Variacs?

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Fastforded
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Variacs?

Post by Fastforded » Fri Apr 18, 2014 11:43 am

I'm reading this article:

http://ultimateclassicrock.com/eddie-va ... ggest-lie/

And in it EVH says:

"On the dial you could crank it up to 140 volts or down to zero. So I figured, if it’s on 220 and it’s that quiet, if I take the voltage and lower it, I wonder how low I can go and it still work. Well, it enabled me to turn my amp all the way up, save the tubes, save the wear and tear on the tubes, and play at clubs at half the volume. So, my Variac, my variable transformer was my volume knob. Too loud, [makes knob turning sound] I’d lower it down to 50."

Would this really work without damaging the amp or transformer?

Someone told me I could use a load plate(?) to raise the volume on the amp without actually making it louder but warned me that it really doesn't work the same, and that load plate will wear out the tubes faster? Maybe even hurt the transformer...

Not sure what's the deal with the load plate and the variac... Any experienced thoughts appreciated...

Thanks
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elvis
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Re: Variacs?

Post by elvis » Fri Apr 18, 2014 12:45 pm

Usually a Variac can only lower line voltage, not raise it. So I am dubious about the 140V thing. Also, running a 220VAC amp at 50V would probably sound horrible. Lowering gain and headroom would maintain tone at lower volume. Lowering only headroom would just make an amp set for flat-out gain clip like hell. It's possible that the tubes debiased, as all the internal voltages would drop ratiometrically. So maybe the preamp gain also dropped with screen voltage.

As far as tube amps, the plate voltage can work over a wide range. The amp will saturate earlier at lower plate voltage, and the efficiency will drop, but it is my understanding that the tubes and support circuitry will be OK. The one thing that could be an issue is the heaters. They generally run AC off a tranny winding. So dropping the voltage would drop the heater power and make the tubes quite cold. Heater power is V-squared/R, so the power would drop as the square of line voltage (Cutting line voltage to half would make the heater power 1/4). That will make the tubes operate poorly and could damage tube life by more than the reduced signal power would extend the tube life. Remember, the standby switch is there to let the heaters heat up fully before sending current through the tubes. Flipping on standby at the same time as power reduces tube life.

Any solid-state circuits in modern amps (opamps, digital switch controllers, MIDI, etc) could be damaged, but most likely they run off a voltage regulator and would be protected by that. However, running AC line voltage low enough could definitely brownout the solid state supply and damage digital components.

So I wouldn't try this on a late-model amp. Old-school amps will probably survive just about anything. Except raising the line voltage. The power supply capacitors and tubes are generally right at their limit for max voltage.
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Fastforded
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Re: Variacs?

Post by Fastforded » Fri Apr 18, 2014 12:58 pm

elvis wrote:Usually a Variac can only lower line voltage, not raise it. So I am dubious about the 140V thing. Also, running a 220VAC amp at 50V would probably sound horrible. Lowering gain and headroom would maintain tone at lower volume. Lowering only headroom would just make an amp set for flat-out gain clip like hell. It's possible that the tubes debiased, as all the internal voltages would drop ratiometrically. So maybe the preamp gain also dropped with screen voltage.

As far as tube amps, the plate voltage can work over a wide range. The amp will saturate earlier at lower plate voltage, and the efficiency will drop, but it is my understanding that the tubes and support circuitry will be OK. The one thing that could be an issue is the heaters. They generally run AC off a tranny winding. So dropping the voltage would drop the heater power and make the tubes quite cold. Heater power is V-squared/R, so the power would drop as the square of line voltage (Cutting line voltage to half would make the heater power 1/4). That will make the tubes operate poorly and could damage tube life by more than the reduced signal power would extend the tube life. Remember, the standby switch is there to let the heaters heat up fully before sending current through the tubes. Flipping on standby at the same time as power reduces tube life.

Any solid-state circuits in modern amps (opamps, digital switch controllers, MIDI, etc) could be damaged, but most likely they run off a voltage regulator and would be protected by that. However, running AC line voltage low enough could definitely brownout the solid state supply and damage digital components.

So I wouldn't try this on a late-model amp. Old-school amps will probably survive just about anything. Except raising the line voltage. The power supply capacitors and tubes are generally right at their limit for max voltage.
thanks!

i didnt pick up on that 140v comment in the article.. but now that you say that, it makes perfect sense.. cant get 140v from 110v...

what exactly controls headroom? I have heard the term headroom several times and thought that was the actual master volume level... IE when the master volume is on "10" there's nothing left, there's no no headroom...
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elvis
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Re: Variacs?

Post by elvis » Fri Apr 18, 2014 3:53 pm

Actually, it is possible to get 140V if the variac is an adjustable step-up transformer. But it is my understanding that mostly they are coils and not transformers.

Headroom is the difference between the max signal that the amp can handle and the actual signal. Master volume can decrease the output signal, adding headroom to the power stage, but headroom is partly determined by the size of the input signal and the preamp gain, as well as the max preamp signal.

So you may clip in the preamp, the power amp, or both. Whichever clips first set the headroom limit.
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Re: Variacs?

Post by elvis » Fri Apr 18, 2014 4:52 pm

Either way, best to check a variac with a meter before turning it all the way up.
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Re: Variacs?

Post by Fastforded » Fri Apr 18, 2014 5:57 pm

elvis wrote:Headroom is the difference between the max signal that the amp can handle and the actual signal.


what determines the maximum signal? Forgive my ignorance, would this be like using a stomp box as a Pre-amp? (increaing the signal strength?)Not sure I'm asking the question the correct way as I am thinking the pickups (exclude active/powered pickups) don't really have that high (voltage) of a signal.. And as far as the "signal" from the pickups, isn't that voltage? I understand some electronics and think when the strings vibrate and that vibration is "induced" into the pickup thru the magnets (coil?)and thats the signal that goes into the amp?
elvis wrote: Master volume can decrease the output signal, adding headroom to the power stage, but headroom is partly determined by the size of the input signal and the preamp gain, as well as the max preamp signal.

So you may clip in the preamp, the power amp, or both. Whichever clips first set the headroom limit.
??? im really confused readfing this... the gain is the preamp and the master volume is the "power stage" of the amp?

BTW....
Found this video (it's in Czech and they are using 220V but you can hear the differences changing the voltages... and i think this variac in the video is the step up transformer as its voltage is above 220V?)

http://www.youtube.com/watch?v=F2NB5bbg ... ata_player

and i saw this article: http://www.mindspring.com/~atlantatubeamp/id23.html

and as a thought, if EVH used a 220V amp and set the variac to 90V, isnt that like running a 120V amp at 49 Volts? how could that even work??

thanks, and im more confused than ever.. wish i took some electronics courses... any good (and easy to understand) articles you could recommend so i can learn about this electronic amplification
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Don
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Re: Variacs?

Post by Don » Fri Apr 18, 2014 10:01 pm

A Variac can raise voltage. Mine has a range of 0-140vac when plugged into an outlet that's at 120vac.
They are based on a ratio and that number is only accurate if the voltage that the Variac is plugged into is the same as the spec on the Variac. When I use mine, I put a meter inline with it (between the Variac and the device that's plugged into it).

The tweed mode on a Mark I reissue and other Mesas works kinda like a Variac. It's a separate primary tap on the power transformer that is similar to setting a Variac at about 90vac.

I have heard that lowering the voltage too far can be bad for the tube's heaters though I don't know if that's true.

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