What Is A Variac?
Moderator: jingle_jangle
What Is A Variac?
What is a variac and how is it used in the world of amplifier repair?
Life, as with music, often requires one to let go of the melody and listen to the rhythm
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soundmasterg
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A variac is a variable AC transformer. In the US anyway, the wall voltage is anywhere from 110v to 130v sometimes, although it is supposed to be 120v. With a variac, you plug it into the wall, and plug in an amp under test to the variac, and you can vary the AC voltage going into the amp anywhere from 0v to usually around 150v. If you have done some work on the amp, and you aren't sure if everything was done correctly, or if the amp has a preexisting problem and you are testing it, bringing up the voltage slowly will allow you to have less of a danger of blowing something. Its best to have a light bulb in series with the variac and amp so that you can see visually when there is a problem like a huge current draw due to a bad cap or something because the light bulb will light up very brightly, even when the AC voltage out of the wall is low. If there are no problems, then the light bulb just lights up minimally. Variacs have a current limit, and you don't want to exceed that with the device under test or you can blow the variac. Typically, a 5 amp variac is fine, and a 10 amp variac will pretty much cover any tube amp you are going to test. I have a 5 amp variac myself.
The popular myth is that Eddie Van Halen used a variac to get his brown sound when in reality he was joking about that in an interview to throw people off. You could use a variac to run the amp at lower voltages to get a different sound, but you also have to rebias the amp when you do this to the new set of conditions, or damage to the tubes can result. You can get variacs from places like Mouser for a couple hundred dollards, and hamfests and surplus stores sometimes sell them cheaper.
The popular myth is that Eddie Van Halen used a variac to get his brown sound when in reality he was joking about that in an interview to throw people off. You could use a variac to run the amp at lower voltages to get a different sound, but you also have to rebias the amp when you do this to the new set of conditions, or damage to the tubes can result. You can get variacs from places like Mouser for a couple hundred dollards, and hamfests and surplus stores sometimes sell them cheaper.
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sinisterick
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Disclaimer: some of what follows may not be completely true and I defer to soundmasterg on this. I'm pretty new to the forum but I sense he really knows his stuff. Hopefully he will correct anything wrong here.
Things I've read and/or heard about variacs
Variacs (also called an autotransformers) - good for that old amp you just discovered that probably hasn't been turned on for years. Less of a shock to gradually go from 0 volts to 120 volts (or whatever amount) than to do it in a millisecond by just plugging into the wall and flipping the power switch. While turning it up you can watch the amp for warning signs of problems and "catch it" before you take out a lot of circuitry. Of course, the safest thing you can do with that amp would be to not turn it on and bring to a qualified tech. Finally, I've heard that tube amps have no problems with variacs but they're not recommended for transistor amps.
Things I've read and/or heard about variacs
Variacs (also called an autotransformers) - good for that old amp you just discovered that probably hasn't been turned on for years. Less of a shock to gradually go from 0 volts to 120 volts (or whatever amount) than to do it in a millisecond by just plugging into the wall and flipping the power switch. While turning it up you can watch the amp for warning signs of problems and "catch it" before you take out a lot of circuitry. Of course, the safest thing you can do with that amp would be to not turn it on and bring to a qualified tech. Finally, I've heard that tube amps have no problems with variacs but they're not recommended for transistor amps.
No, I don't know where Costello is.
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soundmasterg
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Pretty much most of what you said is fine except the last part Abbott. I'll elaborate a bit.
An old amp most likely has old electrolytic caps, and if it hasn't been used, these caps leak their electrolyte out, or the caps stop filtering the AC ripple to ground so the amp hums, or the caps explode when power is applied to them. A variac will slow things down and if you go up on the voltage slow enough, (10 volts an hour) then the caps may reform enough to where the amp will function ok for awhile.
Best bet though is to replace any old electrolytics in an old amp before you even power it up. Since they have to be replaced anyway, why not do it before you power the amp up and risk blowing out tubes or power transformers? Of course if you aren't sure about your own abilities when working with high voltage, then take the amp to a qualified tech for this. Also, if you are running the amp up slowly on a variac and you have a bad cap, you don't have much time before it explodes from when it first starts showing problem signs. If you aren't very experienced with this, then you'll blow some caps up and scare the hell out of yourself and make a big mess and possibly damage the amp.
If you think about how tubes operate, you can see where running a tube amp on a variac without adjusting the bias for the new conditions will cause problems. Say you build an amp, and decide on a bias level. This is with a normal AC wall voltage of 125 volts. Now you take a variac and move the AC wall voltage down to 100 volts. The DC voltage on the tubes doesn't track evenly with the AC, and what used to be the correct bias is now nowhere near correct, and so your tubes are operating either very close to cutoff, which sounds terrible, or very close to burning up, which sounds great but ruins tubes quickly. If you reset the bias at this new operating point, then the amp will run fine, as long as the heater voltage is close to the 6.3v required to heat the tubes. If you deviate too far from that up or down, then you run into other problems. A cathode biased amp will self adjust its bias voltage to the tubes, so your main concern with those would be the heater voltage when running them on a variac. If you're just testing an amp and aren't going to run it very long at a reduced heater voltage, then it isn't as big of a concern.
With a transistor amp, I don't have a whole lot of experience yet, but I would guess that since transistors are very easy to overload, running them low or high on a variac wouldn't be recommended. Sometimes transistor amps have a regulated power supply, so this would give a bit of a safety margin, but still it is probably not a good idea. Maybe Charlie can comment on this part since my experience isn't with transistors?
An old amp most likely has old electrolytic caps, and if it hasn't been used, these caps leak their electrolyte out, or the caps stop filtering the AC ripple to ground so the amp hums, or the caps explode when power is applied to them. A variac will slow things down and if you go up on the voltage slow enough, (10 volts an hour) then the caps may reform enough to where the amp will function ok for awhile.
Best bet though is to replace any old electrolytics in an old amp before you even power it up. Since they have to be replaced anyway, why not do it before you power the amp up and risk blowing out tubes or power transformers? Of course if you aren't sure about your own abilities when working with high voltage, then take the amp to a qualified tech for this. Also, if you are running the amp up slowly on a variac and you have a bad cap, you don't have much time before it explodes from when it first starts showing problem signs. If you aren't very experienced with this, then you'll blow some caps up and scare the hell out of yourself and make a big mess and possibly damage the amp.
If you think about how tubes operate, you can see where running a tube amp on a variac without adjusting the bias for the new conditions will cause problems. Say you build an amp, and decide on a bias level. This is with a normal AC wall voltage of 125 volts. Now you take a variac and move the AC wall voltage down to 100 volts. The DC voltage on the tubes doesn't track evenly with the AC, and what used to be the correct bias is now nowhere near correct, and so your tubes are operating either very close to cutoff, which sounds terrible, or very close to burning up, which sounds great but ruins tubes quickly. If you reset the bias at this new operating point, then the amp will run fine, as long as the heater voltage is close to the 6.3v required to heat the tubes. If you deviate too far from that up or down, then you run into other problems. A cathode biased amp will self adjust its bias voltage to the tubes, so your main concern with those would be the heater voltage when running them on a variac. If you're just testing an amp and aren't going to run it very long at a reduced heater voltage, then it isn't as big of a concern.
With a transistor amp, I don't have a whole lot of experience yet, but I would guess that since transistors are very easy to overload, running them low or high on a variac wouldn't be recommended. Sometimes transistor amps have a regulated power supply, so this would give a bit of a safety margin, but still it is probably not a good idea. Maybe Charlie can comment on this part since my experience isn't with transistors?
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soundmasterg
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Yes, most variacs will have a fuse, but if you have enough of an overload, it could damage the variac before the fuse blows, so I still wouldn't over current the variac if possible. One of those things that makes perfect sense when working with amps is to be overly cautious and safe, because the alternative is either damaged gear or damaged bodies!
The only drawback I have heard with slowly powering up amps and radios etc. on variacs is that it stresses the power transformers and tubes. I don't know how true this is and have done this before but I know some radio experts who won't do it because of this reason. One thing that could be done is to pull all the tubes which would decrease the load on the PT's and the tube problem would be a moot point.
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soundmasterg
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If you pull the tubes, then there wouldn't be any stress on the transformers either since there would be hardly any current. The only problem with pulling all the tubes and firing up the amp would occur when full AC voltage is applied. Some of the power supply caps could then be over voltage. If the amp was designed properly and the power transformer is the correct size for the circuit it is supplying, then testing it with the tubes in circuit shouldn't be too much of a problem when running at lower voltages.
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sinisterick
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karl_teten
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Abbott, you post reminded me of a funny memory.
I lived in the Middle East during the 70's. Saudi Arabia in 1976 and Turkey in 1977. My parents purchased home appliances in the States and brought them over to Turkey while we were there. The fridge and washer/dryer worked fine but it was the record player that played slightly slower than standard.
Our LP's sounded like someone dragging their finger on the turntable. He he he he he
I lived in the Middle East during the 70's. Saudi Arabia in 1976 and Turkey in 1977. My parents purchased home appliances in the States and brought them over to Turkey while we were there. The fridge and washer/dryer worked fine but it was the record player that played slightly slower than standard.
Our LP's sounded like someone dragging their finger on the turntable. He he he he he