Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: eleventeen on June 23, 2016, 01:01:40 pm
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Here is a simple device I've been wanting to build for a long time. The purpose is to avoid blowing lots of expensive fuses when troubleshooting a device that is giving you trouble with the power supply. You've already done the "lamp limiter" test and determined that the amp under test is not a total short; but now, you don't want to blow 4 * 75 cent fuses getting the amp power supply to work properly.
In essence, it is two alligator clips sticking up across which you clip in a piece of very small wire. You get the wire from a braided shield from a scrap of coax wire or pull a strand out of a scrap of stranded wire. The wire is your fuse. Of course, this rig would be phenomenally dangerous, so I took the following safety measures.
The red "ON" LED is on any time the device is powered up; fuse blown or not, switch on or not. The yellow LED is wired across the fuse and will light if the fuse is blown....provided your "amp under test" is plugged in. In the amp under test, you install an oversize fuse so it will not blow. The yellow LED will ONLY shown "blown fuse" if there is a load plugged into the "load" outlet on the device.
Both alligator clips live inside a plastic pill bottle and body of the bottle is attached to the base. To change the tiny-wire "fuse" you unscrew the bottle from the base and change your wire. Because the red LED is lit up any time the device has power, it is dangerous to unscrew the insulating bottle any time the red LED is lit up. If you are feeling brave, you switch the switch to "off" which breaks both line wires into the device. DPDT switch, of course. The red LED remains lit even when the switch is turned off just to remind you that the device is still plugged in. Or, you can unplug the whole device.
If the device is plugged into an active source of AC power, one or both of the alligator clips potentially has power. Because unless you yourself wired and tested the AC outlet and know it to be properly wired, it possible that the good fuse inside the amp under test allows the line "hot" to pass thru the power transformer and thus will "hot up" one of the alligator clips. Of course the "ON" switch is a DPDT which switches both conductors, so this cannot really happen, but I like to be super safe. .
Yes, you are going to have to "calibrate" what size wire represents what amount of line current. Get a multi-bulb light fixture and screw in 100 watt bulbs until the fuse blows. 100 watts is a touch less than 1 amp.
(http://i172.photobucket.com/albums/w32/ttm4/fs_01_zpsfk7wytgg.png) (http://s172.photobucket.com/user/ttm4/media/fs_01_zpsfk7wytgg.png.html)
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Or for $1.58 just get a circuit breaker. Heck, for $14.72, get one of every size that Weber carries. Couple gator clip pigtails and just clip onto your fuse holder for testing. Better yet, just replace the fuseholder with the breaker.
http://www.tedweber.com/amps/fuses-circuit-breakers
(http://www.tedweber.com/amps/fuses-circuit-breakers)