I was sick of my houses line voltage being way too low in summer so....(With Pictures)

Michael Inglis

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Pictures at the bottom...


The line voltage in my house swings dramatically throughout the year and that means im constantly rebiasing my power tubes in a futile attempt to keep them in the ideal bias range. So i was going to buy a 10v center tap transformer and build a "voltage stabilizer". My design would have allowed me to boost or buck the voltage by 5 and 10 volts (so it had a boost or buck switch and a 5 or 10 volt switch). But that really would only get me in the ballpark so i abandoned that idea and just got a $50 variac online and did some mods.

The variac/auto transformer is a 110v input, 500VA model. So it can handle a little under 5 amps which is perfect for my amp and effects pedals. It can boost by about 22 volts which is perfect since during the summer my line voltage gets down to 100 volts and other parts of the year as high as 130 volts. The analog meter/display was surprisingly accurate but not precise enough when the goal is to get to within plus or minus 1 or 2 volts of a given voltage. Not to mention its actually really helpful to be able to see how much current is being drawn by the load after the variac. So i put a digital meter display in its place. The new display can measure Voltage, Current and power/Watts.

Next up im going to change the on-off switch (all i had on hand) for a on-off-on so i can have the 3 positions as *variac on, *everything off, *line voltage (and the line voltage position will still be monitored by the display so ill know if I need to turn on the variac). I also have to get another fuse holder cause they decided to glue the nut to the threads on the stock one. But thats not a big deal since they are cheap. I just soldered in the fuse and put heat shrink over it until the fuse holder i ordered gets here. And that will likely go in-between the input plug and output jack

So if anyone is dealing with similar issues i highly recommend this setup. Its ridiculous how excited i am to know my amp will always see 120v from now on lol. I already installed an optional choke in the amp (with a switch to select the choke or the stock resistor) and my next project is to pick up two millivolt displays and install them in a 1590BB that will connect to the bias test points in the amp via a TRS jack that i will mount on the back of the amp where the other jacks are. That way i can always see exactly where the bias is set at on each power tube pair. Is it necessary? For fucks sake of course not! Lol. But knowing i can monitor all that and keep it in the ideal dissipation range is something i find very satisfying.


Before mods-

Variac Before Mods.gif


After Mods-

Variac with Mods.jpg
Variac Demo.jpg
 
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Michael Inglis

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@Michael Inglis ,
Nice job! Where did you source the digital meter?
TIA,
Gene

Believe it or not i just got it off Amazon for about $10-$15. I found out after i bought it that its the meter display most guys recommend for this type of project so i got lucky (although i was a little disappointed to find that im not the first person to think of the idea lol). The only downside is that these type of AC meter displays run off the power they are reading so they are technically only capable of reading 80v-260v. Which isnt ideal if the goal is to use it with a variac that goes all the way to zero. But i was able to find a modification that allows the meter to read all the way down to 0v very accurately.

If you are still interested the part number for the meter is "PZEM-061". I hope that helps!
 

Gutch220

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why can't those meters get it's power PRE-variac adjustment? Maybe you can wire it that way.
I have a similar variac with the digital voltage display. I don't really need to know the other three things.
 

william vogel

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I’d suggest that if your voltage fluctuation is 100-130 volts, you remove the cover from the main panel and check the feed from the meter. Fluctuation is common but being as high as 130 would suggest you’re close to a substation and low as 100 far away. It’s not common to have that much voltage drop. This could be an indication of high resistance in the circuitry. I’m glad you got the variac to make a stable voltage but many of your household appliances may be unhappy with the voltage also.
 

Pete Farrington

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Yes, contact your power supply company, that amount of variance may be exceeding the limits.
Over at the Hoffmann forum, sluckey had a thread with a fix for the feed to the meter.
 

junk notes

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Unknowingly and unfortunately, the factory where we played at had extremely high voltages, and eventually fried my Plexi. never again!
At the time I was recommended a power conditioner/voltage regulator costing up to five - six hundred dollars, but I went with a (dual) variac setup (ala EVH) and is now my protection and insurance that it will not happen twice! I modded the variac as you did with a small and simple digital voltage meter display to visually monitor the incoming wall voltage.

(The older amps were rated at 110v-115v running off the 120v wall voltages.)

This appears to be a good plug and play idea for a variac option IMO, but not cheap.
He is seemingly excited about this revelation..


 

AtomicRob

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I have a variac and also built a "vintage voltage adapter" which I use for when I'm not playing at home. The variac is too big and too risky of getting bumped to a different setting to use at a gig...

The vintage voltage adapter just has two levels, -6 and -12 (which are really -5% or -10%) but of course if you needed more precision you could add another bucking transformer in series with smaller secondaries, like 1.5/3v. Then with two switches you could dial in 1.5, 3, 6, 7.5, 9, 12, 13.5, 15. And if you added another switch you could make it + or -. Or you could use a multi-tap transformer with all the values you wanted, although those are a little hard to find.

Of course the really boss move here would be to use relays instead of switches, and control those relays with a little Pic or Arduino which watches the incoming voltage and adjusts the relays to achieve the desired output voltage. Which is basically what cheaper commercial AVR units do, although I think those usually have a custom transformer with multiple secondary taps as opposed to a boost or bucking configuration. But it's just switching taps to maintain a desired voltage range. (edit - not mechanical relays - probably something like an scr or ssr)

I'm fairly experienced with microcontroller programming, I'm tempted to build one as proof of concept even though I a) don't need it, andb) don't have time to build it! But it's also possible to buy them for <$100 so if you really needed something like that look for "automatic voltage regulator" on amazon or wherever, there are tons of them.
 
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