Using variac always with Scott 222c

[Zach Nelson]

I heard a guy say that he always runs hit Scott through a variac to keep it at 117v. Is there any real reason for this? I understand wall sockets can be 120-125v, just curious if that was enough to damage a amp that has be restored.

 

[GD70]

I do this. Probably not necessary, knowing my Scott is always getting 117 volts just gives me peace of mind.

 

[Zach Nelson]

I do this. Probably not necessary, knowing my Scott is always getting 117 volts just gives me peace of mind.

What would happen at 120v?

 

[hogwylde09]

Power transformer and output tubes run cooler. Didn’t aim the temp gun, before and after but definitely lower temps. With 115 volt line bucker.

 

[Nyjblbob]

Runs cooler and tubes last longer

 

[drew_t]

What would happen at 120v?

Nothing worth worrying about, IMHO.

 

[Dandy]

Use a bucking transformer rather than a variac long term.

 

[StepheK]

If you are really concerned, APC makes a voltage stabilizer that holds the voltage just under 120V and can be set for 110 for gear with 110 primary.

 

[kcline]

I run this combo bucking and variac.

 

[AlTinkster92]

A Bucking transformer is your friend here, with a variac you could accidently bump the dial and increase the voltages disastrously.. Remember the variac is a transformer within itself. With a Bucking you plug it in and forget it. Hope this helps!

 

[trainbuftony]

Theres more than one way to drop the voltage and the variac is probably the most expensive and not necessarily the best.
I use a cl 80 thermistor on many of my vintage builds if the heater voltage is too high on wall voltage.
The best solution is to build a bucker.
Find an industrial electrical enclosure and put a filament transformer inside, wired as a bucking transformer, and enough outlets to run your amps. People will use filament transformers with 12v center tapped windings to drop either 6 or 12 volts. Seems like the right way to accomplish the drop..
127 volts mains becomes a harmless 115 volts when you buck 12v.
If your voltage is higher than that, you should call your electric company.
The bucker, while expensive is by far the best way to safely run your vintage amps.
Theres multiple tutorials online.

 

[Zach Nelson]

Theres more than one way to drop the voltage and the variac is probably the most expensive and not necessarily the best.
I use a cl 80 thermistor on many of my vintage builds if the heater voltage is too high on wall voltage.
The best solution is to build a bucker.
Find an industrial electrical enclosure and put a filament transformer inside, wired as a bucking transformer, and enough outlets to run your amps. People will use filament transformers with 12v center tapped windings to drop either 6 or 12 volts. Seems like the right way to accomplish the drop..
127 volts mains becomes a harmless 115 volts when you buck 12v.
If your voltage is higher than that, you should call your electric company.
The bucker, while expensive is by far the best way to safely run your vintage amps.
Theres multiple tutorials online.

I'm not so sure I have the knowledge to build one. I can do repairs and stuff but I just don't understand exactly how it works. Got any good links to a page on how to build one?

 

[trainbuftony]

https://www.tdpri.com/threads/bucking-transformer-build.494003/
This is what you want.

 

[Zach Nelson]

https://www.tdpri.com/threads/bucking-transformer-build.494003/
This is what you want.

Perfect. Thanks man.

 

[dogwan]

I just built a pair of Bucking transformers for my Dynaco MK III's. Definitely the way to go.

I used Hammond 166N6 6.3vct 4 amp filament transformers ($13.68ea. at Mouser) so the I could choose between 3.15 or 6.3 voltage drop (nominal).
With the wall voltage at 119.4 volts at test time I got a measured 4.6v and 8.6v drops under load (115.3v and 110.8v, or 3.8% and 7.2% drops respectively).
Using the upper socket (the 4.6v drop) I ran each amp for about 30 minutes and then tested the voltages. Heater voltage was sitting at 6.22vac, and B+ was sitting right at 482vdc.

Without the bucking transformer in place I was getting 6.48v on the heaters and 505vdc on the B+.
The Dynaco manual specifies 475vdc for the B+.
I noticed the power transformer ran much cooler and the output tubes were also a bit cooler and weren't as bright.

Even though the amps would tolerate the 120v (my place varies between 119v and 122.9v regularly), I am much more confident in the longevity of these old amps now.