Help building a modified "AC line voltage corrector"....

Michael Inglis

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Id really appreciate some help from someone who has a better understanding of transformers than i do. I understand the basics but i want to have a better comprehension of what im doing before i start the build.

So my houses line voltage goes from 112v in the summer to about 126v in the winter and everything in between. From day to day hour to hour i never know what it will be. Ive had the electric company out 2 times and both times they give me a line about how they will surely fix it and a month later its back to the same inconsistent service. So ive been using a cheap Chinese "voltage stabilizer" that kicks on once i get around 6V above or below 120v and it corrects by about 10volts. That works ok but its not ideal.

I want to be able to choose when i boost or buck the line voltage or just leave it as it is. So im planning on building something similar to the GeoFx "Vintage Volt Adaptor" which is basically what youll see in the picture i posted. But i want to modify the purpose to boost or buck rather than have two cuts. So my understanding so far is that if i reverse the 6.3v secondary positions i will be boosting rather than bucking, correct? And to do that i had planned on using a quality dpdt switch. So there will be one switch that selects the level of boost/buck and one switch that selects whether it boosts or bucks and one switch that can bypass all that to just provide line voltage.

I already picked up a meter display that will monitor the volts, current, power/watts and freq. And i am under the impression the recommended transformer in the diagram/schematic below will suffice since the mains fuse on my DSL100HR is 4 Amps. (I checked the datasheet and it is listed as a 115v 6.3vct 4A transformer).

Originally i was sourcing my own transformer but i kept running into what seems like an inconsistentcy. Some transformers that clearly have 6.3v secondary's were being labeled as 12v center taps. Could you clarify this? Was this just a mistake on the part of whoever posted the listing? Wouldnt the center tap be more accurately labeled as whatever the secondarys are since its half of their combined total? Or am i missing something?

I can get pretty far with my own research but sometimes a little clarification can be really helpful with projects like this. Id really appreciate all the information anyone is willing to give on the subject. Thanks!
 

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Jon Snell

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You could of course use a motorised Variac but wait ... why bother it makes little difference to electrical equipment as the input voltage ratings are usually +20 -10% so are covered for any event.
DC is simple, employ a Power Factor Correction circuit to keep the DC voltage constant but for AC mains ... that is a whole can of worms and without using an invertor will be difficult to acheive electronically.
 

william vogel

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What you’re wanting to do is not going to be something you can make easily. You really should get a 10 amp variac, install a voltmeter in it to monitor the voltage and adjust accordingly. Whatever you use needs a current rating about 2.5 times the maximum draw of the amp because the amp isn’t a constant current draw and it’s changing demand will cause variations with an underrated device. If you really want stability, they make AC power supplies that monitor and automatically adjust. These things are really expensive, like $5000. Watch Premiere Guitar Rig Rundown for AC/DC. They use them and it’s explained in the video. Because your fluctuations are extreme (112-126), there’s no cheap and easy fix for the situation. A variac will allow you to maintain the desired voltage but it will require adjustment for complete stability. The really expensive AC power supply also outputs a nearly perfect sinus AC waveform but only the $5000+ units achieve this level of refinement to the output. The cheap inverters use pulse width modulation and output a square wave output that mimics the AC and would be noisy in an amplifier. I have already gone down this rabbit hole to figure out a solution and it’s either constant maintenance with a variac with dirty power or $5000 for a power supply. My voltage is not as unstable as yours but mine is dirty. The power company CANNOT and WILL NOT stabilize the grid for you. It would require a sub station being built close to your location. The problem is that you’re far enough away from the sub station and the loads around you are making the voltage rise and fall due to the resistance/impedance within that section of grid. You must be close to the end of the line in the distribution network and getting the fluctuations associated with the distance from the sub station.
 

Michael Inglis

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What you’re wanting to do is not going to be something you can make easily. You really should get a 10 amp variac, install a voltmeter in it to monitor the voltage and adjust accordingly. Whatever you use needs a current rating about 2.5 times the maximum draw of the amp because the amp isn’t a constant current draw and it’s changing demand will cause variations with an underrated device. If you really want stability, they make AC power supplies that monitor and automatically adjust. These things are really expensive, like $5000. Watch Premiere Guitar Rig Rundown for AC/DC. They use them and it’s explained in the video. Because your fluctuations are extreme (112-126), there’s no cheap and easy fix for the situation. A variac will allow you to maintain the desired voltage but it will require adjustment for complete stability. The really expensive AC power supply also outputs a nearly perfect sinus AC waveform but only the $5000+ units achieve this level of refinement to the output. The cheap inverters use pulse width modulation and output a square wave output that mimics the AC and would be noisy in an amplifier. I have already gone down this rabbit hole to figure out a solution and it’s either constant maintenance with a variac with dirty power or $5000 for a power supply. My voltage is not as unstable as yours but mine is dirty. The power company CANNOT and WILL NOT stabilize the grid for you. It would require a sub station being built close to your location. The problem is that you’re far enough away from the sub station and the loads around you are making the voltage rise and fall due to the resistance/impedance within that section of grid. You must be close to the end of the line in the distribution network and getting the fluctuations associated with the distance from the sub station.

I think the practicality of this soltuion is being misunderstood due to a stigma about these type of devices. Id mentioned in the original post ive already been using a device that automatically adjusts the voltage. It cost me $30 and it honestly works perfectly. But i want something more..er...not cheaply made in china lol. It boosts at 116v by 10v and bucks at 126v by -10v. And it didnt make my amp noisier at all and im extremely sensitive and anal about these things. The only noise is the very quiet hum of the transformer which is unnoticeable unless you are right next to it with your ear within 3ft. I added a hammond choke i got off mouser a few months ago and the amp was already so quiet it really didnt change the noise floor, and i crank the ever loving shit out of it cause my neighbors not only dont care they compliment my playing LOL!! The choke did make a postive difference in the feel for sure though. I added a switch to select the choke or the stock resistor and the difference is certainly there and i prefer it with the choke. Anyways...

So heres the rub, the reason its worth making isnt about whether or not the line voltage i have is useable or not. Cause your right, with modern amps it will be fine. But the power tubes wont. Even if i rebias once a week or more (which i did before i got the voltage stabilizer) it still fluctuates enough that i was going through power tubes every 6 months. Now i get about a year and a half or more before i have a power tube fail using EHX EL34's which im quite fond of.

And with the tube situation the way it is now i need my power tubes to last as long as possible, we all do. So ive been biasing even colder than normal and ive found the amp still sounds excellent. So id also rather keep the power at or below 120v so im putting less strain on the power tubes. Conventional wisdom would tell you that all of this is overkill but my experience has shown its not and the difference it makes is absolutely worth it, just ask my wallet. When people talk about these things in theory noise and "clean/dirty" power are always mentioned for why its not worth it but in my case in practice that hasnt been an issue whatsoever.

But theres still a TON of things im unsure of which is why i ask questions. But respectfully i disagree with some of your conclusions. And i really do mean that to be as respectful as humanly possible. Had i not had the experiences ive had i would likely agree with you. Plus this design is almost exactly what a "Tone Preserver" is and those things are highly prized by the people who use them for vintage equipment.

If i had regular line voltage id have no use for this device with my modern marshall but i dont have "regular" stable line voltage and this could come in handy for jam sessions where the power is similar to my line voltage at home. Btw I really liked your theory for why my power is the way it is (being furthest from the substation) and that makes a lot of sense! With that i absolutely agree.

Since posting this i decided to go with a 6Amp transformer with a 5v CT. And i found one that is the right dimensions and is only a dollar more than the 4A 6v CT transformer that most guys use for the -6 or -12 configuration (like the "Tone Preserver" does). So with this transformer i intend on being able to boost or buck by 5v or 10v. I have all the wiring figured about to accomplish this and ive run it over in my head looking for any reason why it wouldnt work and so far i havent found one. But i want to be incredibly thorough. Im not the sharpest tool in the shed sometimes and there are many people smarter than me that might see something i dont.

Hell maybe it was I who misunderstood you? If thats the case please elaborate so i can make sure to get this right. Thank you very much for the response brother!
 

Michael Inglis

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You could of course use a motorised Variac but wait ... why bother it makes little difference to electrical equipment as the input voltage ratings are usually +20 -10% so are covered for any event.
DC is simple, employ a Power Factor Correction circuit to keep the DC voltage constant but for AC mains ... that is a whole can of worms and without using an invertor will be difficult to acheive electronically.

Are you familiar with a product called a "tone preserver" for vintage equipment, this basically exactly that. Ill post a geofx write up about how it works. And yes you are right, with a modern amp i wont technically have an issue but in reality i was going through power tubes every 6 months and now i get a year and a half or more and all i changed was adding a voltage stabilizer that boosts the power by 10v if the line voltage drops below 116v and bucks by 10v if the line voltage goes above 126v. What was killing my tubes was the really bad fluctuations that i was unaware of.

So id set my bias with the amp seeing 120 volts and a few days later id be playing and im red plating. So i check the voltage and its well above 126v. On average it stays between 115v and 126v but every now and then it would drop really low or go up really high. And back then i didnt have a volt and current meter display connected between my amp and the wall so i had no way of knowing there was a problem till i was red plating or a tube went bad.

Now i only had the red plating issue happen about 3 times but all the times the power spiked but didnt cause red plating it was wearing out my tubes. Like i said just by keeping the spikes limited to 126v helped immensely. So if i could choose when the bucking or boosting occurs i can save my transformer and my power tubes a lot of stress.

Since the original post i sourced a 6A 115v transformer with a 5volt center tap. I figured a five volt boost or buck would be more useful but i intend on wiring it so i can also select a 10v boost or buck for when those extreme drops and peaks happen. And having a volt and current meter display monitoring the output voltage will allow me to turn off the device and see what the line voltage is and decide it it would benefit from a boost or a buck.

If there is something you think im not understanding please elaborate. Im certainly not an expert, but im comfortable working inside tube amps with high voltages and im fully aware of the risks. I regularly do modifications to SS amps and occasionally modify tube amps. I also build pedals and guitars lol. But i have enough humility to know i should always seek out people who know more than me to run my ideas by. It not difficult to overlook things when your self taught and work on your own.

And thanks for the reply!
 

william vogel

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I think the practicality of this soltuion is being misunderstood due to a stigma about these type of devices. Id mentioned in the original post ive already been using a device that automatically adjusts the voltage. It cost me $30 and it honestly works perfectly. But i want something more..er...not cheaply made in china lol. It boosts at 116v by 10v and bucks at 126v by -10v. And it didnt make my amp noisier at all and im extremely sensitive and anal about these things. The only noise is the very quiet hum of the transformer which is unnoticeable unless you are right next to it with your ear within 3ft. I added a hammond choke i got off mouser a few months ago and the amp was already so quiet it really didnt change the noise floor, and i crank the ever loving shit out of it cause my neighbors not only dont care they compliment my playing LOL!! The choke did make a postive difference in the feel for sure though. I added a switch to select the choke or the stock resistor and the difference is certainly there and i prefer it with the choke. Anyways...

So heres the rub, the reason its worth making isnt about whether or not the line voltage i have is useable or not. Cause your right, with modern amps it will be fine. But the power tubes wont. Even if i rebias once a week or more (which i did before i got the voltage stabilizer) it still fluctuates enough that i was going through power tubes every 6 months. Now i get about a year and a half or more before i have a power tube fail using EHX EL34's which im quite fond of.

And with the tube situation the way it is now i need my power tubes to last as long as possible, we all do. So ive been biasing even colder than normal and ive found the amp still sounds excellent. So id also rather keep the power at or below 120v so im putting less strain on the power tubes. Conventional wisdom would tell you that all of this is overkill but my experience has shown its not and the difference it makes is absolutely worth it, just ask my wallet. When people talk about these things in theory noise and "clean/dirty" power are always mentioned for why its not worth it but in my case in practice that hasnt been an issue whatsoever.

But theres still a TON of things im unsure of which is why i ask questions. But respectfully i disagree with some of your conclusions. And i really do mean that to be as respectful as humanly possible. Had i not had the experiences ive had i would likely agree with you. Plus this design is almost exactly what a "Tone Preserver" is and those things are highly prized by the people who use them for vintage equipment.

If i had regular line voltage id have no use for this device with my modern marshall but i dont have "regular" stable line voltage and this could come in handy for jam sessions where the power is similar to my line voltage at home. Btw I really liked your theory for why my power is the way it is (being furthest from the substation) and that makes a lot of sense! With that i absolutely agree.

Since posting this i decided to go with a 6Amp transformer with a 5v CT. And i found one that is the right dimensions and is only a dollar more than the 4A 6v CT transformer that most guys use for the -6 or -12 configuration (like the "Tone Preserver" does). So with this transformer i intend on being able to boost or buck by 5v or 10v. I have all the wiring figured about to accomplish this and ive run it over in my head looking for any reason why it wouldnt work and so far i havent found one. But i want to be incredibly thorough. Im not the sharpest tool in the shed sometimes and there are many people smarter than me that might see something i dont.

Hell maybe it was I who misunderstood you? If thats the case please elaborate so i can make sure to get this right. Thank you very much for the response brother!
Your fine. I’m just understanding that your voltage is infinitely unstable between say 112-126. Building a device that targets a specific reduction or increase is okay but if the input voltage to the device isn’t right where it needs to be to achieve the optimal window of amplifier voltage, it seems like it wouldn’t be very useful. I’m really just suggesting to get a 10 amp variac because you can dial to the perfect voltage. I have a Staco 10 amp unit that I bought off eBay used for $100. I fitted it with a digital display volt meter which was pretty easy and I can dial from 0-140 volts and it’s useful for testing, measurement of components, there’s a lot of things that would help justify owning a variac. I enjoy building things that help me with amps and tone but I also like having things that make it easier to achieve my goals and sometimes it’s just better to buy a complete unit that attempt to build something that’s not quite as favorable. I don’t see anything wrong with what you want to build but I don’t think it’s going to achieve the results you need. It’s cool.
 

Michael Inglis

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Your fine. I’m just understanding that your voltage is infinitely unstable between say 112-126. Building a device that targets a specific reduction or increase is okay but if the input voltage to the device isn’t right where it needs to be to achieve the optimal window of amplifier voltage, it seems like it wouldn’t be very useful. I’m really just suggesting to get a 10 amp variac because you can dial to the perfect voltage. I have a Staco 10 amp unit that I bought off eBay used for $100. I fitted it with a digital display volt meter which was pretty easy and I can dial from 0-140 volts and it’s useful for testing, measurement of components, there’s a lot of things that would help justify owning a variac. I enjoy building things that help me with amps and tone but I also like having things that make it easier to achieve my goals and sometimes it’s just better to buy a complete unit that attempt to build something that’s not quite as favorable. I don’t see anything wrong with what you want to build but I don’t think it’s going to achieve the results you need. It’s cool.

Thats entirely reasonable. And i should have added that i agree with you that a variac would be more practical. And i currently dont have one and i really need one as a bench power supply. I built a bench supply using a DROK modual for DC voltages but for AC i could really use Variac. And i have no doubt in my mind ill end up getting one soon for exactly that reason. I think i just enjoy building things myself sometimes and that is a factor that is swaying my decision. In hindsight i really should have explained why I wasnt getting a variac.

The cost of the voltage corrector will be $64.79 after tax and if i were to estimate the cost of the materials ill need but already have it would probably be another $20 at most. So that would be just about the same cost as either a used Variac or one of the cheap Chinese Variacs rated for 5 amps. So if i was going for accuracy and cost then the Variac would make more sense but i really enjoy building things like this myself. Hmm, i wonder what i would need to build my own Variac? I have some googling to do lol. Thanks again for weighing in!
 

william vogel

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Thats entirely reasonable. And i should have added that i agree with you that a variac would be more practical. And i currently dont have one and i really need one as a bench power supply. I built a bench supply using a DROK modual for DC voltages but for AC i could really use Variac. And i have no doubt in my mind ill end up getting one soon for exactly that reason. I think i just enjoy building things myself sometimes and that is a factor that is swaying my decision. In hindsight i really should have explained why I wasnt getting a variac.

The cost of the voltage corrector will be $64.79 after tax and if i were to estimate the cost of the materials ill need but already have it would probably be another $20 at most. So that would be just about the same cost as either a used Variac or one of the cheap Chinese Variacs rated for 5 amps. So if i was going for accuracy and cost then the Variac would make more sense but i really enjoy building things like this myself. Hmm, i wonder what i would need to build my own Variac? I have some googling to do lol. Thanks again for weighing in!
Look on eBay for used Staco units. They are used in industrial applications for speed control etc.
 

XTRXTR

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You need a Bucking Transformer:
and on his website:

He has versions that change the line voltage with a switch. 110, 117, 120. Of course it's dependent on the variations of wall AC.
 
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william vogel

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You need a Bucking Transformer:
and on his website:

He has versions that change the line voltage with a switch. 110, 117, 120. Of course it's dependent on the variations of wall AC.
How would the bucking transformer be better than the variac?
 

XTRXTR

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Shape and weight. Much easier to fit a small light weight box into your power rig than a Variac that is cylindrical and quite heavy.

The variac is built for 0 to 130-140 Vac so its quite hefty for serious current draw at low voltages your amp won't operate on.

A Bucking Transformer is built for a couple of different voltages so it does not need the hefty winding. You could fit it into a combo cab or your rackmount case very easily.

The main application is that the amp is running at lower than high wall AC.
 

Michael Inglis

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You need a Bucking Transformer:
and on his website:

He has versions that change the line voltage with a switch. 110, 117, 120. Of course it's dependent on the variations of wall AC.
That is the same design I posted on my original post. But my question was really about the practicality of using it to boost and buck. I also decided to go with a 6 amp 5v CT transformer so the variations are plus or minus 5 or 10 volts. I think t5 volts is more practical than 6.3. And I sourced my own transformer and found one that's almost the same cost as a 166N6 which is the 4 amp 6.3v CT version most people seem to use. Likely due to cost since the hammond transformers are all pretty expensive in comparison except the 166N6 which is the cheapest option with acceptable current capabilities.

With all that said....and I'm really surprised I'm coming to this conclusion...I think I might just go with a cheap variac for now. It would cost $2 less than the parts I need to build my version and as William Vogel has mentioned it would be variable down to the volt. I really prefer building my own gear for stuff like this but I really need s variac anyways and I also would still be doing some mods to the variac to make it even more ideal for my purposes.

But you mention size as being a reason to build this project over using a variac and your absolutely correct. But I didnt plan on putting it into my Marshall's head anyways.

I'm still deciding which way to go though. Payday is Thursday and in sure I'll have decided by then. Thanks for the reply though. That specific post I hadn't seen and was absolutely helpful.
 

Michael Inglis

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How would the bucking transformer be better than the variac?
William you've made a good point. I was fully aware of a variac as an option before you mentioned it but I'm under the impression if the input voltage is low, let's say 100v, than the output could only be boosted up 10 volts if the variac is designed to be used with 120v input and 0-130v output. Because the winding number limits the amount of boost. Is this correct? But even with that said....I'm really starting to lean towards a cheap 5 or 10 amp variac. Like we talked about before, it would come in handy for a lot of future projects and I really need one. It's a matter of safety with a lot of the projects I do. Hell even this project would benefit from a variac upon initial testing so I dont fry the transformer and melt the enamel if something were wired incorrectly by accident or if the transformer was already faulty. I like to repurpose salvaged components a lot also and a variac would help by bringing them up to operating voltages slowly.

So I'm still on the fence but payday is Thursday and I'll likely have decided by then. I really appreciated your reply!
 

william vogel

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William you've made a good point. I was fully aware of a variac as an option before you mentioned it but I'm under the impression if the input voltage is low, let's say 100v, than the output could only be boosted up 10 volts if the variac is designed to be used with 120v input and 0-130v output. Because the winding number limits the amount of boost. Is this correct? But even with that said....I'm really starting to lean towards a cheap 5 or 10 amp variac. Like we talked about before, it would come in handy for a lot of future projects and I really need one. It's a matter of safety with a lot of the projects I do. Hell even this project would benefit from a variac upon initial testing so I dont fry the transformer and melt the enamel if something were wired incorrectly by accident or if the transformer was already faulty. I like to repurpose salvaged components a lot also and a variac would help by bringing them up to operating voltages slowly.

So I'm still on the fence but payday is Thursday and I'll likely have decided by then. I really appreciated your reply!
Mine is rated for a sweep of 0-140 volts with a 117 volt input. You’ll just have to look closely for it’s rating. Mine would definitely take your stated voltage range and be able to achieve 120 volt output.
 

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