Rbertalotto.... I just might snag that too for my other projects.
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    New2Tubez


    Posts : 184
    Join date : 2018-03-20
    Location : NY

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    Post by New2Tubez Sun Mar 14, 2021 12:26 am

    Maybe this will help:

    https://blueglowelectronics.files.wordpress.com/2018/11/bucking-transformer-slides.pdf

    I'm using the recommended by Bob 5A circuit specialists variac but am considering the bucking transformer. Blueglow has lots of info and youtube content.
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    Rbertalotto


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    Post by Rbertalotto Sun Mar 14, 2021 4:38 pm

    That "Brown Box" is awesome.....but at $350....a bit rich.
    Dave_in_Va
    Dave_in_Va


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    Location : Mid. VA

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    Post by Dave_in_Va Sun Mar 14, 2021 7:33 pm

    That "Buckminister" (simple version) is the one I built a couple of times. It cost about $35 to build one.
    Knocks my 123 or 124 VAC down to about 115/116.
    I run my VTA ST 70, SP 14 and PH15 off the bucker and have been using it for about 4 years.
    By the way, that bucker info is from Rob Robinette's 5E3 site.

    https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer
    Roy Mottram
    Roy Mottram
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    Post by Roy Mottram Thu Mar 18, 2021 4:16 pm

    i'd go with a bucking transformer for sure. A year or two ago, I research every power conditioner I could find, from $200 to $5000.
    NONE of them will keep a voltage at a set level. All they do is shut down if voltage is more than about 130vac or lower than 110vac

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    Tom Pickett
    Tom Pickett


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    Post by Tom Pickett Thu Mar 18, 2021 6:44 pm

    tubes4hifi wrote:i'd go with a bucking transformer for sure.  A year or two ago, I research every power conditioner I could find, from $200 to $5000.
    NONE of them will keep a voltage at a set level.   All they do is shut down if voltage is more than about 130vac or lower than 110vac

    Yes... To me a bucking transformer is the best way to go and it's permanent.

    The bucking transformer post link I made the other day is good for bringing the voltage down from 124 to 116 but sadly is only good for a max of 4 amps so that in my opinion is not enough amps for the VTA 120.

    Im in the same situation. I measured 122v AC on my line yesterday and I still think thats to much and will be hard on the tubes for the 120 after the voltage has been stepped up.
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    Hops


    Posts : 85
    Join date : 2019-09-21
    Location : South Detroit, Michigan

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    Post by Hops Fri Mar 19, 2021 7:34 pm

    I run a Variac that Bob suggested. I have a kill a Watt ( that Bob suggested ) plugged into the Variac. I have a small portable Ground Fault Interrupter ( that someone on this forum suggested ) plugged into the Variac. And I installed a parts express grounded power cord into my ST-120 ( groiund wire to one of the transformer mounting bolts ). I look at the voltage reading on the Kill a Watt periodically -- if it is above 118 I turn it down. If it is too low I leave it along, because the ST-120 is overkill for my small room anyway. Sounds good, and I am well protected against shorts etc and power interuptioons.

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