Variable AC Power Supply

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.

Petter
 
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
 
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 sucks...it 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).
 
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:

later
Bruce:geezer:
 
Hey Bruce....yep you are correct. I just like being anal about readings. I'm not very familiar with tube gear...if I'm off about a volt or so on the bias it bothers me. But I do believe your right about the tolerance of tube amps...can be off a bit I guess, just as long as it's not that far off eh.

Figured out the wattage rating I would need for this amp....wow...@ 10A....2200 watts. He he he, can we say fridge size!!!


Rino Odorico:bigeyes:
 
tolerances in mains

Hey Bruce....yep you are correct. I just like being anal about readings. I'm not very familiar with tube gear...if I'm off about a volt or so on the bias it bothers me. But I do believe your right about the tolerance of tube amps...can be off a bit I guess, just as long as it's not that far off eh.

Figured out the wattage rating I would need for this amp....wow...@ 10A....2200 watts. He he he, can we say fridge size!!!


Rino Odorico:bigeyes:

Rino I remember cutting my teeth on them tube amps and our very trusted AVO7 was only 1Kilo ohms per volt and the accuracy? 10% on A.C. so yer mains could be 200volts up to 260 volts and nobody was worried about it:D:cool: