Ok alors je pourrais utilisé un Variac avec un ampli avec une lampe redresseuse. Je dois juste l'utilisé avec plus de précaution que avec un ampli avec une rectifier SS.
Voila différentes informations que j'ai trouvé sur des forums anglais. (Citation de différente personnes)
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the_random_hero : They are sort of known because of one man - Van Halen. They can cause some issues with your amp though, so I wouldn't suggest doing too often. Keep in mind that you can't use them with any amp which has a tube rectifier.
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LEVEL4 : ''First, [Eddie Van Halen] used an Ohmite Variac, a variable transformer that could lower or raise the voltage going into the amplifier (see photo for what a typical Variac looks like). Edward set the variac to approximately 90 volts, thereby reducing the amount of input voltage going to the amplifier (see the Marshall Super Lead article for more information about variacs and attenuators) and allowing the amp to run more reliably. A key element often not considered today when running vintage Marshall amplifiers is that many that were made for export to the U.S. were designed to run at 110 volts and current U.S. outlets run at 120 volts. As a result, while there has been much talk about the dangers of using a variac, in many applications, it obviously serves a benefit.
According to Gerald Weber of Kendrick Amplifiers, Inc., in the October 2000 issue of Vintage Guitar, he states, 'You cannot harm your Marshall (or any other amp) by running it at lower-than-normal voltage. The opinions you've heard concern running the variac at higher than normal levels."
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Glide : On the Plexi forums I read where someone else purchased a Variac to run in tandem with the Power factor pro to stabilize the voltage for an old Marshall they had. He said he wouldn't play anywhere without both of them.
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Soldano16 : I've asked the question and received diff answers as to at what point the amp should be rebiased to the new voltages it will see. Also folks have mentioned that if you bias for a very low voltage and then forget your variac and plug into 125v, you might not like what happens to your amp.
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AdmiralB : Note that if your amp is correctly biased at normal wall voltage, variacing down will make them overbiased (running cold) - which won't hurt anything; however if you bias them in the reduced-voltage condition, they'll be hot when you go back to standard supply.
Lowering the voltage to some amps will totally change the character - the Komet is one example of this, it runs very high preamp voltages and variacing down loses the touch-sensitivity.
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shredaholic : The main disadvantages with using a variac are on an EL34 it'd knock a few thousand hours of the life of the tube, as can be seen via this diagram of the effects of varying heater voltage.
THATS why he(Van Halen) used the variac in the first place - to reduce the stress on the output tubes caused by the resistor, and also the reason for the 6CA7's - they were more rugged. Presumably he found he liked the tone better with the variac and decided to keep it after he ditched the power resistor.
Try sticking a bigass 1K resistor between the OT primaries of a Marshall and you'll discover Van Halen I tone right there and then - the only problem is not that the resistor caused OT mismatch (as it was placed between the OT primary, not after the amp), but that when it causes the tubes to work harder than normal you will get a lot of tube failures, thus often taking out the OT. The big power resistor and his dummy load are two different things - the power resistor massively reduces the volume, so he needed a dummy load and slave rig to INCREASE the volume for gigs.
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OlinMusic : All I can say is the effect of the Variac is pretty impressive. I really like the way it plays, feels and records. It's a nice option to have available. It's a VERY underrated component in "browning out" an amp, if you some people incl. me. It's not the only solution, but it works fast and easy.
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1973Marshall : I am a user of Variacs for 3 years.
If you want the BROWN SOUND, it will truly get you that squishy, hazy tone. It's GORGEOUS. If you set your amp (resistors, biasing, etc) it only gets better. NEVER go below 90 or you can strip the tubes. Never goose the voltage, its dangerous, ALWAYS stay between 90 & 120.
115 is ideal.
110 will show you how great vintage amps. Most vintage amps were designed around 110 wall voltages, and it will make them sound sweeter and more open and natural.
A variac can balance out bad wall voltages. Many techs feel they are unsafe esp if you use them to play with voltages. Most prefer a line regulator. I can tell you, dropping a Marshall below 110 - sayyyyyy 105 can be verrrry nice. EVH used to go between 90 and 110 as his mood hit him.
It does sweeten the tone and brown it out, give you that haze and sponge. You will hear some volume drop, but not by tons. It can allow you to get some interesting distortion at earlier points in the volume dial, but it is NOT as effective as a HOT PLATE.
The Varaic is an awesome way to regulate voltage- say 110 for vintage amps. Peter Stroud swears by them.
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FullerPast : If you want to really do it right, you need both a regulator and a variac. The regulator gets you a steady source, and the variac gives you a percentage of that steady voltage. Otherwise, as John said, you will only get a percentage of a varying source from the wall.
As for proper design voltage, amps are designed with a nominal voltage in mind, but need to be able to accomodate the full range of typical wall voltages seen, at least over a range of 10V. This generally means that vintage amps will be running hotter than they were intended, and modern amps may be set up to be safe but perhaps have less than optimum tone at say 128V.
Remember that there is about a 4x multiplication on plate voltage for every 1V change in line voltage. This can be very significant.....an amp running at 125V can have 40 or 50V more on the plates than one running at 115V. That's where the regulator and variac can make all the difference in fine tuning your tone.
A variac by itself can be dangerous if you set your voltage higher than the wall voltage and then the wall voltage drifts up. That can push the amp voltage to dangerous levels. It can also be bad in the other direction, if you set the voltage to be low and then the wall voltage drops, causing cathode stripping. That's why it's good to have a regulator in front.
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u?_Z3ro : variac for attenuation. marginal at best. think 3 dB. an attenuator or powerscaling (both is very good) is a much better solution.
certain amps respond better to browning out by using a variac than others. as mentioned above, if U wanna run an amp at 95 volts U're gonna have to bias the tubes accordingly. mostly tho', browning out an amp will result in the amp performing sub optimally. there are a few amps that will really respond nicely to lower voltages under certain conditions.
apart from the cathode stripping problems already mentioned (several times) i think that amp builders design the amp and voice it for a certain voltage and really expect the amp to be used that way. i'd say, more often than not, they are correct. older amps do sound better at 110-112 volts. some amps w/ a proper bias sound really quite nice a 95 volts. it's by no means a universal thing that browning out an amp will make it sound better. i'd say it's more the exception.
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