New 300B monoblocs blowing fuses

[steve]

Hi all,

Having trouble with a pair of WAD 300B PSE mono blocs I have just completed. Both blocs are blowing fuses 3 out of 4 times when I switch on. I have double and triple checked the wiring and all seems fine. No dry joints, missing or incorrectly placed components.

I have checked for rectifier damage and shorted caps – all OK.

When I do manage to get the amps to switch on they stay on and work fine. All the voltages are well within 10% of the stated values. Sig gen and scoping outputs produce nice clean sine wave. (Fuses can blow even with no input connected).

I have tried swapping valves around and this makes no difference to the fuse failure rate – it’s about the same on each bloc. The fuses really fail i.e. they are black.

If I power the amp on with just the 5U4GB’s all seems to be OK.

The valve configuration is JJ 300B’s Electro Harmonix 5U4GB, Philips 5814A/ECC82 and GE 6AU6WC.

I recall the old WAD forums had several posts regarding blowing fuses but I am reluctant to raise the rating just at the moment. Fuses are slow blow T1.6A.

The mains TX’s seem mechanically very noisy (far worse than KiT6550) but there is only minor hum from speakers when in operation.

Anyone any suggestions I am getting a little frustrated and I am running out of fuses ?

Thanks in advance.

[John Caswell]

Hi Steve,

First of all check the power line voltage, I would guess from your comments it is about 250Vac, in which case initially up the fuse rating to 2A slo blo. If this stops fuses blowing then I suggest an inrush current NTC will cure the problem.
The problem is certainly inrush current and it needs to be limited. I have given suitable NTC types elsewhere on this BB but if you can't find them look for the thread re 6550C fuses blowing. In the meantime I will dig out the type nrs again.
And here they are: I have used the following types from Farnell to aid the 6550 "fuse blowing at switch-on" problem: 606-753, 606-765, 606-777. Although I specified the 6550 amp they of course will work equally as well in any amp

Regards

John aka Dr John

[steve]

Hey thanks John - that was quick a reply 10 mins from post.

Mains input voltage is 243V. Will look up the thread.

Looks like I won't get to listen to these babies for the first time tonight though.

Thanks.

[soulminer]

Hi Steve. I had exactly the same problem with my 300B PSEs. I just replaced the stock fuses with some 3.15A time-lags, and it has been fine since. Never blown another fuse in the near 2 years I have been using them. Good luck. Cheers.

[John Caswell]

Soulminer
This is not a good idea, as it increases the chance of serious problems. The fuse is there to protect the amp and I suggest that you insert some sort of surge limiter in the power line input of the amp.

Regards

John aka Dr John

[NealG]

I've always used 5A T fuses in my original 300B PP as it used to always blow the 1.6A standard fuse after a number of power ups. The 5A fuse has not blown in over 5 years of use!

A side affect is I think the amp. sounds better with the 5A fuse....deluded? Maybe but I'm not the only one....

I don't think WAD built in enough derating for the fuse, this article illustrates why: http://www.jaycar.com.au/images_uploaded/fuseprmr.pdf

I'm curious to know what sort of fault condition would 'benefit' from having fuse rated at xA vs one of say 3*xA. If its wildly overrated then I can understand why...John?

[soulminer]

Hi Dr John. I know what I have done is seriously bad practice, and looking back, I should not really have posted such information. The thing that is so strange though, is that everyone that experiences fuses blowing on switch on almost always has voltages well within the WAD specs, so why do they blow in the first place if all is correct with the amp? Is it not a case of incorrect calculations on the part of the PSU designer, or should WAD have included a surge limiter in the kits maybe ?
I concur that it would be safer to fit a surge limiter, but in my case I have just never got round to it, thinking, like Neal, that any kind of fault condition that was serious enough to wipe out my amp would just as easily take out a 3.15A fuse as a 1.6A fuse. My three pin mains plugs on each monobloc only have 5A fuses in instead of 13A too.
You have made me think though, and I may investigate replacing the fuses altogether, with those small resetable fuses you can get that look like VDRs. Any thoughts on those ?

[steve]

Hi John,

Maplin N99BT inrush current limiter plus 2A slow blow did the job nicely. In fact I am going to fit one to my 6550 kit that I am giving to a friend (for a reasonable fee) as his wife does not like clunks or strange noises from electrical equipment.

Neal I think I can answer your query over the say 3*xA. As an example if you look at the primary windings on your mains TX you will find they actual windings are quite thin. Using a 3*xA fuse means that should a fault develop someplace in the amp, the primary windings may act as the fuse instead of a correctly rated piece of fuse wire inside the fuse cartridge. Thus your nice expensive mains TX ends up in smoke. A correctly rated fuse protects the components in your amp – yes you are right a serious fault will probably blow an overrated fuse in any event but the fact is you may end up replacing more than just the fuse after correcting the problem.

I would recommend Dr Johns fix to all who have solved the problem with larger fuses.

Anyway thanks all for posting.

The 300B PSE sounds fab – I just have to correct the really loud mechanical buzz on the mains TX. Got some mounting gasket material earlier so I will play tomorrow.

[John Caswell]

Hi all,
In a general reply, as an example, if the bridge rec for the heaters or HT should go S/C - quite common - then a 1.6A fuse would be much more likely to blow than the 5A, obviously, and quite likely save the power tx whereas the 5A would last long enough to make sure the power tx went shorted turns or overheat or .
I do not like changing fuses willy nilly especially increasing as there has to be a reason for the original designers fuse rating.
Fuses tend to blow on the peak if the mains cycle and toroids tend to make it worse, and of course fuses also get "tired" and fail. If you look closely at a fuse when the amp is switched on you will often see it sag or even glow dull red for a few mS, usually a sign that it is near its peak rating due to surge.
So I reiterate, if the fuse blows at switch on don't just up its rating willy nilly look for the cause, there alsways is one

John aka Dr John

[Richard]

I listened to 1.6A, 2.5A, 5A and 10A and the latter 2 sound better than the first two. The 10 didn't sound better than the 5 so I settled on that. It never blew either.

The Consonance I bought - which was supposedly a quality amp, £1600 retail, - had, yes, a 5A fuse fitted as standard, no other protection.

When I was arranging a heater bridge for a DC supply on Kit88 I blew several 5A fuses due to the filter caps I was using but there was no problem with the TX.

Really I don't think you'll protect anything with a 1.6A fuse that won't be protected by a 5A fuse as the extra protection given by the low rated one is is against increased steady current draw and the most likely fault will be a catastrophic one. Your choice of course.

Rich