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DCR of small value fuses?

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Posted on April 27, 2022 at 11:54:42
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012
 I am repairing a tube amp & there are fuses in the bias ckts. for the power tubes. (KT88s) The fuses (125mA) that survived the failure measure 3.4 ohms & new fuses measure 6.6 ohms. BTW I measured the fuses with a 22 ohm resistor in series. I don't have instructions for adjusting bias on this unit & am concerned about replacing some fuses with these new ones. I didn't notice, when I ordered the fuses but, is the DC resistance in the spec? So, should I be concerned about this variance (3.2 ohms) in the new fuses?
Thanks,
Kev

 

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Older Slo-Blo fuses have a small resistor inside, posted on April 28, 2022 at 12:37:47
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015



This is a 1/10A 250V fuse. It has a 33 Ohm resistor inside. Indeed, it measures 33 Ohms DCR.

 

RE: Older Slo-Blo fuses have a small resistor inside, posted on April 29, 2022 at 08:35:05
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012
 These are all fast acting fuses.
So, I see that others are measuring some elevated value of DC resistance on low value fuses. FYI the 12A mains fuses measure .1 ohm. The 125mA fuses are in the self biasing ckts. of the KT88s. One post stated that it would not be a factor in a bias ckt. Should the fuses, at least, be all the same DCR?
BTW This amp is all tube KT88 output, 90W/channel in class A & is has a 12A fast blow fuse in the mains. Is that normal? The rec. value is not posted on the unit & I don't have a manual. BTW it is set up for 120V.
Thanks,
Kev

 

125mA Fuse, posted on April 28, 2022 at 10:19:47
Triode_Kingdom
Audiophile

Posts: 10042
Location: Central Texas
Joined: September 24, 2006
 Eaton's specification for the "Typical DC cold resistance" of a 125mA AGC fuse is 10.825Ω. I just checked one that I purchased from Digikey last month. It measures 11.2Ω using a 4-wire ohmmeter with 25mA flowing. I calibrated the meter against a 10Ω 0.1% resistor before taking the measurement.


 

Thank you TK nt, posted on April 28, 2022 at 10:44:52
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

You have a measurement problem, posted on April 27, 2022 at 13:58:53
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 The DCR of a fuse, regardless of rating, is under 0.3 ohms. You ether have bad contacts between your resistor and fuse, your meter is not sensitive enough, dirty contacts, etc.

Don't fret about DCR of a working fuse

 

RE: You have a measurement problem, posted on April 27, 2022 at 15:11:17
Uncle Mike
 Fuses will degrade with use and abuse in time. I've seen this more than once in industrial settings where even the high resistance of a fuse would shut down a system. I found mirrored systems where we found one fuse blown and pulled the fuse to check in it's doppelganger and found very high resistance. The new replacement fuses were minimal resistance.
Pictureguy has a good discussion of this in the Tweakers Asylum.

 

RE: You have a measurement problem, posted on April 27, 2022 at 20:13:06
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 The fuses I just tested are all NOS.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: You have a measurement problem, posted on April 27, 2022 at 14:14:14
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 "The DCR of a fuse, regardless of rating, is under 0.3 ohms"

How did you come to that conclusion?

I just measured some 1/16 amps fuses at over 4 ohms. Some 1/4 amp fuses at over 2 ohms. Some 1/2 amps fuses at .7 ohms.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: You have a measurement problem, posted on April 27, 2022 at 14:03:22
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012
 Thanks. This really surprised me.
Thanks,
Kev

 

RE: You have a measurement problem, posted on April 28, 2022 at 06:58:33
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 Slow-blow fuses have relatively high resistance - 1/8A Littelfuse is 30 Ohms. Look up the data sheet - it's specified. A few ohms won't make a difference in the bias circuit (or anywhere else a low-value fuse is used).

 

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