Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: capn_starsky on June 13, 2020, 10:46:02 am
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Hey all! I've got a question regarding monitoring with a variac. My father is letting me borrow his old variac and was wondering if anyone had any suggestions for in line monitoring (mainly current draw.) What sort of meters do you all have recommendations for. I currently only have one DMM and would like another meter for for monitoring for shorts that I don't have to modify the variac to use or tie up my meter by running it in series. Thanks!
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good read here;
https://el34world.com/Forum/index.php?topic=25028.0
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Precisely what I needed!
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Here's another option that simply plugs into the variac (or wall) and you plug a device/amp into the meter box. Very easy to build and use. Cost about $20 for everything you see. Amazon has a variety of those cheap meters priced about $15. I prefer the ones that use a CT to sense current.
Here are the links to both units seen in the attached pic...
https://www.amazon.com/gp/product/B079JNY5VD/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B01EWW1RJ4/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
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This is a great little digital meter and I mounted mine to the variac enclosure similar to the others. The real benefit is the ability to read the RMS voltage and current is a steady state operation.
While the meter displays accurate readings, the caveat is that it is not a good indicator of real time measurements.
What I mean by that is–if you are putting an amp under test for faults, or monitoring the mains current draw to look for shorts, an analog panel meter (or LED meter) has a much faster response. Often the situation calls for a quick reaction from the technician. The speed of an an analog meter can be the difference between saving a critical component or not.
A moving coil meter can a much better way quickly seeing if everything is "normal". Digital meters are a better way of reading accurate measurements. I rely heavily on both kinds.
Rod Elliott has an interesting article on metering here:
https://sound-au.com/articles/meters.htm
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I put the 061 on my Variac but the video I watched didn’t say to CUT the lead of the cap. :BangHead: I blew the fuse and just disconnected the wire and called it good. Now I can get the meter down to zero. Thanks!
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One more question, just got my hands on said variac, it's an old one with a two prong output, any way anyone can think of plugging in three prong cords to it? I was thinking of just cutting the ground lug off of a short extension cord to be used for this purpose ONLY and being careful and clenching certain orifices :laugh:
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look at post #3;
re-do your 2 prong to make it a 3 prong using a Jbox, then add the 3prong recept to plug amps into
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One more question, just got my hands on said variac, it's an old one with a two prong output, any way anyone can think of plugging in three prong cords to it? I was thinking of just cutting the ground lug off of a short extension cord to be used for this purpose ONLY and being careful and clenching certain orifices :laugh:
Sure. Does your variac have the same enclosure as the ones posted above? Assuming it does, Here's what you need to do..
Remove the existing 2-prong power cord which supplies AC to the Variac. install a new 3-prong chord to the Variac. Make sure that the correct wires go to the correct connections:
• The Neutral connections (White wire in the power cord, variac transformer, and outlet receptacle) should all connect together. A variac is an auto transformer, and is not isolated. (In UK and EU neutral is Blue)
• The Live conductor (Black wire in the mains power. Brown in UK and EU) should be connected to where the other mains wire was connected to the variac coil. Be careful to leave a little bit of lead on to show you where it was. This should be the ONLY connection to this point.
• The Earth conductor (Green in US. Green & Yellow in the UK and EU) should be crimped with a proper ring terminal. Drill a dedicated suitable hole in the variac metal enclosure or panel. Clean or scrape any paint or coating off around the interior drill hole to expose the bare metal. Insert a star ring terminal onto the screw stud first, then the earth ring terminal, and screw them down using a nylock and/or shake proof washer. For convenience, you can also run the earth connection from the outlet to the same stud. I can't remember if UL recommends doing this or requires a separate connection, so it might be worth double checking. But if you use the same stud, I would separate the two ring terminals using another star ring terminal, or nut/star ring terminal combination.
Off the record, while this is the correct way of doing it, if you don't have a good crimper (or any crimper at all), then using a solder tab is better than no earth at all. Just make sure you use a thick enough wire gauge for your replacement power cord, AND fuse the output of the variac!! this will save your variac from failure if there is over-current at low voltages!! A fuse on the mains side isn't a bad idea either, but if will not blow if something happens at low AC voltages
Here are some images illustrating this: