# Variac



## Business (Jul 30, 2013)

I just got an email from Jim Kelley (not Kelly the football player) telling me most amps "back then" were designed with a line voltage of 110V in mind. He told me using a variable transformer (variac) of 8-10A was recommended with his older amps, and probably many amps from that period. Anybody use a variac? Can anyone recommend a specific brand or model? 

Cheers


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## ronmac (Sep 22, 2006)

Variacs are adjustable voltage devices, and quite expensive and bulky, usually used for test and repair. A power transformer would be more practical to recreate a fixed voltage at the output.


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## YellowBrick (Mar 7, 2015)

I agree with Ron. You don't need the variable part of the variac. Look for something like this instead http://www.amazon.com/VCT-VT-500J-Japanese-Transformer-Converts/dp/B000PC4JL4


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## Business (Jul 30, 2013)

Wouldn't voltage from a variac fluctuate less than with a regular transformer?

I was eying this model, which isn't that expensive:

http://www.ebay.ca/itm/3-kVA-VARIAC...632?pt=LH_DefaultDomain_0&hash=item487c2dfba0

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Ocean7 said:


> I agree with Ron. You don't need the variable part of the variac. Look for something like this instead http://www.amazon.com/VCT-VT-500J-Japanese-Transformer-Converts/dp/B000PC4JL4


I actually have one of those; it converts from ±117 to 100, not 110
Plus, it's only about 5A, which isn't enough for an amp


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## greco (Jul 15, 2007)

If you decide on a Variac...this thread might be of some help/interest:

http://www.audiokarma.org/forums/showthread.php?t=287117

One that shows a digital readout of AC amps and volts would be very cool.










I had one and gave it to an amp repair fellow locally. 
It was a gift to me from an estate but I never used it. 

BTW...These little guys weigh more than you might imagine.

Please keep us updated...I'm interested.

Cheers

Dave

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Business said:


> Wouldn't voltage from a variac fluctuate less than with a regular transformer?


I hope someone answers this question as I am wondering the same....and I also suffer from extreme curiosity...LOL.

Cheers

Dave


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## Louis (Apr 26, 2008)

I had a $100 Chinese Variac with my Blankenship Variplex
and a high end version would have been better imo


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## YellowBrick (Mar 7, 2015)

Business said:


> Wouldn't voltage from a variac fluctuate less than with a regular transformer?
> 
> I was eying this model, which isn't that expensive:
> 
> ...


A variac is not a very accurate device to start with. You have to dial the correct output voltage first (don't rely on the sometimes included voltmeter, read with a proper DMM) and then you have to make sure nobody messes up with the adjustment while you are away. They are also bulkier and way heavier than they look. If you gig with your amp it's going to be a pain to carry around (and remember to adjust the output voltage again when you are all setup on stage).

5A is not enough? What kind of amp do you have? A 100W Marshall draws about 3.5A tops iirc.

How does your amp sound with that 120-100V transformer?


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## ronmac (Sep 22, 2006)

A variac is a suitable device for the workbench, but I don't recommend it for a live device. They have the ability to change the voltage available in a wide range (typically 0~140). Having the setting changed in error could have disastrous outcome.

Why not change the power transformer to achieve the target output voltages.


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## Business (Jul 30, 2013)

According to what I'm reading, the variac should be able to deliver twice the current the amp draws (to be on the safe side)
I don't even want to try the 120-100V converter.

I'm not looking to gig with it, only use at home with vintage amps (changing the power transformers in those is out of the question)
I'm not interested in the whole "Van Halen" dropping voltages shenanigans, I just want to provide 110V to those amps, as apparently it's better for the filter caps (according to Mr. Kelley). And the original Kelley amps have really high plate voltages, which added to today's cheap quality 6V6 tubes isn't a good combination. Lower line voltage = lower plate voltage, as I understand it


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## jb welder (Sep 14, 2010)

Business said:


> Wouldn't voltage from a variac fluctuate less than with a regular transformer?


No it would not, it is not a regulator. To regulate AC line voltage, units cost around $1000. But you shouldn't really need regulation, our voltage is usually good to +/- 5% which is pretty reasonable.
For home use, when it's all said and done a variac is a fairly simple and inexpensive option. You can also build or buy something like a "vintage voltage adapter" either into the amp or as a stand-alone box. It is a "bucking transformer" that reduces the line voltage to some preset amount, or may have switchable options.
Here's a bunch of links to articles and discussions of various options.
http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm
http://ctgelectronics.weebly.com/cruncher---line-voltage-reducer.html
http://sound.westhost.com/articles/buck-xfmr.htm#s30
http://shop.vintagesoundworkbench.com/
http://www.guitarscanada.com/showthread.php?50585-Voltage-regulator-Tone-Preserver


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## Moosehead (Jan 6, 2011)

Business said:


> According to what I'm reading, the variac should be able to deliver twice the current the amp draws (to be on the safe side)


I think you are confusing voltage and current. 

The wall socket delivers 15 amps. Over 4 times needed according to the previous example (100w marshall draws 3.5A). The amp will only use as much current as it needs. Surplus current is left unused for other devices on the circuit.

From what you are saying you want a decrease in voltage, plenty of examples have already been given. 

Im no vintage expert but ive played through some 60's/70's amps and a 50's fender champ all plugged into whatever receptacle was nearby. I couldnt tell ya if there was a 10 volt difference but I can tell you that 50's champ sounded fricken amazing at 120v.

I think you're chasing your tail on this one.


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## Fader (Mar 10, 2009)

If you don't mind doing a mod to your amp, you may want to consider a Variable Voltage Regulator.

http://guitarkitbuilder.com/install-variable-voltage-regulator-guitar-amplifier/


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## Business (Jul 30, 2013)

Moosehead said:


> I think you are confusing voltage and current.
> 
> The wall socket delivers 15 amps. Over 4 times needed according to the previous example (100w marshall draws 3.5A). The amp will only use as much current as it needs. Surplus current is left unused for other devices on the circuit.
> 
> ...


No confusion here; there is a current surge when powering an amp up, a 5A variac actually offers 5A at 100V, but 4.54A at 110V
Power up an amp requiring at lot of current, and there goes your Variac fuse. 

As far as an original Jim Kelley amp goes, the plate voltage is ±450V, which is enormous for a 6V6, also considering that newer 6V6 tubes can't handle that type of voltage. A variac will lower that, making possible tube failure less of a problem


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## Fader (Mar 10, 2009)

A variac will also lower your filament voltage. The VVR lowers the B+ only.


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## YellowBrick (Mar 7, 2015)

So what's the current draw of your amp? Do you know it? Or the fuse rating as a hint?

Why don't you want to try that 120-100V transformer that you have?


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## Business (Jul 30, 2013)

Ocean7 said:


> So what's the current draw of your amp? Do you know it? Or the fuse rating as a hint?
> 
> Why don't you want to try that 120-100V transformer that you have?


The amp uses a 3A Slo-Blo fuse; you can find info about the amps here:

http://www.jimkelleyamplifiers.com/docs.htm

I don't really want to drop the voltage as low as 100V. 
I trust the transformer to work with my Japanese DJ Mixer, but I wouldn't use it with an amp


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## greco (Jul 15, 2007)

WOW! ...this thread has certainly become much more complicated and confusing (to me) than I expected.

jbwelder's links were interesting and very informative/educational.
It was fun (and helpful) to read the the GC forum thread from 2012.

Cheers

Dave


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## jb welder (Sep 14, 2010)

Fader said:


> A variac will also lower your filament voltage. The VVR lowers the B+ only.


This is part of the problem that needs to be solved. A power transformer that is designed for 110 or 115V will have proper heater voltage (6.3V) when the line voltage is vintage level. At modern line voltage (125V here) the heater voltage goes too high. This reduces tube life, adds heat, and adds extra load on the heater winding of the power transformer.
VVR is good for reducing plate voltage, but may not help reduce the voltage on the filter caps, and will not reduce heater voltage, which is sometimes required.


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## Business (Jul 30, 2013)

So I just bought this one:

http://www.ebay.ca/itm/3-kVA-VARIAC...632?pt=LH_DefaultDomain_0&hash=item487c2dfba0

Total cost of 183$ shipped from Hamilton

I'll get back to you folks when it gets here


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## jb welder (Sep 14, 2010)

Nice. That should be bigger than you'll ever need, a wall outlet will only put out 2.4kva on a 20A breaker, only 1.8 if the breaker is 15A.
Don't go by the voltage meter unless you verify it is highly accurate under various line voltage conditions.


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## Business (Jul 30, 2013)

jb welder said:


> Nice. That should be bigger than you'll ever need, a wall outlet will only put out 2.4kva on a 20A breaker, only 1.8 if the breaker is 15A.
> Don't go by the voltage meter unless you verify it is highly accurate under various line voltage conditions.


Thanks for all the info you've provided in this thread
I'll set the voltage on this one with a DMM at 110V, then won't mess around with it unless I change the outlet


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