New 300B monoblocs blowing fuses

[John Caswell]

Hi all,
Well I must disagree, you are throwing away a safety factor

John aka Dr John

[steve]

I second Johns comments. By overrating the fuse you risk damage to internal amp components. I personally don't think its worth risking your amp components, your safety or rare occasions the possible fire risk for a slightly better sound.

[NealG]

Well I remain unconvinced, I can understand if the fuse was increased to 10 or 15A but I still think the WAD fuse (for my 300B) was not derated enough. It has a slow start in the form of the GZ rec. and yet the 1.6A would blow after a few power ups. The 5A fixed it and I feel safe that if a primary or secondary is shorted it will still blow quick enough.

Incidentally, Lee, the fuse in the plug is there only to protect the mains lead, it does nothing to protect your kit.

[NealG]

I’ve made some measurements of the AC current my 300B draws.

From cold the inrush current hits 2.5A approx. it then settles down to a constant 1.35A draw at idle.

At 25degC fuse temp and 75% rating the fuse should therefore be a 1.8A fuse. WAD chose 1.6A and it keeps blowing. 2A would have been more suitable. However, that is at 25degC. The actual fuse temp after a few hours use is going to be a lot greater, I will have to measure it but I suspect 45-50degC would not be out of the question normally.

In this case the fuse should be derated by a further 10% or so to make a 2.2A fuse. We could build in a little more margin for higher temps so let’s make it 20%, a 2.5A fuse should therefore suffice….

However, the inrush current will still stress the fuse from cold so I would expect a 2.5A fuse to blow eventually…possibly…

5A therefore seems excessive, but I’m prepared to live with it. Perhaps a 2.5A fuse and inrush limiter is the ideal answer…

[Greg.]

Hi Neal,

Very diplomatic yet well constructed answer that's sucked me in. Actually I'm using a 3amp fuse with no problems. I did have a few blow at 1.6A. I converted to 5A on my KaT88 and could hear the difference. 300B PP has been running well now for sometime. I'm inclined to move to 5A accordingly. I always wanted to do this but James' advice held me back.

Best wishes,

Greg

[John Caswell]

Hi all,
The difficulty lies with measuring the inrush current and also at what point of the mains cycle you actually switch on,as th inrush current can easily exceed 10A or so. Using a DMM to measure this is useless, an AVO 8 on the 10A is much better as from it you can get a
However in the absence of one of these elegant devices I will stick by my original comments.

John aka Dr John

[Richard]

No-one has explained what the 1.6A fuse is able to do better than the 5A fuse in practice. Under what fault condition will it better protect the amp? For example if the valves are aging and one starts to run away and overheat a 1.6A will not blow until the valve goes catastrophic. We saw plenty of that with JJ KT88s. A failed electrolytic or other short condition will blow a 5A instantly. To damage the mains TX an overly high current would have to be drawn for an overly long time (both relative terms) and I can't see anything in a valve amp that will do this. Being safe means not leaving a running valve amp unattended.

Rich

[James]

Hi,

I think that in the case of a hard fault, such as a dead short on the primary or secondary side of the transformer, the difference in protection would be minimal, as the fault current would blow the fuse rapidly.

My thinking is that a softer fault, of some ohm's resistance or impedance, could cause a danger. Examples are SC capacitors or valves.

In the case of a 5A fuse, it is probable that at least a kilowatt would need to be dissipated at the fault point for a significant period of time before the fuse would blow. A KW can and will cause a significant amount of smoke to escape!

As Our amplifiers are used in a domestic setting, and usually the input fuse is the only form of protection (No secondary HT fuses), i would prefer to limit the potential fault energy to a couple of hundred watts, rather than a couple of KW,s.
Although the risk is small, i find that a amplifier pouring smoke for any length of time reduces the spouse acceptance factor significantly!

James

[Greg.]

OK, I give in. I'll stick with 3 amp. James you're always so so logically sensible I must correct that and get you ****ed one night

Best wishes,

Greg

[Richard]

Hi James

I do understand what would seem to be good sense but still can't see an sc cap or valve disippating several hundred watts so that a low value fuse would blow!

You, John and Steve are of course right to go for the lowest value which will connect the amp to the mains reliably and I realise my logic must seem stupid. However, I did think about it (believe it or not ) and came to the conclusion that a 1.6A fuse wasn't going to prevent anything more than a 5A. It was also safe in that it's the correct rating to protect the mains supply. I could clearly hear the benefits of the 5A fuse and it wouldn't blow at switch on as the 1.6A one did. (By blowing at switch on a 1.6A fuse is clearly showing that it is under-rated for this particular application.) So all things considered I settled on 5A.

John provides the solution for people who do not want to increase the value.

Rich