Weird fuse issue

[Randall]

I have a 1967 Deluxe Reverb in with a blown fuse. I had it a few weeks ago for the same issue. I replaced the fuse, and ran it on my Kill A Watt meter for hours and hours with no issue. I tested all the tubes for shorts. I probed and prodded. It ran at just over 1/2 amp flawlessly. I gave the amp back and said sometimes fuses just go bad for no apparent reason.

Customer brought it to a gig. He turned it on and went for a drink. When he came back the power lamp was off, and the 2 amp fuse was blown, so he brought it back to me. Same deal, replaced the fuse and it is running fine at 0.58 Amps.

I don't know what to do. I can't find anything wrong with the amp.

[Jon Snell]

If it is the mains fuse, always use a Time Lag or Anti Surge fuse but of the identicle value. Also, ensure any power outlet points are good and have no loose connections, especially extension leads.
If it is the HT fuse, that can be tricky. I would suspect ouput valve issues.

[g1]

Please post correct schematic.

[Randall]

https://ampwares.com/schematics/deluxe_reverb_ab763.pdf

It of course has a slo-blo fuse. I have no way of knowing what is going on at his gig venue. All I know is the amp runs fine at 0.58 amps when in my shop for hours and hours. There is no HT fuse in a Deluxe Reverb. I have run all the tubes through my Hickok military tube tester, and I find no evidence of any shorts.

The amp has 2 amp slo-blo fuse printed on the back panel, but the schematic states 1 amp. It has twice had a 2 amp slo-blo fail, which makes it all the more odd.

I am leaning toward either there is a power surge at his gig, or a very unlikely coincidence of two faulty fuses.

[g1]

It's quite common to have tube failures that will not show up on a tube tester. Most often because the tester does not use high enough voltages, and because we don't test them (on tester) for sustained periods of time. Most likely it would be power tubes or the rectifier to cause fuse blowing.
But you are running it for hours in the shop, so that should show such issues. Sometimes you need to test with program type material (music) rather than a constant signal like a sine wave.
Sometimes a turn on surge will do it, but this one has 2A fuse so that should be less likely.
Two faulty fuses is unlikely, but very rarely you could have a batch marked with wrong value, or an oddball 'slow' rating that is not the usual slow type.

[Randall]

I'm getting too old for this shit.

[Jon Snell]

It's quite common to have tube failures that will not show up on a tube tester. Most often because the tester does not use high enough voltages, and because we don't test them (on tester) for sustained periods of time. Most likely it would be power tubes or the rectifier to cause fuse blowing.
But you are running it for hours in the shop, so that should show such issues. Sometimes you need to test with program type material (music) rather than a constant signal like a sine wave.
Sometimes a turn on surge will do it, but this one has 2A fuse so that should be less likely.
Two faulty fuses is unlikely, but very rarely you could have a batch marked with wrong value, or an oddball 'slow' rating that is not the usual slow type.

Unless the supplier is unknown or fleabayamazun maybe.

[Mick Bailey]

Slo-blo fuses vary considerably in operation and manufacturers produce curves for each of the many types within this category. All fuses are not equal - their construction varies considerably - springs, flat strips, composite wire links, spirals wound over a former and the M-spot type with what looks look a blob of solder on the wire link. Those can fail abruptly if they're repeatedly operated under surge conditions just over their nominal rated capacity. That type is best avoided for amp use. Where I've had problems with fuses blowing I always examine the type and stick to Bussman, Littelfuse or Schurter - preferably ones that have the thermal spiral.

A good while ago after experiencing problems of random blowing with no detectable fault, I did some test on fuses from a random selection and found that the crap that the market is awash with can be extremely unpredictable - some blowing instantly at just a little over their rated current and having no slo-blo characteristics at all (despite being marked as such). Some blew at way under their rating, and others would hold a massive overload current while the link continued to glow red. The branded fuses all behaved consistently and predictably.

[eschertron]

Customer brought it to a gig. He turned it on and went for a drink. When he came back the power lamp was off, and the 2 amp fuse was blown, so he brought it back to me. Same deal, replaced the fuse and it is running fine at 0.58 Amps.

So many unknowns in this scenario. Dismissing the possibility that someone messed with the amp while he was away, I'd be curious...
1. if he plugged/unplugged any peripheral equipment after turning the amp on, external speaker cab, loop FX, etc.
2. if the other band members were setting up, maybe pulling the wall voltage down while they were firing up their equipment
3. if (as you suggested above) the line voltage at the venue is just 'flaky'
4. tubes that jiggle loose on the way to the gig that you don't experience in the shop
5. what else in the environment there that could be different than in your shop

'nuther question: does it ever blow a fuse while he is playing?

for the amp to blow just sitting, no signal, while in a controlled environment there's no issue? That suggests to me there is something at the venue that is NOT controlled. JM2C

[52 Bill]

Some rectifier tubes (power tubes as well) will fail only when hot and vibrated. Turn the amp on and let it warm up to full running temp and then give the amp a good Enzo whack and see if you get a reaction. I will sometimes thump each tube to see if they react to the vibration.