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How Much Variac Do I need?

Discussion in 'Home Brew' started by ElectronTubesRule, Sep 19, 2011.

  1. ElectronTubesRule

    ElectronTubesRule Active Member

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    Ok so my target peak output is 1000 watts at 2500V at 800ma. So given that I am using 110 power service in my home I think I need 9.0909......amps at 110 volts to power the amplifier. So do I need to have a variac that is able to handle that type of load? I am assuming no. I am guessing that powering idle with no transmission is more then enough to test the circuits and how well they are put together and the individual components in the circuit. Is this a wrong assumption.

    So far I have come across units in the 2.5 amp to 7.5 amp used. Oh and a lot of like new still in the box 240V unit's.....I am thinking some Ham figured he would go to 220/240V in his shack and never got around to it based on how many of them I have come across new still in their seal box. I am not planing on running 220/240 any time soon, not going to use my basement dryer supply or move out to my pole barn where my Lincoln Welder is either so 110/120 is what I need. LOL

    Also if I keep my plate voltage and bias down low and do not drive it hard I could easily get the power needed down to 5 amps of I absolutely need to test it under load with less then 100% power.



    Thanks for the input!!!
     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    First off your math is wrong. 2500 volts at 800 mA is 2000 watts DC input. That requires 8.33 amps at 240 volts AC or 16.66 at 120 volts AC just for the plate supply. Unless you have a special 20 or 30 amp 120 volt outlet you are screwed. Not a good idea running anything over a kilowatt on a 120 volt line anyway due to current draw and line regulation.

    Secondly, why do you want/need a variac?
     
  3. ElectronTubesRule

    ElectronTubesRule Active Member

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    Well that goes to show I should not be doing math @ 3 in the morning.....LOL

    My home can support more then 30 amp's at the wall but the breakers in the power box are prob. 15-20amp. I can though replace the breaker on what ever circuit I decide to put the amp on......

    Man I am glad you pointed out not only my bad math but the HUGE OVER SIGHT on my part!!!!

    Well I can build a smaller amp with lower output!!!

    If I use a tube with more gain I can get the power needed down a bit as well.

    I could also pull power from two different circuits in the house use two transformer's wired separately on separate circuits in home. Not ideal I do understand but it would get around that issue.

    If I build a one tube amp it will be in the 350-450 watt range which is still more power then a 13 year old just getting started needs. If I had to I guess I could tap into the stove or dryer power lines. My pole barn is 40x80 and not heated so 1/2 the year it would be too cold to use the power service out their. I have a huge old red Lincoln Stick welder that runs off 220/240. I know I have up to 230 amps on the dial and my lights never flicker. The problem is that the places that have that service are in the wrong spots in the home for convince. I would need to buy different transformer though since I do not have any 220/240 50/60 transformer's just 110/120 50/60 transformer's.
     
  4. ElectronTubesRule

    ElectronTubesRule Active Member

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    Well I was going to get one a Variac that is so I could slowly ramp the power up. Most of the home brew builds I have read about they used one to slowly bring the power up in stages to see if the circuit's where sound. I too wondered why one was needed seeing how I never say one used in the Electronic repair facility I worked at for 2 years as an apprentice and the only time I saw them do anything like that was with variable power supply usually on 12V radio's or amps when looking for bad cold solder joints and grounds and such.....If they thought they had them all they would normally start lower on the power and slowly turn the voltage up often taping on the board with a plastic alignment tool while watching for any issues. So I too wondered about a Variac but it seemed like a good idea to have one since so many homebrew guy's seem to have them and use them.

    So in this case it truly is a case of monkey see monkey do on my part.
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    You do realizethat you need at least 12 gauge wire for 20 amps and 10 gauge wire for a 30 amp circuit don't you? I highly doubt that you have outlets wired with those gauges. Also a standard 15 amp household outlet is only rated for 15 amps.


    Higher gain does not mean less power demand requirements. It simply means less driving power required from the exciter. Power output is what determines the power requirements of the final amp.

    You can derive 240 volts from two 120 circuits if they are on opposite sides of the incoming AC lines but it is not a great idea to do and use it that way. I have done it when testing 240 gear when 240 volts was not available but I would never run anything from it.


    Different situation altogether. A welder is basically a variable transformer with current limiting. The stick voltage is much lower that the AC line voltage and if you are drawing 200 amps at the work end of the welder then it will still be drawing less than 30 amps from the 240 volt line. Check the fuse/breaker rating on that welder circuit and you will see what I mean.
     
  6. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Variacs are good for testing gear and looking for faults but a poor idea in an amp power supply as it results in poor voltage regulation in high current draw situations.
     
  7. ElectronTubesRule

    ElectronTubesRule Active Member

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    I got lucky!!! I checked my power box and I have a fairly even mix of 20amp,30amp,40amp breakers. I actually have a few 50 and 60 amp circuits but I did not count those since I saw a plethora of 20,30 and 40 amp breakers. Now I just have to figure out which circuits are which inside the home they are not labeled. So it looks like a draw of just under 17amps is not going to be a problem for me.
     
  8. ElectronTubesRule

    ElectronTubesRule Active Member

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    Oh I am sorry I must have not been clear. I was just going to use it to test the circuits in the amp. I had no intention of leaving it in the amp or in between the amp and wall outlet only for testing.
     
  9. Kamikaze

    Kamikaze Active Member

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    A properly sized variac shouldn't be that much of a voltage regulation issue.
    A marginal one could be.
     

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