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Thread: Using a variac for n00bs

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    Using a variac for n00bs

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    Hello, everyone.


    I have one of those newesque Fender EVH 5150III 50W heads and I'm really digging the sound with the loadbox I'm using. Now, I also want to get a variac. Not exactly for lowering volume but because I know you can fool around with the amp tone by lowering its voltage. I think it gets a neat sound by going around 10V lower than the specified voltage from the clips I've listened to. The problem is I don't know anything about electrical engineering or theory. There's loads of variacs on ebay and the cost range is from around 40 to 500. How do I know which one to get? Are they all the same? Why the huge cost difference? How safe is it for the amp? Oh, and I live in Europe so I need a different voltage from the US.

    Any info is really appreciated, thanks.

    PS. I did read all the old variac topics but I didn't really get any definite answers on how to go about this. I NEED to know.

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    What will help your tone is running the design voltage. A variac can help with that

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    Greetings Keryi,
    Regarding the Variac, I come at it from a linear TV repair tech angle. It doubles as an ISOLATION transformer. By isolation I mean that the variac completely separates Line noise, transient hum, and other nasties from entering your equipment. Nasties like the voltage spike from your refrigerator/freezer compressor, water pumps, air conditioners, even an aquarium pump !
    The Variac also gives a smooth constant voltage for your equipment. Your tone will be electronically BEST with the proper and constant voltage.
    That said, you should know that lowering the voltage below optimal operating levels to change the tones forces ALL the circuits to strain and work harder to produce the sound it was designed for. This strain can produce neet but usually unwanted noise... This strain WILL also shorten the life of your amp due to the strain of components functioning beyond their designed capacities... How much will it shorten the life depends on the quality of the individual parts within the circuits.
    WARNING: !! If you go ABOVE the operating voltage of your amp you WILL fry it ! This goes double if you are using digital equipment.
    My best thought is that varying the voltage is a shortcut to disaster.

    -Analog guy in a digital world-

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    Beyond that, many don't know what the service is feeding the area for voltage. You don't, particularly, want 120v at your 110v amp, either

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    I messed with one on my Twin.. [> years.. YEARS ago-before-I knew squat /<].
    Anyway, this makes me curious now, if Mesa's Mk V Variac mode reduces voltage to the whole amp? Or maybe just in places in the circuits? I'll have to ask sometime.
    I haven't gotten into the feature ('soft and spongy' isn't where I'm at 90% of time, plus it being global' isn't applicable as a 'one ch effect.

    I do know it needs to be large enough to safely dissipate the power attached.
    Placebo stomps 96k ....... Recent projects
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    Variacs DO NOT give isolation!!! They are variable 'Auto-transformers' and there are two dangers to your amplifier.

    1) Yes, overvolts. UK mains is nominally 230V but almost everyone gets 240 and a bit. Unless you meter the output (dangerous) you have no idea what the variac is putting out and if mains is at the allowed +10% 253 gets stepped up to transformer saturating volts!

    2) Under running valve heaters, especially power valve heaters leads to their early demise.

    Then, most of the cheaper variacs are not enclosed and therefore lethal voltages are exposed. You can buy metal enclosures but then the whole thing would need PAT testing.

    You need to know the 'VA' rating of the amplifier and then get a variac of about 20% above that. If you don't understand the concept of AC 'Volt/Amperes' don't mess with this.

    You don't need any kind of mains filtering or surge protection in the UK unless you live on a remote farm and Bros' MIGs carts in the barn.

    Variacs DO NOT regulate the supply (if anything they ADD impedance!) That needs a Constant Voltage Transformer although I would guess the job is done these days by double conversion inverters?

    Dave.
    Last edited by ecc83; 1 Week Ago at 22:46. Reason: one Joe, no brekky yet

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    2) Under running valve heaters, especially power valve heaters leads to their early demise.
    Coincidentally I happened to get an answer for the question I asked from the Mesa forum re the Mk V '90 volt 'variac mode. Apparently it does not reduce voltage to all the circuits.
    From Bandit over there.

    Variac mode feeds power to the PT on a different tap of the primary side, thus it will affect everything on the secondary side of the power transformer. However most of the accessory circuits for the switching logic control and the sensitive preamp tube heater circuits that are DC will remain consistent as they are regulated so there is plenty of headroom on those circuits to maintain consistent operation at full power or variac power.
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  9. #8
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    Quote Originally Posted by mixsit View Post
    Coincidentally I happened to get an answer for the question I asked from the Mesa forum re the Mk V '90 volt 'variac mode. Apparently it does not reduce voltage to all the circuits.
    From Bandit over there.

    Variac mode feeds power to the PT on a different tap of the primary side, thus it will affect everything on the secondary side of the power transformer. However most of the accessory circuits for the switching logic control and the sensitive preamp tube heater circuits that are DC will remain consistent as they are regulated so there is plenty of headroom on those circuits to maintain consistent operation at full power or variac power.
    In that particular case it is ok (but I would like a specific model number and a schematic if possible) . Most of the Blackstar amp range have DC, regulated pre amp heaters but there is the further problem that if you reduce the AC mains input too much the heater regulator will 'drop out' and you will have no heaters at all!

    It is also very unusual to DC heat power valves. The current requirement is much higher and there is no hum reduction advantage.

    There still remains the issue of 'pinkies' in un cased variacs.

    Dave.

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    Something like this I would drop the voltage on the phase splitter and plate supply with adding resistors in series and a dpdt switch that shorts the resistors across to defeat the low voltage mode.

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