Poor man's Variac - a light dimmer?

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Poor man's Variac - a light dimmer?

Postby Writer Frog » Tue Jul 07, 2009 9:08 pm

I got a few old vacuum tube amplifiers of unknown condition and origin.
I like to power up these things slowly and gently at first to see if they work at all. One problem. I haven't got a Variac.

So, being a cheapskate that I am, I intend to build a quick-and-dirty "poor man's" Variac using a 600W light dimmer switch that you can buy at Walmart for $4.99.

Looking at the output waveform from this dimmer, it appears to adjust the duty cycle of the sine wave by chopping off the leading edges of each half-cycle of the sine wave:
Image

Does this waveform do any harm to a vacuum tube amplifier? (???)
/Matt
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Postby TomMcNally » Tue Jul 07, 2009 9:35 pm

Hi Matt ...

A light dimmer will make the transformer hum real
loudly, smoke, and burn up. Don't try it.

An alternative would be to wire a light socket in
series, and insert different wattage bulbs, the
bulb will glow in proportion to the current being
drawn. It would be a good "buffer" to see if
the amp has a shorted transformer, etc.
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Postby Geek » Tue Jul 07, 2009 10:39 pm

EEEEEEEEK! =:o

Been there. Glad I had a fire extinguisher handy.

Dimmers work OK for making controlled temp soldering irons though ;)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby Writer Frog » Wed Jul 08, 2009 12:59 pm

Hi Tom and Greg,

Thank you for the great points.
I realized that the "chopped" sine wave could theoretically contain 3x the base current in 3rd, 5th and higher harmonic components. That could heat up the transformer quickly.
I was originally concerned about the effect of the pulse-y AC current on the filaments/heaters and EMI, but an overheating power transformer is a far more serious and immediate problem.
What surprised me was that this cheap dimmer that I bought seemed to have no filter of any kind. The waveform looks like a raw chopped sine wave with sharp edges.

Thank you again.
Last edited by Writer Frog on Wed Jul 08, 2009 3:36 pm, edited 1 time in total.
/Matt
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Postby erichayes » Wed Jul 08, 2009 3:04 pm

Had to go dredging through the archives for this--and I can't remember what it was that I PROMISED I would post, but FWIW:

Hi All,

I was going to save this for the treatise, but Ed opened the door.

When I was a starving college student in the late '60s, I bought, repaired and sold 1930s vintage radios to augment my parental stipend. Since the cost of a variac was hysterically prohibitive, I came up with the "Saint Nick Variac".

Take two or three 15 light strings of the old indoor-outdoor christmas lights--the ones that use the large flame shaped bulbs with the intermediate sockets. I believe these things are still being manufactured, but if you don't want to wait until Labor Day for the Christmas season to start, you might check thrift stores or yard sales.

Now, getcha a standard single gang electrical box, an ordinary duplex receptacle with screw connections and a plate of your choosing, if desired, as well as a line cord harvested off of some dead soldier clock radio or hair dryer or whatever.

If you look at the receptacle closely, you'll see that there are tabs between the screws on each side that can be broken, thus separating the two outlets. This is so you can make, say, the upper outlet switched and the lower always hot, or for split circuits in a kitchen.

Thread the line cord into the box, break either of the tabs (but not both) and connect the line cord to the two screws that are now disconnected from one another.

Plug the strings of lights together (daisy chain, outlet strip, etc), and plug them temporarily into a wall outlet. Loosen each bulb until it goes out. Then plug the mess into one of the St. Nick outlets, and the amp or radio under test into the other.

Plug the St. Nick into a live outlet, turn the power switch of the apparatus on, and screw in one bulb. Then, methodically screw in one bulb at a time in accordance with the guides I PROMISE I'll submit forthwith.

With two strings of 15 lamps, this will give a ballast of around 200 watts total, which should be plenty for bringing up most reasonably sized amps and radios.
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Postby TomMcNally » Wed Jul 08, 2009 3:48 pm

if:

Image

then:

Image

Image
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Postby Ty_Bower » Wed Jul 08, 2009 4:43 pm

erichayes wrote:Take two or three 15 light strings of the old indoor-outdoor christmas lights...

Now, getcha a standard single gang electrical box, an ordinary duplex receptacle with screw connections and a plate of your choosing...


It sounds like this is moving towards some variation of the "dim-bulb tester", but with some built in capacity for adjustment.

http://bot-thoughts.blogspot.com/2009/0 ... ariac.html

http://antiqueradio.org/dimbulb.htm
"It's a different experience; the noise occlusion, crisp, clear sound, and defined powerful bass. Strong bass does not corrupt the higher frequencies, giving a very different overall feel of the sound, one that is, in my opinion, quite unique."
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