# Variac



## J-75 (Jul 29, 2010)

I need to get a variac. What current rating would you suggest - is 4 A. enough, or should I go higher?
Cost is not a big issue, more size & weight.


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## jb welder (Sep 14, 2010)

You mentioned size and weight so I'm wondering what you will be using it for.
Generally they are for bringing up a faulty amp with low power till the fault is found. Testing of a working amp into a load is done without the variac so you don't need high current available for that.
Something like a Twin or other 100W amp is usually fused at around 4A, and would only be using near that much at full power, so I think for troubleshooting purposes the 4A variac would be fine.
All that being said, the unit I have is 10A.


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## smorgdonkey (Jun 23, 2008)

...and if you are doing it for the Van Halen thing...wasn't that just a can of malarkey to throw people off?


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## Wild Bill (May 3, 2006)

smorgdonkey said:


> ...and if you are doing it for the Van Halen thing...wasn't that just a can of malarkey to throw people off?


Yes, it was!

It's another case of a little knowledge can be a dangerous thing. Someone hears how a variac will raise or lower the line voltage fed into the amp. They also hear from some self-appointed internet amp guru that this will raise or lower the plate voltage, changing the tone of the amp.

This in itself is quite true! A lower plate voltage on the EL34s will give more of Eddie's "brown sound".

However, if one bothers to actually READ some textbooks and STUDY how things work they learn that the variac is changing the voltage into the power transformer of the amp BUT THAT POWER TRANSFORMER DELIVERS MORE THAN JUST THE HIGH VOLTAGE FOR THE RECTIFIER!

It also delivers the filament voltage to light up the tubes! The tubes are designed to run on 6.3 vac. The variac will also change this voltage. If someone digs deep enough they will find curves showing how tube life is affected by filament voltage. If the voltage is too high or low the tube life falls off. Only a little bit will DRASTICALLY reduce your tube life!

Guys like London Power offer circuit kits that will allow changing ONLY the plate voltage! You do have to know what you are doing to install it in your amp, however. If you go this route, make sure the tech you choose knows the difference between resistance and reactance.

Wild Bill / Busen Amps


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## mhammer (Nov 30, 2007)

This only serves to illustrate, for me, why one should generally be sceptical of musicians' "explanations" of how their gear does what it does. Such explanations can be woefully incomplete. Not because they are trying to preserve trade secrets, or anything like that. Rather, their strengths lie in playing their damn instrument, not in teaching electronics to people who will need to understand something well enough to be certified professionals. Once in a while you'll run into someone who _does_ understand, and _can_ explain, in fulsome fashion (I was knocked out by Brad Paisley's grasp of his gear), but they are the rare exception.


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## gtone (Nov 1, 2009)

If you have excessive line voltage in your home, rehearsal space or where you gig, a variac can be useful to bring that down a bit to protect valuable vintage amps/gear. Most of the older stuff was designed to operate between 110-120V and a little trip around your home with a DMM, fr'instance, you may find readings of 118-125V or even higher typical today. There may be a huge difference in component life between say 126V and 113V with some gear...


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## mhammer (Nov 30, 2007)

...Which is why there is a market for "power conditioners".


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## jb welder (Sep 14, 2010)

Power conditioners are great for cleaning up the AC and spike/surge suppression, but generally don't offer voltage regulation unless you get into the very expensive ones (unless things have changed recently).
For vintage gear that you need to keep the heater voltage right (and B+), a variac or installing a bucking transformer are probably the most economical options.


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## mhammer (Nov 30, 2007)

But do variacs provide a precise _absolute_ output voltage, or do they simply provide a relative/divided voltage, predicated on a_ presumed _wall voltage?


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## jb welder (Sep 14, 2010)

Your suspicions are correct, the variac gives a percentage of the actual wall voltage (up to around 120% I believe).
So you do have to measure and adjust for every scenario, and hope the wall voltage doesn't change much during each use.
To get and maintain an absolute line voltage would require an AC voltage regulator, like a furman AR-1215 @ ballpark $500, but they do not have adjustable output, you get 120V.
So to get an AC regulator with a vintage 115V out, or adjustable output, you would need to spend even more.


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## Wild Bill (May 3, 2006)

mhammer said:


> But do variacs provide a precise _absolute_ output voltage, or do they simply provide a relative/divided voltage, predicated on a_ presumed _wall voltage?


Mike, they provide from 0 volts to the original supplied voltage. It's expected that you will put a meter on things to see the actual output voltage.

Basically, it's just a big transformer winding with a tap that slides across that winding, changing the turns ratio. Also, it is an autotransformer - it has only one winding. There is no isolation.

Wild Bill


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## mhammer (Nov 30, 2007)

Wild Bill said:


> Mike, they provide from 0 volts to the original supplied voltage. I*t's expected that you will put a meter on things to see the actual output voltage.*
> 
> Basically, it's just a big transformer winding with a tap that slides across that winding, changing the turns ratio. Also, it is an autotransformer - it has only one winding. There is no isolation.
> 
> Wild Bill


And that's likely the part that enthusiastic hero-emulators might skip.

It's not so much the information that IS provided on the net, but the information that is too often omitted.

Mark


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## jb welder (Sep 14, 2010)

I'm still waiting to hear what J-75's intended use for the darned thing is :smile-new:.


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## J-75 (Jul 29, 2010)

Sorry guys, been away for a while.
My intention was to provide isolation on my bench - for safety.
(I had no idea that there was a stage/performance side to these devices.)
So, I gather that if they only have a single winding, they will not offer what I'm looking for.
I have to find a 1:1 isolation transformer instead.

Anyone have recommendations - what do you use?


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## jb welder (Sep 14, 2010)

Not sure what the safety issue is here, chassis should stay grounded for safety. When they used to have hot chassis TV's is when guys used iso trannies for safety.
Nowadays you would only really need one if you want to work on the primary side of a SMPS (switch mode power supply).
Personally, I don't work on SMPS's (or hot chassis or widow-maker amps), so I don't have or need an isolation transformer.


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