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Rino odorico 4th September 2002 01:07 PM

Variable AC Power Supply
Anyone know how to make a "VARIAC"? Or can they be purchased less exp.?

Rino Odorico

Petter 4th September 2002 03:27 PM

Making would not be a good idea. Small units can be had for about $100-200. Sometimes you can find the surplus which would be a good idea, particularly if you could get one which was already in a box.

It is not an isolation device, so if you intend to build into an amp, consider an isolation transformer as well.


hitsware 4th September 2002 03:42 PM

you could try a light dimmer .....

Rino odorico 4th September 2002 03:59 PM

A light dimmer? How much current will one of those take?

This might me an idea, all I would like to do is just hold back about 40VAC to bias one of my tube amps (bench has higher voltage). Also just to test/run my Zen.4 for initial setup.

Think this could work?

HDTVman 4th September 2002 04:17 PM

Hi Rino

A light dimmer is not such a good idea. They generate a LOT of noise, are not desiged for that type of load and offer no protection in case of problems.

I would use a light bulb wired in series with the hot power lead of the amp under test. By using different wattage light bulbs you can customize the test rig for any voltage you want at the amp. Just remember to use regular 120 VAC bulbs.

AC plug hot-----------bulb--------------AC socket hot

AC plug netural-------------------------AC socket netural

It's as simple as this. Give it a try, this will work and could save your equipment.

later Bruce

Rino odorico 4th September 2002 04:53 PM

Wow, thanks Bruce.

Never thought about using a light The only thing is that the one amp I'm trying to bias is a Tube amp (makes no difference I think), and it runs on a 220 line.

220 light bulb?

HDTVman 4th September 2002 05:11 PM

220 bulbs for 220 gear, 120 bulbs for 120 gear.:idea:


cyclotronguy 4th September 2002 06:39 PM

Light bulb
"Lightbulb"...... that's how I spell current limiting. Good for reforming electrolytic caps in old equipment too.


Rino odorico 4th September 2002 07:47 PM

Trying to figure out the math needed for my project. What I have is a wall outlet @ 240VAC, what I plan on doing is droping the Total Voltage 20VAC. So that I have a Total Voltage of 220VAC.

For the life of me, I can't remember how to calculate what resistor I need to drop the line voltage 20VAC. The amp pulls a max total of 10A @ 220VAC. I'm only using this voltage drop to obtain a correct bias (bench outlet sits a 240VAC). So what I'm saying is that, can someone show me the formula to determine the 20 volt drop needed. The 10A rating of the amp is the max draw, she's not coming anywere near that to bias (well I hope not).

HDTVman 4th September 2002 09:04 PM

Ok, now I see what your trying to do. In order to calculate the correct dropping resistance for what you want to do you need to know the current at idle.

Most tube gear isn't that picky. I think I'd just set the bias with the AC line at 240VAC. If the power supplies are regulated then it won't matter if the AC line is 240 V or 220 V. If not then both the B+ supply and the C- (bias) will move up and down the same percentage (it's less than 10%) and it should be close. :xfingers:


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