quick fuse question

[Motherfuzzer]

My Vibro Champ needs a 1.25a 250v fuse.

I found some 1.5a 250v fuses.

Can I use them?


Thank you.

 

[Kyle B]

yes it will work. But you shouldn't do it. If you're looking to replace a fuse, presumably that means one blew...and putting in one with a higher value means whatever caused it to blow will get further stressed. Is my presumption accurate?

It's generally not wise to up the value...though in a pragmatic sense, 1.25 is indeed really close to 1.5. If you asked about a 4A replacement, the answer would be a resounding NO! If I were at a gig, and the only option was the 1.5, I'd use it (with intent to replace it ASAP)

There's also a time factor to consider... some fuses blow really fast, some have an intentional delay. Most amp fuses are of the delay variety...that prevents nuisance blowing at power on.

Fuses are really cheap. Go to any good hardware store, they'll have a drawer full. Or, if you're not in a hurry, you can get them off ebay for almost nothing.

 

[kjt1776]

Good advice

 

[VaughnC]

It's risky to replace a blown fuse (especially with one with a higher amp rating) until it was determined what blew the original fuse as fuses don't typically just go bad. And for tube amp mains, you'd also want a slo-blo type fuse as the tube's filaments have less resistance when cold, so there's an on rush of current when the amp is first turned on...and, consequently, a regular (fast blowing) fuse might blow when the amp is first turned on.

 

[kjt1776]

Very true, good advice again, the blow fuse usualy from my experience will blow again very shortly after replacement

 

[Kyle B]

And for tube amp mains, you'd also want a slo-blo type fuse as the tube's filaments have less resistance when cold, so there's an on rush of current when the amp is first turned on....

Is the primary inrush really from the cold heaters? I always thought it was from charging the empty caps ???

 

[VaughnC]

Every solid state amp I've worked on always had a fast blowing fuse in the mains...and they have filter caps too. Anyway, that's what we were taught in tech school back in the day when they were still teaching tube circuits .

 

[Kyle B]

Not fair ... I had to learn tube theory on my own!

SS has much lower voltages too...and not as many caps either.... EL caps have gotten better too, less ESR. So maybe not necessarily = anymore ? I get the inrush on a 120v bulb...but a 6v heater? I'd have guessed the transformer would saturate and self limit

I might just have to test this one out.... You got me wondering now.

 

[zenas]

I've used fast blow fuses of the right value many times in tube amps. Just because they were the only fuse available in the right value and no body stocks the slow blows here in Podunk. When those blow there's always been more going on than the wrong type.
I finally got around to stocking fuses myself. So if you're in Podunk OP stop by and I'll give you one.

 

[UsableThought]

I've always heard that filaments do add to inrush current. As you remember I built a few heater supplies, and the topic always came up - it wasn't just the big reservoir cap. For example Randall Aiken mentioned filament inrush a couple of times, including in this comment, when he was giving me tips on building a heater supply around a switcher module.

Aside from that - when talking about SS vs. tube . . .

SS has much lower voltages too...and not as many caps either

Not as many caps? I dunno but I got the impression from reading teemuk's book that a good SS power supply should be made quite stiff via lots of filtering. From p. 180:

Because solid-state amplifiers benefit from having a massive rail capacitance this creates a problem of slow startup and shutdown. In some amplifiers it takes several seconds before the supply voltage has risen to its maximum potential and all voltages in the circuit have settled.

I got the same impression from Rod Elliott's comments on SS power supplies, e.g. the one for his Project 27 guitar amp.

And of course lots of filtering will cause lots of current inrush, so teemuk devotes a couple of pages to strategies to deal with that, e.g. relays, thermistors, etc. Plus he recommends slo-blo fuses - p. 184:

Mains fuses should be “slow-blow” type (US type “T” or euro-type 2 or B, C and D). It is highly recommendable to use an inrush current limiter with every transformer that has a higher V A rating than 300.

And he reinforces that opinion in this thread I found over on M-E-F:

BUT who says solid-state amps have only fast blow fuses...? I've seen plenty with slo-blo and for the good reason that the inrush surge can be ludicrously high in them as well. They don't have filaments alright, but the reservoir capacitances are often substantially larger than in tube amps and the rectifiers are almost without question solid-state and "instantly conducting". Also, throw in a toroid with high VA rating and the stage is set for massive inrush current draw.

 

[Kyle B]

I've always heard that filaments do add to inrush current. As you remember I built a few heater supplies, and the topic always came up - it wasn't just the big reservoir cap. For example Randall Aiken mentioned filament inrush a couple of times, including in this comment, when he was giving me tips on building a heater supply around a switcher module.

Aside from that - when talking about SS vs. tube . . .



Not as many caps? I dunno but I got the impression from reading teemuk's book that a good SS power supply should be made quite stiff via lots of filtering. From p. 180:



I got the same impression from Rod Elliott's comments on SS power supplies, e.g. the one for his Project 27 guitar amp.

And of course lots of filtering will cause lots of current inrush, so teemuk devotes a couple of pages to strategies to deal with that, e.g. relays, thermistors, etc. Plus he recommends slo-blo fuses - p. 184:

Absolutely the filaments would contribute to inrush....just never considered they'd be the dominant source. I've said it many times... Even after 25 years of dealing with electric circuits, I learn something new every day kinda why I like what I do so much, never gets boring!

 

[pdf64]

I too am a bit sceptical about cold heaters causing a big inrush current at startup, at least in our application.
As the HT winding usually seems to be a more significant proportion of the PT load than does the heater.

 

[swiveltung]

Be sure to get slo blo fuses whatever you get. I wouldn't hesitate to use 1.5 V... but that's me.

 

[VaughnC]

I too am a bit sceptical about cold heaters causing a big inrush current at startup, at least in our application.
As the HT winding usually seems to be a more significant proportion of the PT load than does the heater.

Huh? The heaters require much more current than the B+ supply and the heaters pull current immediately on power up. Obviously, the HT voltage is higher than the heaters...but the tubes don't even start to pull current until the tubes are warmed up. Yup, the filter caps take a big drink on power up with a solid state rectifier...but cold filaments take a bigger one. And, with a tube rectifier, it has to warm up before the filter caps take their drink .

 

[UsableThought]

I'm too foggy right now to take this very far - but you could draw a crude graph of both filament & cap inrush of various sizes pretty easy, yes? That would give a semi-precise answer. Or even just take the peak number for both and compare.

I remembered just now that in an old thread of mine, @reaiken had commented on inrush from cold filaments for a little heater supply he had built & had calculated the max current inrush for a set of filaments just from their cold resistance - I'd have to do the math here to make sure the cold inrush does not include the steady 10R resistor he added (he added that only to stress-test the capacity of the supply).

I just hooked it up to two 6V6s (in series) and four 12AX7s (wired for series 12.6V), and an extra 10 ohm power resistor, for a total 2.3A load current (not counting cold-start inrush current) and it started up just fine from a cold start every time. The cold DC resistance is 2.4 ohms, so the inrush current is 5.25A, excluding the inrush current of the discharged filter caps.

I don't myself have the math to calculate total current inrush for a typical setup of heaters plus all the filter caps - presumably it has been reduced to mere algebra, but I'd have to find the equations & sit down with them for quite a while. I'd probably cheat and use a PSU calculator. But youse guys with your skills & experience could do a simplified comparison in a jiff, yes?

I do remember that back when I was calculating current inrush for a DC heater supply, using an online calculator, peak inrush for the PSU alone (not counting cold heater draw since heaters would have been represented as a fixed load) was shown as higher than just 5A, more like 8 or 10A! But that's a fuzzy memory so doesn't count. Someone could have a try with Duncan's PSU calculator, which seems capable of a reasonably full modeling. Or I could try & replicate my heater supply calculation, which might count as proof by falsification if it worked out the way I remember it.

Anyway it could be done. And graphing it to get the area under each curve might be better than just comparing peaks - only because we're talking a slow-blow fuse & differing times for filaments to fully heat up vs. caps to fully charge.

 

[UsableThought]

Yup, the filter caps take a big drink on power up with a solid state rectifier...but cold filaments take a bigger one.

Okay, so I'm getting sloppy in the argument I will give here - but I bet it stands up. It's my contention that very likely, the heaters DO NOT draw more inrush than the filter caps in a typical tube amp - at least, not for peak draw. My evidence is the "big demo" PSU file that I just pulled up in Duncan's PSU Calculator; screenshot is below.

You'll see that this model represents stages (with tubes) as current draws, plus the PSU with a modest res cap of 220uF, plus filters for the stages. Most of the current draws are constant, but two are stepped to change after 1 second: I1 increases from 70mA to 210mA, and I2 increases from 5mA to 20mA. I don't know tube amps nearly well enough to guess what these stepped values represent. The only thing I'd venture to say is that they don't represent cold filaments, because in that case, the draws would decrease over time, not increase. So the model does not seem to have any components representing filaments in their cold state. I don't know what effect the choke has; I wouldn't know a choke if it bit me.

If I am reading the labels correctly, the graph in red shows current across the reservoir cap, and we see at startup that it's quite high - about 18A till it calms down to 6A or so for steady operation. Because we don't have "cold filaments" represented, this excess current pull can only be due to the modeled filter caps. Again math is my weak point, and I'm not sufficiently familiar with tube amps to readily guess the number and type of tubes depicted here . . . but so long as this tube set is not too far off from the tube set that reaiken tested in his heater supply (two 6V6s in series, and four 12AX7s, also in series at 12.6V), then we've got a big disparity. Try to imagine how many tubes we'd need to get a cold filament draw of anywhere close to 18A!

Sloppy, sloppy. But clean it up and I bet heaters still lose the cold draw contest.

Now my novice question: Might it be that one wrinkle is that if the caps are initially seen as dead shorts, then their draw is going to start off HUGE for just the initial period? This makes immaterial how much current the heaters pull vs. the B+ - whether at startup or when the amp is running steady. I.e. it's not "filaments vs. B+", it's "cold filaments vs. discharged big caps".

Second novice question: If we could graph the cold filament draw separately, and thus compare areas under the curves, might filaments do better in the race? Here we are considering peaks only. Filaments take awhile to heat up . . . but we'd really need the shape of the curve to know.

 

[J M Fahey]

Every solid state amp I've worked on always had a fast blowing fuse in the mains...and they have filter caps too. Anyway, that's what we were taught in tech school back in the day when they were still teaching tube circuits .

WHEN did you go to Tech School?

 

[VaughnC]

WHEN did you go to Tech School?

That looks about right...I got my associates degree in electronic technology 1973 .

 

[reaiken]

Capacitor inrush is not calculated by assuming a dead short on startup, it is limited by the transformer winding resistance, the ESR of the capacitor, and any wiring resistance.

Filament inrush is determined by the cold resistance of the filaments.

 

[UsableThought]

Capacitor inrush is not calculated by assuming a dead short on startup.

Thanks. I got that phrase from my reading somewhere - in fact I think I've encountered it in more than one place - but it sounds like it wasn't meant to be literal, but figurative - "almost a dead short" would be more accurate?

EDIT: I can't find the original references, but I found another - same sort of usage. This is from a page on circuit analysis by Rod Elliott:

At least one model of Fender amp had the standby switch between the valve rectifier and main filter caps. When the amp is switched out of standby, the result is that the rectifier valve sees as near as damnit to a dead short.