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How do I determine the fuse current rating for a new amp?

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Posted on August 1, 2015 at 13:34:33
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 I just finished building an amp and it blew the fuse I had installed. The amp has 8 el84 tubes in PPP with 3 12ax7 for the driver/splitter and 2 5v4 rectifier tubes.
At the start of the build I just used a 3A fuse as a reasonable value. But after a total of maybe 50 hours of play it just blew the 3 amp fuse after about 4 hours of continuous use. I am thinking my fuse is too low in amperage so I was wondering what factors I need to know in order to make a good choice for fuse rating. Thanx in advance and best regards, Dak

 

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RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 13:44:48
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 It sounds like you need a 4 amp fuse.

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

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 14:36:40
vinnie2
Audiophile

Posts: 4481
Location: North Carolina
Joined: September 28, 2013
 Going on the principal of "teach a man to fish", you need to add up the current requirements of all the tube filaments (look in the rca tube manual or on line) and then throw in a little extra. Tre has you in the ball park I think, probably 4 or 5 amps.

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 15:01:24
Alpha Al
Industry Professional

Posts: 2958
Location: N. Carolina
Joined: February 16, 2004
Contributor
  Since: December 3, 2015
 I usually measure AC current draw using an AmpClamp, then install a fuse about 20% higher to account for cold turn-on surges. Also, with tube amps, Slo-Blo fuses are your friend.

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 15:44:06
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Ah, the empirical method. Unfortunately, I don't have that kind of meter. If I use my Fluke dmm on the current function, how do I hook it up? Do I remove the leg of the AC line and connect my meter in its place or??? cheers, Dak

 

RE: I don't think that is right, posted on August 1, 2015 at 15:51:05
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 If I go by the "fish" method I would have 4 amps for the 2 5v4, 4.8 amps for the 8 el84, and about 1 amp for the 3 12ax7. Then I would need to add the B+ current consumption to that. That would add up to about 10 amps. I don't that is the right way to calculate the fuse rating. regards, Dak

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 16:03:32
lugnut1
Audiophile

Posts: 132
Location: Austin,TX
Joined: September 24, 2011
 I would go up from 3A in 1/2 amp steps till you find one that works.

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 16:51:41
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 "connect my meter in its place"

Connect the meter in series with one leg.

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

 

RE: I don't think that is right, posted on August 1, 2015 at 17:04:04
Tre'
Industry Professional

Posts: 17296
Location: So. Cal.
Joined: February 9, 2002
 Just as an example to get you started...........

So the 4 amps at 5 volts for the 5v4's would only be 20 voltamps (volts times amps)

20va at 120 volts would be 167ma. (.167 amps).


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

 

RE: I don't think that is right, posted on August 1, 2015 at 18:10:17
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 OK, great info. Sorry for my spotty understanding of basic electronics. I was never good at formulas. cheers, Dak

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 20:42:18
Triode_Kingdom
Audiophile

Posts: 10045
Location: Central Texas
Joined: September 24, 2006
 The maximum power consumption of the tubes (including rectifier drops) should be about 175W. That's 1.5A with a 117VAC line. I would double that to accommodate transformer and other losses, then add 20%. The result is closest to a 3.5A fuse, and as someone else mentioned, a slo-blo type (MDL) is appropriate. A 4A might also be OK, but in that case, I would try a standard fast-blow first. If it doesn't blow during the turn-on surge, you're OK. Bear in mind that if you continue having problems with the fuse blowing, there might be something wrong with the amplifier.

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 1, 2015 at 20:55:37
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Thank you TK, I will probably have to order some as the nearest I have is 5 amps. Maybe I will use a 3A slo blo until I can get a 4 amp unit. cheers, Dak

 

RE: How do I determine the fuse current rating for a new amp?, posted on August 5, 2015 at 06:42:21
Tom Bavis
Audiophile

Posts: 961
Location: Upstate NY
Joined: May 25, 2007
 Use alligator clips on the probes. Most meters have a separate input for the highest current range (10A on my Fluke 77). Clip the leads to the power switch terminals and plug it in - switch should be OFF so that current goes through meter. 1.2 to 1.5 the running current should be OK for a slow-blow fuse. An inrush limiter like GE CL-80 can reduce the current surge at turn-on.

And don't forget to put the meter leads back to the V/Ohms position when you're done - otherwise you'll blow the meter fuse next time you use it - and those Fluke fuses are EXPENSIVE.

 

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