variac?

[Luke]

hi everyone,
Has anyone ever made their own variac? I was thibking about trying to make one- is it hard (and dangerous)? are the schems out for them?
thanks,
Luke

[Ansil]

how low do you want to go.  geofex has a cool article on droppin voltage for oldermodel amps.. i will see if ican find the thread.  ultimately if you drop it too low for too long it can fry the tubes..  but hey  tone is tone.
http://www.geofex.com/Article_Folders/vintvolt/vintvolt.htm


i am working on a verision of this for that 90v eddie vanhalen sound.

[Joe Davisson]

You must use an isolation transformer before a variac. Without it, the variac is connected directly to the mains, which is extra-dangerous to you and your equipment. Don't even try it, trust me. Variacs don't enjoy the iron-core isolation of regular transformers, meaning if you touch the ouput you are directly connected to the mains.

If it's even worth bothering with, make sure the variac is rated for higher amperage than the amplifier, and that the isolation transformer is rated higher than the amplifier. For example: 10A isolation -> 6A variac -> 4A amplifier. In this case, putting 4A fast-blow fuses between the mains, isolation, and variac will protect the transformers.

One time I blew up a decent variac that I had connected directly to the mains, following a bad schematic in a book. I've since learned to get a second opinion when dealing with high-powered circuits. While it really was neat watching the fire and smoke work it's way around the coil, the nauseating burnt lacquer smell lingered for WAY too long. Anyway...

-Joe

[Ansil]

so is RG'S scheme wrong joe.

[Joe Davisson]

No, that's not a variac circuit! That uses a regular transformer (note the double-coils with the core in-between.) Looks like a really great idea. Pretty nifty Mr. Keen!!

A variac symbol looks like a single coil with a moveable tap, like a potentiometer...

-Joe

[Peter Snowberg]

Three words.... Don't try it.

You can't just "make" a variac. You need a BIG torroidal core and there is no room for error in the winding. Even if you get past that part, making the commutator would be a major pain in the butt. I've repaired a lot of them over the years and they're somewhat precision devices.

Now using them gets into other issues. They make the absolute best light dimmers you can even get and they produce Zero hum in the process. They're great for slowly powering up something on the test bench. However, they're a really bad idea to use with amps.

If you want to drop the voltage to the original device specs, R.G.'s transformer article is great and will do the job perfectly.

If you use a variac to ajdust you amp voltage, you are guaranteeing that your tubes will be incorrectly biased.

The tubes will die early, and they could take a transformer with them if your amp is not fused, depending on the variac setting (you can wire most of them to boost the mains voltage by 15%).

If you want to drop the voltage to get a "browner" sound, replace the power transformer and re-bias ALL the tubes accordingly. There is no free lunch unfortunately.

Take care,
-Peter

[Luke]

thanks everyone,
I think I will leave it alone. I was just thinking about getting more power amp distortion from my Marshall JCM900, without exessive volume. I guess its not really possible.
Thanks again guys,
luke

[Peter Snowberg]

If you want to drop your output section voltage, you can use a big zener diode to drop off 50-100 volts (be sure to heat sink it and use an isolated mounting), but you have to re-bias after you add it. You can also put a high quality DPDT switch in there to selectively short out the zener and select between two bias pots. A high current rotary switch will allow you to choose even more if you use multiple zeners and a bias pot for each position.

There's almost always a way. ...maybe not with continuously variable parameters, but don't give up hope. If you want the effect, it can be done.

Take care,
-Peter

[Ansil]

aparnetly we can do it with the way shown to us by RG and Peter  thanksx!!!:)

[Andy]

I've got this variac for trouble-shooting purposes.  It's HUGE and HEAVY.  I have never used it.  It was given to me.  What does putting the voltage lower do for the tubes?  Make them work harder and sound hotter?

[Ansil]

dropping the voltage lowers headroom.

[Peter Snowberg]

What does putting the voltage lower do for the tubes?  Make them work harder and sound hotter?

Actually, the tubes won't work as hard, so with proper bias they should last longer. The sound is.... well.... browner :?. The breakup is earlier and to get really subjective, the resulting response is "slower". Sorry for the audiophoolery term there. The distortion gets "chunkier" in the examples I've heard.

Picture big industrial machinery in a lawn chair.

Take care,
-Peter