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What is the least detrimental way of reducing the input AC
from 120V to 115V for a 115V rated power transformer?
Besides the obvious variac of course.
Follow Ups:
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Hi.120VAC is only 4.34% over the rated 115VAC.
For the power transformer built nowadays are normally rated up tp 20% safety margin using 155C class F magnetic wires & 130C PE interwinding insulation sheet, no danger of overloading the iron at all.
Most if not all, tube HV & heaater/filaments are rated with min 10% saftety margin, so it should not cause any problem of blowing the tubes.
However, strictly for sonic issue, some may not like to overdrive the tube heater or filaments, it may be desirable to reduce the increased heater voltage.
Let me take the overdriven situation of my ST-70 since day a couple years back, the household AC mains voltage is 120V-125VAC but the power amp is rated for 117VAC. Over 6.84% all the time!!!
Only a couple of month ago, I decided to install a DIYed wooden core CM choke to reduce the excessive 0.45VAC from the heaters. I also added a 0.1uF200VAC polyester film bleeder cap to the CM choke for the bonus filter effect. Somehow, I find the reduced heater voltage, now set back to normal 6.3VAC gives better sound.
Don't know your app. and how well it would suit, but the first thing that came to mind reading your post was a thermistor (NTC) - sizing it/them so it stabilizes with the right amount of DCR. Would do double duty as a soft(er) start device. (I did this a few years ago and it worked fine at the time)'just had another thought - If the mains voltage varies a lot would it work well to use a PTC device that would increase resistance as the circuit draws more current (with rise in mains VAC)? You wouldn't get the soft start and sizing it would take some experimentation - though I think the manufacturer's engineer would help you work it out. I haven't tried it myself and can imagine reasons it may not work but I think the idea is at least worth a look.
I used an isolation transformer (used off ebay) that had an adjustment switch 105, 115, 125 for the input voltage. That way I just set it to 125 and I get about 110 to 115 at the output(when loaded). The voltage does creep up a few volts under no load. I think it is either 250VA or 500VA. I was getting some ground noise due to my setup. But I may try it out again when I get finish recapping my Mark 3 amps since the B+ is over 500 volts when my AC in is 124V.
As already posted you could "buck" it but I wouldn't bother. Utilities vary widely from season to season and even hour by hour. I have a line monitor on my bench and it ranges from 117V on some summer evenings to 128V during spring and fall days. It's usually about 123V or so.
Unless there is a problem using it with 120 volts don't bother.The problems would be primarily:
voltage too high on secondary causing problems in the load circuits
excitation current too high causing transformer heating or growling
The drawing is an old one where I show how to reduce 126VAC to 120VAC. A 5V transformer wired this way would reduce 120VAC to about 115VAC. Note that only the secondary winding carries significant current, so it can be a relatively small transformer. If your amp draws 1A of current then a 5V @ 1A transformer should do, though you might go for a deluxe 3A version.Obviously, you need to get the phase right or you'll add 5V instead of subtract.
If you happen to have an unused 5V winding on your power transformer then you can use it without adding an a extra bucking transformer. Just wire it in series with the primary.
I agree with Eli's post, but if you still feel that you must tweak the input voltage, you can use a smaller 120vac-to-5vac transformer as a bucking device. Connect the 5vac winding in series with the 115vac primary of your main transformer. Together these series windings are then connected across the full 120vac line. Hook it one way, and you'll get 125vac ("boost") across your main transformer’s primary, not what you want. Hook it the opposite way ("buck"), and you'll drop five volts almost losslessly, from 120vac to 115vac. You’ll need to experiment (measure) first. The 120vac primary of the buck transformer must be connected across the full 120vac line of course, of course. This approach results in much lower DCR than just sticking in a series dropping resistor.
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Brian
Dan,I don't know if you should even bother. Yes, average line voltages are higher now, but the specification has always been 105-125 V. for US house current. Equipment is supposed to work under those conditions.
Eli D.
Yep, that is exactly the spec in my 1960's Heathkit Manual, 105vac to 125vac. Conversationally, the general public said one-ten or one-fifteen because that was plenty adequate to convey the meaning; we did not actually measure our wall outlets back then or SAY 105 to 125.
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