Are NTC thermistors safe, always in the circuit?

[schoolie]

I want to put a CL-60 between the fuse and power switch on my bassman, so I can use fast blow fuses and maybe save some wear and tear. From what I've read, the Cl-60 should ideally be bypassed after start-up, and can explode when it fails. Any opinions? I know Fender uses them in their modern versions.'

Thank you for your help!

[R.G.]

I want to put a CL-60 between the fuse and power switch on my bassman, so I can use fast blow fuses and maybe save some wear and tear. From what I've read, the Cl-60 should ideally be bypassed after start-up, and can explode when it fails. Any opinions? I know Fender uses them in their modern versions.

The devil is always in the details. Design/selection of an NTC thermistor requires some analysis. The NTC makers have helpful application notes to help engineers do this bit of design. To do it properly, you have to know the maximum current that your amp will pull under maximum operating conditions and idle conditions, and the maximum internal box temperature under maximum external air temps ( i.e. - how hot does it get inside the amp when you're playing on stage in 105F heat in the sun, that kind of thing). Once you know that, you can select an NTC which will conduct the max current without burning up or exploding, and which will 'turn the corner' of limiting at power up and then heating up til it's negligible.

It's not horribly complicated, but it does take some information and calculation to say yes, it will or no, it won't for any given NTC.

NTCs can be and are sized to stay in the incoming AC line for many applications, including amplifiers. So yes, it can be done. It may or may not be possible for your amp and that CL-60 (or whatever.)

There is no substitute for knowing the numbers.

[schoolie]

Appreciate your helpful reply, R.G. Unfortunately, I have no idea what the thermal conditions would be in the chassis, and I guess I would need to model the power supply with thermistor to know the maximum current. This is a 5f6a circuit and Fender uses the CL-60 in their Bassman RI, so I thought it would be a good choice, and it's rated at 5A maximum.

[R.G.]

A current meter on the incoming AC (I like the Kill-A-Watt unit, neat toy with lots of uses) would tell you a lot of what you need to know.

[Chuck H]

Another issue is "Why do you want to include this in your amp?" Are you having, or do you anticipate problems that this new circuit would solve? Or do you just want to confound the circuit because it can be done? What is the goal?

[schoolie]

Another issue is "Why do you want to include this in your amp?" Are you having, or do you anticipate problems that this new circuit would solve? Or do you just want to confound the circuit because it can be done? What is the goal?

Good question. I thought that this might allow me to use a fast-blow fuse on the AC and offer a little more protection. Also, it's fun to experiment a bit with new gadgets, but I don't need to do add this. I just like buying new parts from Mouser

[Enzo]

And what would be the advantage of using fast blow fuses?

You should realize that the fuses in amps are there to prevent the amp from catching fire and burning down your house, not really to protect much inside the amp.

That amp has gotten along without it for 50 years now. hard to imagine it all of a sudden needs one.

If you just want to experiement, then go for it.

[exclamationmark]

Usually guitar amps don't really need slow-starting circuits (I don't particularly like NTC's either), but they can be helpful to tame inrush currents. A novel approach I saw somewhere was to stick a power limiting resistor in series with the primary (chosen to limit however much current you want), and then a relay in parallel with the resistor. The relay was a slow 50ms+ type, and was connected to the heater windings (I'm not quite sure whether it was rectified or had a time delay on it though). When the thing powered up, the resistor was initially in series with the power supply, but after a suitable delay, the relay shorted out the resistor.

[Chuck H]

I had an old CB radio that used a thermistor circuit to pause HV to the tubes until the filaments warmed up. The thermistor circuit failed long before the rest of the radio did. It was old, and components can drift. It got to where, on cold days, I would turn it on and leave the room to make coffee, or lunch, or whatever, and return hoping that it decided to operate. I finally bypassed the circuit. I suppose I could have repaired it instead, but I was so frustrated with the waiting that I chose to dissable it. That, and I wasn't doing electronics at that time so I wasn't sure how to do much but dissable it.

[J M Fahey]

Living in Argentina, many "not so usual" parts are not available over the counter everywhere, so I ended to not using them.
They *can* be special ordered, of course, but a lot of my customers live in faraway Provinces, and I want to make things easy for them.
Whenever posible, I design with standard , available anywhere parts, which when necessary they can get locally.
Not much of a National Authorized Service Network .
Typically it's "Joe the TV repairman over the corner" who does the dirty job.
So, for example, no Slow Blow fuses, nor NTC Thermistors used.
My breadwinner 100W SS amp used for ages a 1A Fuse in a 220V line, meaning it will blow with nominal 220V x 1A = 220W (or VA).
Given that the normal consumption reaches 140/150W and maybe 180W when Heavy Metal or Punk overdriven, fine.
The (available everywhere) fast blow 1A fuse worked for ages (there are 25 Y.O. amps still with the original fuse), and did not self destruct at turn on.
*BUT* about 8 or 10 years ago, I changed the design of my power transformers.
Found that modern Silicon Steel is *very* good, can be pushed *much* more than what everybody realizes, and allows far fewer copper wire turns.
My transformers now weigh 30% less than old ones, have about 40% less resistance, work cooler , and, ahem !!! , are cheaper.
BUT (there is always a butt making trouble .... sorry, I meant "but"), old faithful 1A fuses started blowing at turn on.
Not always, but, say, once a month.
After some experimenting, I upgraded (in that case, and because of the new transformer) to fast blow 1.5A fuses.
So far, so good, flawless performance.
I've been doing that for 8 or 10 years, so the amount of samples now is significative.
Perfectly stand the turn on thump (capacitors charging), blow instantly on shorts.
Dead boards almost never have blackened parts, meaning its size is perfect.
As a side note, I always ship an extra 1.5A fuse , taped to the power cord.
For some unscrutable reason, small faraway Mom and Pop "Radio/TV" shops carry 1 and 2A fuses , but not 1.5A.
Oh well.

Back to your amp: am I suggesting you do the same, use a 50% higher Fast fuse?
No way, use what´s been tried and true for ages.
Now, if you were in " A Small House in the (Pampa) Prairies" .... maybe you'd have to.
Good luck.

[schoolie]

Thanks again to everyone for sharing your experience, and common sense It sounds like I am creating a problem that doesn't exist then attempting to solve it Time to just finish this build, so I can get back to playing guitar.