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  #1  
Old 05-15-2011, 04:38 PM
mbear2k mbear2k is offline
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Fusistor Replacement - RCA 14PD8054

A set I am working on calls for a 5.6 ohm, 5 watt Fusistor. Can I simply replace with a 5.6 ohm 5 watt resistor with a fuse in series? When I calculate out the fuse value, it sounds very small.

The set is an RCA 14PD8054 with a hot chassis. This fusistor is the first thing off the AC line as feed/protection for the B+ supply. Should I assume 120 volts (line voltage) x .041 amps (potential fuse) = 5 watts (resistor rating) for my fuse calculation? Then select a small slo-blo fuse as close to .041 amp as I can find?

Thanks
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Old 05-15-2011, 07:02 PM
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wa2ise wa2ise is offline
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Quote:
Originally Posted by mbear2k View Post
A set I am working on calls for a 5.6 ohm, 5 watt Fusistor. Can I simply replace with a 5.6 ohm 5 watt resistor with a fuse in series? When I calculate out the fuse value, it sounds very small.
That will work, but the fuse should be around 1.5amps or so. That's what I see in various B&W tube sets using a resistor and fuse. 0.041A is way too small.
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Old 05-16-2011, 03:16 PM
Gunslinger Gunslinger is offline
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DO you have an RCA part number?
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Old 05-16-2011, 03:55 PM
mbear2k mbear2k is offline
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As a matter of fact: 100117
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Old 05-16-2011, 05:26 PM
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Quote:
Originally Posted by wa2ise View Post
That will work, but the fuse should be around 1.5amps or so. That's what I see in various B&W tube sets using a resistor and fuse. 0.041A is way too small.
I'd second this. The primary function of a fusistor is as a fuse. The secondary function (especially in tube sets) is to limit inrush current with the resistance it offers, providing a "soft-start".

The wattage for the resistor will be different (as you've discovered) because the heat dissipation of the resistor is much more efficient than that of a thin wire as in a regular fuse - so it can have a higher current without damage than its wattage implies.

If you can't get a replacement, you can do as you suggested, but for current rating, you should go 1.5-2x the current rating on the back panel of the set (if present). If you can get an actual current measurement, even better.

The reason you don't want an exact match is thermal stress- Inrush current (somewhat limited by the resistor) will be higher than normal draw, and even if it does not exceed the fuse rating, it causes the metal wire inside the fuse to flex from thermal expansion as it heats up- leading to premature failure and "blown" fuses when there's nothing wrong.
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Old 05-16-2011, 07:06 PM
mbear2k mbear2k is offline
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Sounds good - thanks for the info. The rating for the set is 1.1 amp, 125 watts at 117 volts. But I will do the electrolytics, then do a current test and see where I am with modern AC voltage. I'm guessing either a 2 to 2.5 amp fuse?
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Old 05-16-2011, 07:57 PM
DaveWM DaveWM is offline
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I have a predicta that has on of those, next time I pull it I may end up making my own out of a thermistor/fuse/resistor. The resistor will be sized to drop the B+ to what ever it should be after the thermistor warms up. The fuse will be a fast blow since the theremistor will reduce the inrush, that way the fuse can be set to just above the max B+ current for max protection.
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Old 05-16-2011, 08:23 PM
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Quote:
Originally Posted by mbear2k View Post
Sounds good - thanks for the info. The rating for the set is 1.1 amp, 125 watts at 117 volts. But I will do the electrolytics, then do a current test and see where I am with modern AC voltage. I'm guessing either a 2 to 2.5 amp fuse?
That sounds like it should be good - If you can get a peak current reading at turn-on, you can use that to better determine the safety margin you need - If it's too much larger you might want to up the wattage and resistance of the resistor a little bit - It's generally agreed that the the softer this old gear warms up, the better; many people have made modifications so that their tube filaments don't get a power-on current spike, like DaveWM is suggesting.

You'd probably want to be a bit more careful with a SS rectifier, but with a tube, the HV won't appear until it's good and warm along with the others, so a slightly slower start has no ill effects.

I probably wouldn't worry too much about which of those to get - short conditions usually cause current draw in excess of that 0.5A difference, so you'd be protected either way... but if you already have them on hand, then I'd go with the 2 and replace it with a 2.5 if it blows under normal operation.
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Old 05-16-2011, 08:28 PM
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Findm-Keepm Findm-Keepm is offline
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Look anything like this? http://www.talonix.com/images/WMF5.6.jpg
or is is rectangular/box shaped, like Belimiter Fuse Resistors?

The Belimiters were all merged into two RCA part numbers - one 5.6 ohm, and the other 7.5 ohm. Your RCA part number is one that was merged. I don't have my RCA guide here with the part numbers, but can get them if you need them.

Only replace a fuse resistor with another fuse resistor, or similarly sized (ohms/Current Rating) inrush limiter.

Cheers,
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When fuses go to work, they quit.
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Last edited by Findm-Keepm; 05-16-2011 at 08:31 PM.
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Old 05-17-2011, 06:51 AM
mbear2k mbear2k is offline
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Brian - it does look like the one in the picture, except maybe a little bigger and is/was white.

If you have a source for replacement, please let me know. I would prefer to go that route.

Thanks!
Mark
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  #11  
Old 05-17-2011, 11:46 AM
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Findm-Keepm Findm-Keepm is offline
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A Google search (actually a Google Image Search) for "5.6 Ohm Vintage Fuse Resistor" turned up the image - and a couple of sources.

Moyers in Pennsylvania might also have it in the original RCA...

Cheers,
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