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Tube bias ckt. component matching.

172.58.102.177
Posted on April 30, 2022 at 08:49:03
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012
 I had a post here that covered several issues & now I have one final question. In the other post it was established that low value fuses have a DCR of around 3 - 5 ohms. I am repairing a tube amp that has 125mA fuses (4) in the cathode auto bias ckt for the power tubes (4 KT88s). I don't know auto bias ckt designs, but these have an LED in the ckt. I have some 125mA fuses that measure 3.4 ohms DCR (original) & some that measure 6.3 ohms (new). I don't have enough of either to replace all, so I am wondering if I should order more. (6.3ohms) Is it important to have the fuses be all the same value, or is this not an issue. I know it may be hard to answer without seeing the ckt, but I don't have the schematic & haven't drawn up the ckt yet.
Thanks,
Kev

 

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established? who and where did they say that?, posted on April 30, 2022 at 09:26:03
Story
Audiophile

Posts: 10427
Location: NJ
Joined: December 11, 2000
 I just pulled 1 amp fuse out of my parts rack and it measures 0.4 ohms. That's a zero point 4 ohms when my meter reads 0.3 ohms when I short the leads.

So that's at the most a 0.1 ohm resistance so I don't know where you got that info so tell us where



 

RE: established? who and where did they say that?, posted on April 30, 2022 at 09:45:53
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012






The fact that low value fuses measure higher than I expected DCR was confirmed in my other post by another member & in the Mfg specs of the fuse.

The key here is low amp rated fuses. Mine are 125mA fast blow. Until this, I had no idea that a fuse could measure this high a resistance!
Kev

 

RE: established? who and where did they say that?, posted on April 30, 2022 at 10:53:04
Story
Audiophile

Posts: 10427
Location: NJ
Joined: December 11, 2000
 I measured a Buss 0.25 amp and what do you know, 2.0 ohms so you are correct after all. I didn't realize



 

and you should always have extras, posted on April 30, 2022 at 11:00:05
Story
Audiophile

Posts: 10427
Location: NJ
Joined: December 11, 2000
 I had to repair all kinds of stuff and I have about 150 extra's, all values. Nowadays I haven't blown one in years



 

RE: established? who and where did they say that?, posted on April 30, 2022 at 11:13:55
bugeyed
Audiophile

Posts: 55
Location: TX
Joined: October 15, 2012
 I was really surprised, but I guess I have never worked with such low rated fuses.
Cheers,
Kev

 

who and where , posted on May 1, 2022 at 07:56:46
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 Two posts down from this one we're in now. Link below

 

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