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

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

Yes, I would recommend getting a variac.

This is what I got and it works great.

http://amzn.com/B00BXJYPK

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 ...   

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.

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

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

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.

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.

Thanks for reposting that picture and diagram   

-DB

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

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?

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.

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 ...  

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!!"

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.

Thanks for the shematic, most helpful
I've built a bucking transformer today Stripped the secondary from a
300va toroid and rewound it with another 4mm2 wire. Dropped 6V and
seems to be stable and not getting hot or anything. Ac-voltage have
differed from 225 to 238v the last days and that doesn't seem to be ok.
Power transformer goes quite hot and then it hums just to much.

Each channel of M-125 draws 170w so I figured that 300va would be
fairly sufficient. I'm building a second one for the other channel as well
and maybe I will ad some more turns to the secondary to let it drop ~10v.

Any thoughts about if the transformers are big enough are welcome.

Dahlberg,

Your bucking transformer is more than adequate for many M-125s..... Let's try some simple maths for ideal situations. (Ready to be shot down in flames if I make a mistake )

Watts = Voltage * Current
Current = Watts / Voltage

You say that the single M-125 draws 170w and assume your supply voltage is 220V. The amp will be drawing 170W / 220V = 700mA.

If you wish to drop 10V then the total power for the 10V winding will be:-

10V * 0.7A = 7W!

You'd be surprised how small a transformer you can get away with. Obviously the larger the transformer the better the regulation will be but there are sensible limits.

Hope this helps.

Steve.

I have a massive Furman SPR 20i that clamps the output voltage at 121 VAC pretty much regardless of what's coming out of the wall, which is fine for the s/s gear, TV, etc. It would be nice to dial the amp outputs down to 118 or so but there are no adjustments to do so. I lack the parts and/or expertise to build my own bucking xformer, and wondering if there's someone here who might care to build one for me (it would be running a pair of M-125s) and what it would cost. Volunteers? I'd gladly pay you Tuesday for a ham-bucker today. PM or just reply here.
Thanks,
David

Ended up with two like this, one for each M-125. Drops 10v so I should
stay below 230v in the future, 218-228v if this weekend is what's normal.