Silver Supporting Member
My Vibro Champ needs a 1.25a 250v fuse.
I found some 1.5a 250v fuses.
Can I use them?
I found some 1.5a 250v fuses.
Can I use them?
Is the primary inrush really from the cold heaters? I always thought it was from charging the empty caps ???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....
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:SS has much lower voltages too...and not as many caps either
I got the same impression from Rod Elliott's comments on SS power supplies, e.g. the one for his Project 27 guitar amp.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.
And he reinforces that opinion in this thread I found over on M-E-F: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.
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.
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!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:
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 .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.
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 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.
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.Yup, the filter caps take a big drink on power up with a solid state rectifier...but cold filaments take a bigger one.
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?Capacitor inrush is not calculated by assuming a dead short on startup.
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.