Fuse ratings in Tube Amps

[Mylithra]

Working on a guitar amp that keeps blowing its fuse. Its not a consistent thing where you turn it on and pops. Its a 2x 6V6, 3x 12ax7 amp.
The amp's fuse is rate at .5A. I've looked over everything in the amp and its fine. There is nothing wrong with it. I checked for bias drift but it was still biased right where it should be. (60%)
I put the thing on a current meter and the amp just sitting there idling is pulling .45A. When you start playing, it swings above what the fuse is rated for.
Now the fuse is a slo-blow variety so it wont immediately pop but over time, it will.
This seem to me to be running the fuse way too close to its tolerance. An Eaton whitepaper I read recommends 125% for most cases, which puts this around 560ma, still over this fuses rating.
Is 125% a good rule of thumb for fuse ratings? My thinking for this amp, moving to a 600ma fuse would keep it from popping when it shouldn't but still be effective should there be a problem requiring it to pop.
Thoughts?

 

[radardoug]

What voltage is your primary getting? If its 240 volts, that means the amp is drawing 100 watts odd idling. If its 115 volts, its drawing 50 odd watts at idle. In either case, too high.

 

[Mylithra]

It is 115v. There is also a Rec tube in this thing. The 450ma didnt seem to be too out of the ordinary for a total current draw for this amp.

 

[pucho812]

duel 6v6's and three 12ax7's sounds like a fender or fender style amp..
it's not a blues deluxe is it? Fender specs those amps to use a 3A fast blow when doing 120VAC mains according to the schematic I have.

 

[Mylithra]

Its a Two Rock Exo 15. Its a deluxe style amp but not a blues deluxe. The bias in this amp isn't as hot as Fender makes them plus this one is easily adjustable.
If memory serves, you have to replace a resistor in the blues Jr, pro Jr and Blues Deluxe. The rating for the existing fuse came from what's silkscreened on the amp.
I did want to know if there is a certain rule of thumb for sizing fuses as I do need this info when I do my own personal builds. So far, I've been doing the math based on max datasheet ratings and padding it a bit but generally no more than 20%. Ill use the Kill-a-watt device to check my math. As I mentioned, this white paper mentions it should be 25% beyond what your avg current consumption is. I'd like to know what your thoughts on the matter are.

https://www.eaton.com/content/dam/e...ive-device-ratings/bus-ele-voltage-rating.pdf

 

[rock soderstrom]

I did want to know if there is a certain rule of thumb for sizing fuses as I do need this info when I do my own personal builds.

Member Merlin says:
"The fuse should be rated for around 1.5 to 2 times the normal primary operating current."

see:

The Valve Wizard

 

[rock soderstrom]

duel 6v6's and three 12ax7's sounds like a fender or fender style amp..
it's not a blues deluxe is it? Fender specs those amps to use a 3A fast blow when doing 120VAC mains according to the schematic I have.

Fender uses IMHO 2A slow blow for that class of amps.

For instance, the Princeton Reverb with one 12AT7 more uses a 2A slow blow fuse. The same with the Tweed Deluxe 5E3 with one preamp tube less than the TOs amp. Both US 120V version.

But no matter how you look at it, the thread starter's fuse is too small.

 

[pucho812]

Fender uses IMHO 2A slow blow for that class of amps.

For instance, the Princeton Reverb with one 12AT7 more uses a 2A slow blow fuse. The same with the Tweed Deluxe 5E3 with one preamp tube less than the TOs amp. Both US 120V version.

But no matter how you look at it, the thread starter's fuse is too small.

in my schematics for a blues delux it listed "3A fast"

 

[rock soderstrom]

in my schematics for a blues delux it listed "3A fast"

True, but the Blues Deluxe doesn't have much in common with the thread opener's amp. 6L6 tubes, no rectifier tube and a bipolar power supply for the OP amps. The amp class I mentioned is more like the profile he was looking for and is very generously fused with 2A. I would not go higher, rather less. The TO mentioned +25% current are on the other hand too little, at the moment of switching on the amp draws much more current, because the tubes are very low impedance. This will really stress the fuse.

 

[pucho812]

True, but the Blues Deluxe doesn't have much in common with the thread opener's amp. 6L6 tubes, no rectifier tube and a bipolar power supply for the OP amps. The amp class I mentioned is more like the profile he was looking for and is very generously fused with 2A. I would not go higher, rather less. The TO mentioned +25% current are on the other hand too little, at the moment of switching on the amp draws much more current, because the tubes are very low impedance. This will really stress the fuse.

I was just looking at the 2 6l6's and 3 12ax7's

 

[Mylithra]

This will really stress the fuse.

This is what I gather is happening here. The amp operates normally fluctuating below and above the fuse's rating and even though its slo-blo, the fatigue that causes on the fuse element is whats causing it to fail in the end. Thanks for the advice. I think the answer out of Valve wizard gives me the answers I need.

 

[ccaudle]

The amp's fuse is rate at .5A.

The owner's manual says fuse should be 2A. See page 5:
Exo 15 Manual

 

[rock soderstrom]

The owner's manual says fuse should be 2A. See page 5:
Exo 15 Manual

That is true, which also corresponds to reality. However, on the back it says 1/2 amp slow blow, which is very unfortunate because it can be read as 0.5A. What is meant is 1 amp for 230V and 2 amps fuses for 120V countries.

 

[soapfoot]

That is true, which also corresponds to reality. However, on the back it says 1/2 amp slow blow, which is very unfortunate because it can be read as 0.5A. What is meant is 1 amp for 230V and 2 amps fuses for 120V countries.

View attachment 96881

Nailed it. That is very, very confusing labeling.

 

[Mylithra]

That is true, which also corresponds to reality. However, on the back it says 1/2 amp slow blow, which is very unfortunate because it can be read as 0.5A. What is meant is 1 amp for 230V and 2 amps fuses for 120V countries.

yeah... This is exactly how I read this. I feel mildly dumb now but at least I'm not the only one. I wasnt the first owner of this amp. When I got it, it had a 1/2 amp fuse on it so I wasnt the only one who made this error.

 

[rock soderstrom]

I guess you were not the first and not the last one who put a 0.5A fuse in this amp.