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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
Follow Ups:
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.
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Buy Chinese. Bury freedom.
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
I would go up from 3A in 1/2 amp steps till you find one that works.
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.
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
"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"
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.
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.
Edits: 08/01/15 08/01/15
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
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"
OK, great info. Sorry for my spotty understanding of basic electronics. I was never good at formulas. cheers, Dak
It sounds like you need a 4 amp fuse.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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