Peter W. wrote:
ImFritz wrote:Just for shits and giggles I pluged my Emotiva XPA-5 into the variac and its running fine without the variac blowing the 5 amp fuse.
Must be a problem with the ST-120.
I will spare the complete rant, but, fair warning of a mini-rant on V
utotransformers. I will throw in, for giggles, a bit on fuses:
a) "Variac" is a registered trademark of ISE, Inc. purchased from General Radio, issued and renewed since 1933. Any other uses of that word other than on equipment made by General Radio and/or ISE is a copyright infringement and theft of intellectual property.
b) A VA without effective current metering as a bench tool is dangerous at best, worse-then-useless otherwise. Keeping in mind their original purpose was to dim lights, not test electronics. As a bucker, however, that is within their label.
c) From the symptoms you describe, it could still be either - much of how/why the fuse blows is dependent on the type of fuse, and where it is in the circuit.
1. If you had a current monitor on your VA, you would know whether it or the power-amp were at issue. Essentially, a VA should draw 0-current by itself.
2. It very much depends on the _TYPE_ of fuse in use. A standard or fast-blow fuse used on a piece of tube equipment is always at risk, unless sufficiently over-sized as to for the protection of the real-estate rather than the equipment.
3. Whereas I can see the use of a conventional slow-blow fuse (element wound on a fine ceramic rod) in a variac, I cannot see such a fuse being used on a piece of quality audio equipment. The overload-over-time tolerance of these devices is nuts.
4. An audio fuse should be a DUAL ELEMENT fuse chosen to be fairly close to the actual operating current of the item. Such a fuse can handle the starting surge, but will act more like a conventional fast fuse after the surge. These fuses hate short-cycling - so they are not appropriate in a VA - which by design gets turned on and off fairly frequently.
If you have a DE or slow-blow fuse on your amp, and a fast fuse on the VA, it would be a close-run thing as to which one would blow first in the case of a fault - but favoring VA failure first.
To figure this out of a certainty:
* you will need to determine the current draw of the 120 - and how that compares to what it should be.
* you will need to see if adding the VA increases that draw - and if it does, send it back. https://www.ebay.com/p/Heathkit-Ip-5220-Variable-Isolated-0-140-Volts-AC-3-Amps-Power-Supply/1701509401
is presently on that auction site. I keep two (2) of them as they have excellent metering, and work very well. Most useful on tube equipment as it will show with some precision the onset of B+ as tube rectifiers warm.