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    Is VTA 120 in need of Step down or bucking transformer?

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    tkemmett

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    Join date : 2013-10-24

    Is VTA 120 in need of Step down or bucking transformer?

    Post by tkemmett on Sun Aug 10, 2014 3:11 pm

    Is the VTA 120 best operated at reduced line voltage (110-117) rather than the 124-126 I measure at the the outlet? I have read that tube life and sonics are improved with reduced voltage in a lot of tube gear.
    TIA,
    Boyotk

    corndog71

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by corndog71 on Sun Aug 10, 2014 4:19 pm

    Yes, I would recommend getting a variac.

    This is what I got and it works great.

    http://amzn.com/B00BXJYPK

    Bob Latino
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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Bob Latino on Sun Aug 10, 2014 5:36 pm

    tkemmett wrote:Is the VTA 120 best operated at reduced line voltage (110-117) rather than the 124-126 I measure at the the outlet? I have read that tube life and sonics are improved with reduced voltage in a lot of tube gear.
    TIA,
    Boyotk

    The VTA ST-120 was designed to run on 120 VAC and will run OK up to about 122 VAC. If your line voltages are higher than 122 VAC, the amp will still run OK BUT the higher internal DC voltages caused by your higher than normal line voltage will shorten tube life somewhat, especially the life of the rectifier. Above 122 VAC you should use some type of voltage reducing device like a variac.

    Another idea for dealing with higher than normal line voltages is to use a 5U4 rectifier in place of a 5AR4 rectifier. This won't help your higher line voltages but it will drop the amp's internal DC voltages to a more normal level and improve tube life ..

    Bob

    sKiZo

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by sKiZo on Sun Aug 10, 2014 6:49 pm

    As Bob said, they're happy at 120vac, but 124vac is on the high side of happy. Not saying you can't be happy when high ...  jocolor 

    I was averaging 124vac and above here, so I built a simple bucker out of spare parts.



    That was actually a good day at the time ...

    I also followed up with an inquiry directed at the power company. Come to find out, they apparently missed a couple items on their checklist of "things to do when upgrading lines from 4k to 14k - now I'm back down to 120vac, so the bucker is disconnected. I keep the Kill-A-Watt plugged in to an extra socket and glance at it every now and then to make sure there's no new surprises. I'll keep the bucker in reserve, just in case.


    tkemmett

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by tkemmett on Mon Aug 11, 2014 5:17 pm

    Is a 3 amp variac enough for the amp and a preamp?
    Thanks for the replies!

    Bob Latino
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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Bob Latino on Mon Aug 11, 2014 6:44 pm

    The VTA ST-120 draws about 2 amps at idle. If you add a preamp to the mix then you would probably be at or near the limit of a 3 amp variac. IMHO, for an ST-120 you should use a 5 amp or higher variac.

    Bob

    tkemmett

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by tkemmett on Mon Aug 11, 2014 7:01 pm

    Thanks Bob...I sure do appreciate the timely and useful info I have gotten from this forum!

    sKiZo

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by sKiZo on Tue Aug 12, 2014 12:21 pm

    Probably goes without saying, but ditto with a bucker. The transformer I used is rated at six amps with a 120 primary and 6.3 secondary winding.

    Don't quote me, but I believe it originally came out of an Eico something or other.

    Bob Latino
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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Bob Latino on Tue Aug 12, 2014 1:00 pm

    sKiZo wrote:Probably goes without saying, but ditto with a bucker. The transformer I used is rated at six amps with a 120 primary and 6.3 secondary winding.

    Don't quote me, but I believe it originally came out of an Eico something or other.

    sKiZo,

    I know that on one of the posts on the forum here you mentioned how you made your bucking transformer but I can't find that post ? Do you think that you could mention here (again) how you made the bucking transformer? maybe showing a diagram of the wiring and what type of transformer that you used ?

    Bob

    sKiZo

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by sKiZo on Tue Aug 12, 2014 8:06 pm

    A picture is worth maybe a dozen words with inflation ...





    The transformer I used is a basic step down with 120vac and 6.3vac taps. Basically, what you're doing is direct connecting one side of the AC power to an output outlet, and forcing the other side through both the high and low sides of the transformer, in effect, coupling them. You can either boost or cut the output by the value of the secondary winding depending on how you hook it up.

    Can't say where I got the transformer in the first place ... that was a parts bin special. Mine looks a bit more complicated than the diagram because it has a breaker, light, and power switch courtesy of the old power strip I used for a case. Also a junk bin find, but I like the old skool look of the metal and bakelite ... Just lopped out a couple of the old sockets to make room for the transformer and left the rest in place.

    PS ... if you check the before/after readings, you can see I'm actually getting a 7.2vac drop from the bucker. Not sure if that's a tolerance variance or just the way it was made, but close enough to what I was looking for. As said, it was a leftover, and there weren't any labels, other than a piece of masking tape that had turned brown and krinkly over the years.

    DarthBubba

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    Thanks...

    Post by DarthBubba on Tue Aug 12, 2014 9:38 pm

    Thanks for reposting that picture and diagram  cheers 

    -DB

    tkemmett

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by tkemmett on Wed Aug 13, 2014 2:04 am

    What are the pro's and con's of using either a variac or bucker to lower voltage?

    Bob Latino
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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Bob Latino on Wed Aug 13, 2014 8:07 am

    tkemmett wrote:What are the pro's and con's of using either a variac or bucker to lower voltage?

    With a variac you can dial your line voltage up or down and set it anywhere you want between 0 and 140 volts (in 120 volt counties) For a tube amp you want to set it at maybe 115 - 118 VAC. The voltage drop in the bucking transformer can be altered somewhat as sKiZo has said by changing the connection hookups. The variac adjusts voltage much easier - you just turn the dial up or down.

    Bob

    deepee99

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by deepee99 on Wed Aug 13, 2014 1:20 pm

    Bob Latino wrote:
    tkemmett wrote:What are the pro's and con's of using either a variac or bucker to lower voltage?

    With a variac you can dial your line voltage up or down and set it anywhere you want between 0 and 140 volts (in 120 volt counties) For a tube amp you want to set it at maybe 115 - 118 VAC. The voltage drop in the bucking transformer can be altered somewhat as sKiZo has said by changing the connection hookups. The variac adjusts voltage much easier - you just turn the dial up or down.

    Bob

    Bob, as I understand -- pls correct me here -- the variac's output voltage rolls up and down with the input voltage. So if you set it for 118 with 120 input, and the input goes up 4 volts, you're back where you started from, right?
    FWIW, I was getting 123-124 here at the house pretty regularly when the power company installed a new xformer on the pole. I hollered and they tweaked or replaced it, life is much happier now. I haven't blown a Weber in quite some months. Plus I am told that the tube filaments will tolerate a pretty wide voltage swing (+ or- 10%) without much damage. Thoughts?

    Bob Latino
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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Bob Latino on Wed Aug 13, 2014 3:35 pm

    David,

    Yes - as your input voltage goes up the variac's output voltage will also go up. Most people have line voltages that don't vary more than 1 - 2 volts per day but in some parts of the country this may not be true .. I would set the variac's output to maybe 117 VAC. Even if your line voltage goes UP a little the variac probably won't go over 120 VAC.

    Tube filaments (except for the rectifier filament line) should be about 6.3 - 6.7 VAC with 120 VAC coming in. From talking to customers since 2006, it appears that the VTA amps will tolerate higher voltages up to about 122 VAC without incident. When you get up to 123 or higher the amps internal AC and DC voltages will rise and tube life will be shortened.

    You are more likely to have lower and more steady voltages in a city or a large town. As you live further "out in the sticks" line voltages seem to be higher. I had someone who lived out at the end of a long 6 mile road in which he had the only house at the end of this road. He mentioned by Email that his line voltage was 126.7 VAC. He bought a 5 amp Staco variac, set it for 117 VAC out and had no more tube life issues.

    Bob

    corndog71

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by corndog71 on Wed Aug 13, 2014 4:39 pm

    I live on the north side of Chicago and get a fairly steady 125VAC. I remember getting a solid 120VAC in one of the suburbs bordering the city.

    sKiZo

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by sKiZo on Wed Aug 13, 2014 4:42 pm

    Yup ... I used to be one of those "end of line" customers - unfortunately, civilization seems to be creeping up on me. A 600 acre centennial farm across the street sold recently and developers were lining up to piece it out. Ah, well ... progress.

    I've got a 5a variac, but I use that on the hobby bench. I figured no sense tying it up for single purpose voltage reduction when a bucker would do. That's a LOT cheaper if you can scrounge up the parts. Outlet strips are real popular as cases, and the only other expense is the transformer.

    You CAN get a "line conditioner" that stabilizes voltage to a certain degree. Even those can't handle a radical power offset from "normal" for any great length of time - they're more built for transients. And in any case, they tend to be uber expensive.

    If your power wanders a lot, I believe Kill A Watt makes a remote display device that you can plug in and keep the readout handy as a quick reference. Set that up in series with a variac and adjust as needed whenever you power up the amp.

    PS ... what's the deal with variacs lately anyway? My 5a cost me $50 shipped a couple years back ... they seem to have doubled in price since I last looked. Should have bought a boatload of those instead of that Radio Shack stock I guess ...  bom

    corndog71 wrote:I live on the north side of Chicago and get a fairly steady 125VAC.  I remember getting a solid 120VAC in one of the suburbs bordering the city.

    Anywhere near Deerfield? I hear they have a fairly consistent zero VAC ... reigning kings as power outage capital of the world. Controlling voltage there would amount to how fast you peddle.



    "Faster old man! I'm trying to listen here!!"

    Pillo69

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    Re: Is VTA 120 in need of Step down or bucking transformer?

    Post by Pillo69 on Thu Aug 14, 2014 5:21 am

    In the diagram below you can see an enlargement of provided by Skizo, where you can adjust the output voltage with a rotary selector, by a voltmeter see the output voltage.
    You can use a transformer to 24v secondary.



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