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Inrush current limiters, my thoughts and yours.

Carbsandcaps

Carbsandcaps

Super Member
As I watched my fisher tube amp short out, zap like a welder for a full second and literally catch fire for a short time the other day due to a stray wire shorting B+ to ground the fuse did eventually blow but it got me thinking. I think a fast blow fuse is the better option for basically anything.

Can someone recommend a general inrush limiting setup for a tube amp? Maybe a thermistor part number or specification. Should it got directly to the B+ line or the 110v input? Would a delay start be better? The amp in question is an x-202-b using 7591 tubes.
 
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fuse speed is part of it, but fuse rating is also worth considering. Some of this stuff has (IMO) a little too high of a fuse rating, and even a fast blow isn't going to blow all that much more quickly. Might be worth putting an ammeter inline and running it up to full output on both channels and taking note of the current draw. See how that compares with the fuse rating. Possibly you can back it down some to get a little extra protection.

inrush current limiters are also a good idea, and can allow a little lower of a fuse rating because the initial surge isn't there. Just know they get pretty toasty so they need somewhere to live that it won't cook any wiring or nearby parts.
 
An inrush limiter (thermistor) inline can also drop the line voltage by a couple of volts.

A CL-80 works fine for my 6BQ5, 6V6 and 7408 amps.
I installed my thermistor in a 2 x 4 handy box with a pigtail and outlet and then plug my amp into that which keeps the heat, as Gadge mentioned, from the thermistor out of the amp,

Each of my amps has its own thermistor box.
 
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trobbins said:
Perhaps address the elephant in the room first? Prevention is the first step to take - how did a wire with B+ go 'stray' ?
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It was most likely a cut off leg of a resistor or capacitor from a recent rebuild either way I believe it was human error on my part. Whatever it was went up in smoke
 
Since the "how did" of the stray wire was already addressed... The first thing you need to determine is the nominal current draw of your amp in order to choose the proper NTC thermistor. You should not just pick one and install it into your power line. Yes, the CL-80 and CL-90 are popular but that does not mean it is best for your amp.

Once your current draw is known, then you can choose the one best for the application. You should choose an NTC thermistor that is going to be in the 75%~100% amperage range based on the chart in the application guide.
 
OBMG74 said:
An inrush limiter (thermistor) inline can also drop the line voltage by a couple of volts.

A CL-80 works fine for my 6BQ5, 6V6 and 7408 amps.
I installed my thermistor in a 2 x 4 handy box with a pigtail and outlet and then plug my amp into that which keeps the heat, as Gadge mentioned, from the thermistor out of the amp,

Each of my amps has its own thermistor box.
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So yours is inline with the 110v input?
THD+N said:
Since the "how did" of the stray wire was already addressed... The first thing you need to determine is the nominal current draw of your amp in order to choose the proper NTC thermistor. You should not just pick one and install it into your power line. Yes, the CL-80 and CL-90 are popular but that does not mean it is best for your amp.

Once your current draw is known, then you can choose the one best for the application. You should choose an NTC thermistor that is going to be in the 75%~100% amperage range based on the chart in the application guide.
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I guess I’ll get one of those wall adapters that reads amperage. Is the measurement at normal operation or the highest amperage at turn on.
 
Need to measure mains current at condition that presents max current. Then identify what fuses you can buy - this is not trivial if you want to use a fuse that is as close as practical to the max level you have measured. It may or may not be the default 3.2A slo-blo UL rated fuse, but there is a better type of fuse to get you closer to your target, an IEC rated fuse.

I'd then suggest you insert a fuse in the HT secondary winding (to the doubler rectifier) as the better way to address your concerns about such accidents. I'm not a fan of using a NTC for the concern you have.
 
Carbsandcaps said:
As I watched my fisher tube amp short out, zap like a welder for a full second and literally catch fire for a short time the other day due to a stray wire shorting B+ to ground the fuse did eventually blow but it got me thinking. I think a fast blow fuse is the better option for basically anything.

Can someone recommend a general inrush limiting setup for a tube amp? Maybe a thermistor part number or specification. Should it got directly to the B+ line or the 110v input? Would a delay start be better? The amp in question is an x-202-b using 7591 tubes.
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An in-rush limiter would prolly not stop that disaster. Those devices or circuits are designed to limit the start-up current flow into the power supply. Specifically, the capacitors.

They are not safety devices, like a fuse.

But as you found out, a straight hot wire to ground can even beat a fuse. Heck, if you put a straight steel bar from your house's hot to ground --- it'll prolly cause a huge flash and sparks before the circuit breaker trips. Electricity just loves to find the fastest and shortest path to ground.

You need to find out why that bare wire was able to create havoc, in the first place.

Hope the lightning isn't going to ruin your XMAS.
 
Mr Boochie said:
An in-rush limiter would prolly not stop that disaster. Those devices or circuits are designed to limit the start-up current flow into the power supply. Specifically, the capacitors.

They are not safety devices, like a fuse.

But as you found out, a straight hot wire to ground can even beat a fuse. Heck, if you put a straight steel bar from your house's hot to ground --- it'll prolly cause a huge flash and sparks before the circuit breaker trips. Electricity just loves to find the fastest and shortest path to ground.

You need to find out why that bare wire was able to create havoc, in the first place.

Hope the lightning isn't going to ruin your XMAS.
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Ya everything is fixed now but a simple spark then pop when the fuse blew would have been fine but this was an enough to literally cause a small fire. So the goal is to eliminate the need for a slow blow fuse by using an inrush limiter with a fast blow fuse
 
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Thats the cold resistance. As it heats that number drops. There is a chart or something in the datasheet, but its related to current flow. Can't safely pull too much current through one, but at the same time you can't over-rate them or it won't heat up enough to drop the resistance down.
 
If you had a bogey 3.2A slo-blow UL rated fuse, then the likely full rated current of the amp is no more than 70%, or 2.2A. If your fuse blew within about 1 sec, then the mains current was likely more than 2x 3.2 = 6.4A, but no better guess can be confirmed. At that likely fault current, there is also no certainty that a fast blow fuse would blow any faster.

If you introduce some limiting part like an NTC, then you have to go through the NTC design and work out what would happen. Perhaps the arc continues, as the fuse may not then blow.
 
There's no 100% solution for this. Components can catch fire without ever causing a unit to draw the overload current needed to blow a line fuse, even if it's a fast-blow type. Most amps and other gear are used for decades without an incident like you experienced. It's not something that really needs to be fixed IMO.

Jack
 
TriodeLuvr said:
There's no 100% solution for this. Components can catch fire without ever causing a unit to draw the overload current needed to blow a line fuse, even if it's a fast-blow type. Most amps and other gear are used for decades without an incident like you experienced. It's not something that really needs to be fixed IMO.

Jack
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I've heard of houses catching fire --- with the circuit breakers intact. A lot of current in the wrong place (like next to something very flammable) to going to beat a fuse or circuit breaker.
 
I've had a 5AR4, a 6L6, and a 6550 arc in separate instances (which were no fault of the amp design) and in all cases the fast blow fuse I had on the hot leg of the input mains to the power transformer blew very quickly and didn't cause damage to the amp, other than the arced tube itself.

In other words, in my experience fast blow fuses do what they are intended to do as long as they are only slightly over rated for the max current you will see at full power and/or at amp startup.
 
Mr Boochie said:
I've heard of houses catching fire --- with the circuit breakers intact. A lot of current in the wrong place (like next to something very flammable) to going to beat a fuse or circuit breaker.
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My wife and I own a campground and you would be amazed at the abuse a 30 (single-pole) or 50 amp (double-pole) breaker can take before it trips. I have seen plug blades melt away where the copper blade goes into the molded plug. This is usually where the problem emanates from, then the receptacle on my power pedestal gets damaged from the heat and excessive current. If the right conditions are sustained, the circuit breaker may trip or it may not and continue to heat-up (enough to not trip) and then the breaker fails. This condition over many cycles will weaken the breaker and the breaker will then completely fail too.

A circuit breaker is there to technically protect the wire feeding the circuit (upstream), not equipment connected to it.
 
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Carbsandcaps said:
So after doing some math I came up with just around 3600uf total capacitance, 2 amp total load. So it does look like cl-80 or cl-90. The only difference I see is the resistance at 25* Celsius. Cl-80 is 47ohm and cl-90 is 120ohm. Seems like a big difference but I’m not sure what these resistance values are for. Is this how much it will drop the voltage after the inrush?

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The 47 & 120 ohms are cold resistance at room temperature (25C). When the NTC is installed and you turn on the equipment the resistance goes down fairly rapidly and you end up with only a few ohms of series resistance in the power line.

In the below chart, the CL-80 will have 0.49 ohms at 100% rated current capacity which would be 3 amps. You typically want to use the NTC in the 75~100% range to keep the series resistance low which also effects the power dissipation of the selected device (NTC).

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