Hi I am new to the site and have a question about NTC thermistor and choosing the proper one. I want to keep this very simple so please no replies telling me how to make a 20k amp better. I merely need to replace the inrush current limiter in one amp. Mains fuse is 10 amp (300 watt tube monos). I fixed it using a 10d-15 (10 ohm 5 amp 15mm) but 2 have just crumbled resulting in a open circuit. I ordered 2 different types, 10 ohm 7 amp 22mm, 20 ohm 8 amp 22mm
My question is: I ordered bigger ones with a higher amperage rating, and the joules rating is much higher. The accuracy is 20% on one and 25% on the other. Is it ok to go bigger as long as I stay under 10 amps? NTC is placed after mains fuse and before relay board. Anyways looking for some quick thoughts as I am installing today.
My question is: I ordered bigger ones with a higher amperage rating, and the joules rating is much higher. The accuracy is 20% on one and 25% on the other. Is it ok to go bigger as long as I stay under 10 amps? NTC is placed after mains fuse and before relay board. Anyways looking for some quick thoughts as I am installing today.
Tube amps usually don't need thermistors unless you have the heaters wired in series, little usual IMHO in audio services.
It’s used as an inrush current limiter. Amps are hand built in Germany and were designed to use one. I have it bypassed right now but since I have the replacements, I would like to know which one would be the best to install. Again it’s common to use one as a ICL in a tube amp, I’m just wondering if it’s a problem of not enough amperage rating or too low of a joules rating that is causing it to fail. It does not fail right away, sometimes taking a few weeks before it crumbles. Thanks anyways.
I have a hard time believing that the engineer/ designer of a 27,000.00 set of mono block amplifiers does not know what he’s talking about. It’s in place to protect from the current surge when powered on. It’s a NTC thermistor. It’s protecting the same things that the mains fuse does, but it has the current limiting function. I bypassed it right now but want to put one in because it came from the factory with it.
I have a hard time believing that the engineer/ designer of a 27,000.00 set of mono block amplifiers does not know what he’s talking about. It’s in place to protect from the current surge when powered on. It’s a NTC thermistor. It’s protecting the same things that the mains fuse does, but it has the current limiting function. I bypassed it right now but want to put one in because it came from the factory with it.
DigiKey has 7 Ohm 10A and 5 Ohm 12A, 24 and 22 mm sizes in stock. But you may want to talk to the manufacturer first...
I can only see two good options.
As per 20to20, contact them direct. It seems crazy that a supposedly high priced amp has no operating manual with a parts list - were they that stingy?
If they don't respond, then I suggest you really do need to reverse engineer the part, which typically means measuring the idle and maximum continuous AC mains currents, and making a schematic of the power supply (with transformer VA and voltage details and secondary side capacitance details). The current measurements identify that the NTC operates above its min, and below its max current ratings. The Tx VA and voltage ratio, and the capacitance, allows the NTC max Joule rating to be assessed for transformer and capacitor in-rush contributions. If your transformer supplies valve heaters, then the heater current draw can also contribute to NTC Joule capability.
You may want to continue to follow the third available option - which I would call 'suck it and see' - but given the price of the amp, that would seem a bit incongruous.
As per 20to20, contact them direct. It seems crazy that a supposedly high priced amp has no operating manual with a parts list - were they that stingy?
If they don't respond, then I suggest you really do need to reverse engineer the part, which typically means measuring the idle and maximum continuous AC mains currents, and making a schematic of the power supply (with transformer VA and voltage details and secondary side capacitance details). The current measurements identify that the NTC operates above its min, and below its max current ratings. The Tx VA and voltage ratio, and the capacitance, allows the NTC max Joule rating to be assessed for transformer and capacitor in-rush contributions. If your transformer supplies valve heaters, then the heater current draw can also contribute to NTC Joule capability.
You may want to continue to follow the third available option - which I would call 'suck it and see' - but given the price of the amp, that would seem a bit incongruous.
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I have used a lot of thermisters NTC in soft starts over the years. Like you I have tried a few different values.
If it was me (and it was) I would not worry about going higher resistance and more Joules because the NTC will be bypassed in a second or two anyway, and higher resistance = softer start anway. I spent a month looking at the NTC in my Krell clone and replacing it with different values. Measuring voltage drop at startup etc... Turns out these things don't really heat up in practice over a few seconds as far as I could measure across them anyway. Once the big transformers start conducting current the surge should be mostly gone you would think.
If I were you I would be happy with the larger values and more joules as long as it starts up nicely. My Class A amps still "booooing" when the NTC is bypassed but no blown up stuff due to that as far as I know.
After using a lot of these, I am of the opinion that an NTC is not a great soft start load even if immediately bypassed in seconds. Just seems to not start 'softly'.
If it was me (and it was) I would not worry about going higher resistance and more Joules because the NTC will be bypassed in a second or two anyway, and higher resistance = softer start anway. I spent a month looking at the NTC in my Krell clone and replacing it with different values. Measuring voltage drop at startup etc... Turns out these things don't really heat up in practice over a few seconds as far as I could measure across them anyway. Once the big transformers start conducting current the surge should be mostly gone you would think.
If I were you I would be happy with the larger values and more joules as long as it starts up nicely. My Class A amps still "booooing" when the NTC is bypassed but no blown up stuff due to that as far as I know.
After using a lot of these, I am of the opinion that an NTC is not a great soft start load even if immediately bypassed in seconds. Just seems to not start 'softly'.
I have on my monoblock with 4x KT150 a big reserve with 8 x 470uF 550 vdc for each amp I use a Ntc 10 ohm 8 A + 5 resistor of 1 ohm 7 w in series then a relè delayed for 5 seconds to bypass the network. This in series with primary
Walter
Walter
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You need to understand that there is little correlation between price and designer competence (at electronics) but high correlation between price and designer competence (at marketing).mimikamimi said:
Does this amplifier have unusually large reservoir capacitors? Or an unusually large mains transformer? Overspecifying the PSU is an easy way to boost sales and get good reviews; people see the fat PSU and automatically hear good bass.
The great reserve is not for deep bass, or not only. The OT trafo need to be very fine for this job as for the high freq. of course The great amount of Joule helps on dynamic and this not mean necessarly do develop a great power to the load.
Walter
Walter
Well, to be fair to mimkamimi, he never actually told us what the units cost. 27k what? Box tops, secret decoder rings, marbles, pencils. Without a defining symbol, a simple number is meaningless.
Yes. And I even divided by 2 to try to figure out what the value of the thermistor was.
I would retrofit it with the soft start board on DIYStore, this board works great and bypasses the resistance. Those thermistors get too hot for my liking....
I have a hard time believing that an amp that costs so much contains parts that fail after only a few weeks of use.
Since on one has asked, what type of amps are these?
Have you contacted the engineer/designer and asked him why the thermistors are so unreliable? Don't the amps have a warranty?
Thermistors are not true soft-start devices. They influence current draw for only a very brief period of time. I would be tempted to just remove them and then use a variac to power them up. This would provide a true soft start. It would also be easier on the rest of the components than whatever momentary delay a thermistor can provide.
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You do not even tell us your mains voltage and ask for Tech answers? 😕
Start by posting that vital information.
Also post a picture and physical dimensions of original surge protector.
Any inscriptions on it?
Also measure DC resistance of power transformer primary.
Is it a toroid?
EDIT: add a back panel picture showing mains voltage, consumption, fuse rating, etc. and if available upload a User Manual copy.
The more/better data you provide, the better answers you´ll get. 🙂
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NTC inrush limiters usually work for maybe 4 - 10 cycles and then things settle down especially if they are bypassed by a time delay relay. Get the most Joules you can at the closest resistance if the are breaking.
The beauty of tube rectifiers is they do this all by themselves.
The beauty of tube rectifiers is they do this all by themselves.
Some usefull info here - Inrush Current and here - Soft-Start Circuits
On a pair of monoblocks I made I used a thermistor for a few reasons 1) to reduce the fuse value and 2) as protection not for the amp but to stop any tripping of RCD's etc. The amps had big toroids, high capacitance (1100u) and series OP valve heaters.
I used the "bung in the biggest round black thing wot I've got" approach.
Andy.
On a pair of monoblocks I made I used a thermistor for a few reasons 1) to reduce the fuse value and 2) as protection not for the amp but to stop any tripping of RCD's etc. The amps had big toroids, high capacitance (1100u) and series OP valve heaters.
I used the "bung in the biggest round black thing wot I've got" approach.
Andy.
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