Laney TI15-112 undersized HT fuse?

[tdlunsfo]

I have this newish Laney on the bench with a blown HT fuse. Fuse doesn't look too bad with an open right in the middle, so I can tell it isn't defective and it isn't obliterated like I've seen in a direct short. Curiously, I disconnect one of the HT leads and hooked up my mA meter and with no signal it is around 65mA which appears to be mostly the plate current on EL84 pair (bias is bang on at 70%) but when I crank it full volume we are at 120mA's on the HT circuit. This is a timed 100mA fuse so I see this as a problem. I checked the tubes in my hickock and they tested perfect. Caps look brand new (whole amp does really, not even dusty) but I couldn't check ESR without removing the board. It's a 15W amp, so not beyond the realm of possibility that someone is trying to gig with it and cranking it full blast.

What ya'll think? Could it be possible with all of todays technology that this fuse is sized a little too close to the edge? Or... maybe those caps are going south or I am missing something else.

Note: I couldn't find the schematic on this thing and haven't called Laney yet but it appears to be a fuse in-between the main filter and the OT.

[Helmholtz]

120mA at full power is absolutely normal for a 2xEL84/15W amp. So no signs of leaky filter caps. High ESR won't increase current.

[nickb]

It seems to me a common target of cost-cutting is the rating on the power transformer and I suspect that is the reason for the fuse rating. If it were a higher rating the transformer would overheat. As it is, if you play it loud for a little too long, then phut!....the fuse goes.

[tdlunsfo]

The power transformer on this unit is pretty small

[Kevina]

Under EU/UL laws, the fuse rating is chosen so that under fault conditions, the mains transformer core temperature does not exceed a given figure.

The unit must remain safe, and pose no fire hazard, even under fault conditions..

If you change the fuse rating to suit what you believe it requires, then you could be held responsible for any future mishap; do I need to point out what this could mean?

[tdlunsfo]

I hear you Kevina. So how would you handle? Feels a little funny throwing in a new fuse then telling the customer not to turn it past seven.

[Helmholtz]

Do we even know if the 100mA fuse is original specification?

BTW, a slow blow fuse must stand 1.5 times rated current for at least one hour.

[tdlunsfo]

Yeah manual states HT fuse is T100mAL and also listed on back of amp.

[Jazz P Bass]

Here is a schematic from the VBC15.
Same fuse: T 100 ma
How about lowering the bias?

[nickb]

The most effective and expensive solution is to fit a higher rated transformer but there are legal implications. The cheap solution is to limit the drive, say by using a fixed equivalent of a post phase inverter master volume.

Incidentally, the fuse is legally there normal to prevent fire in the downstream equipment. Thus it's the mains fuse that protects the transformer legally from fire. Using the HT fuse to save the transformer HT winding from damage is something of conjecture on my part as I do not know what was in the designer's mind, but it seems reasonable.

Talk to Laney, I've found them very helpful.

[tdlunsfo]

I was going to lower the bias like Jazz suggested. I think a 120mA fuse would do just fine but somewhere down the road someone will put another 100mA back in it since its stamped on the back of it, plus I don't want the UL police to come throw me in jail.

[Jon Snell]

If you know more than the designers and electrical safety laws, go for it.
Be very aware that if an incorrect fuse is fitted, YOU are liable for third party damage!

Personally speaking, I have seen many "faulty" amplifiers because they are fitted with fuses purchased through fleabay that do not comply to IEC 60127-2/3 or equivalent compliance in your country.
Purchasing fuses from a reputable seller like RS, Farnell, Mouser etc will save you a lot of heart ache.

Always replace like for like.

[Mick Bailey]

The 100mA rating for a 'T' type fuse should be adequate for the amp. I did some testing a while back to determine the characteristics of fuses and found that many cheap fuses did not behave predictably and did not comply with the specification. I've had quite a few instances where nuisance blowing was just down to junk fuses and nothing more. The only issue with running small fuses close to their maximum continually is they tend to fatigue due to heating.

[Leo_Gnardo]

I don't want the UL police to come throw me in jail.

Uh-oh, better beware the UL police. They'll look around your place and notify the pillow, mattress and furniture-tag police if any of those "Do Not Remove" tags are missing or illegible.

[pdf64]

Maybe it’s a fuse rating systems incompatibility issue?
My understanding is that UL types have a lower overload capacity than IEC. So in this case, a true IEC T type 100mA may cope with a sustained 120mA ‘kerrang’, whereas a UL type may not.
‘T’ implies an IEC type, which is consistent with Laney being designed in Europe, and with Mick and Jon’s findings.
See p2 of https://www.dalmura.com.au/static/Va...p%20fusing.pdf
According to my reading of that, a 133mA slo blo UL type fuse would be equivalent to a T100mA IEC fuse. But don’t cite me as your expert witness!

trobbins may be able to advise better.