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Old 31st January 2009, 03:50 AM   #1
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Default Inrush current limiting heater filaments

Would limiting the inrush current of heater filaments extend their life?

Assuming there is a benefit, would an inrush current limiter do the trick?

I'm just thinking about incandescent light bulbs dying when they are first turned on.
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Old 31st January 2009, 04:05 AM   #2
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IRCL device are relatively inexpensive. CL-90 and CL-80 are popular choices. Many vintage power transformers were designed for line voltages which are slightly lower than today's typical 125 volt line, so even the slight "hot" resistance of the NTC device might be appreciated. I can't see how it could hurt any, and the cost is low.

On the other hand, when was the last time you saw a heater filament incandesce as brightly as a common household lightbulb? When was the last time you saw a vacuum tube fail because the filament went out? I believe the most common mode of old age failure is loss of cathode emission, not filament burnout.
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Old 31st January 2009, 04:21 AM   #3
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The application I had in my head was a tube rectified B+ and a separate filament transformer, and just current limiting the heater circuit, and letting the tube rectifier do the normal slow warm-up, giving the heaters the same slow start treatment.

I relatively new to tubes, so I haven't had any die yet, but when I turn on my Baby Huey with JJ EL84's, a couple of them glow really bright for a second or two.....I understand that this is not that uncommon for some tubes.
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Old 31st January 2009, 04:25 AM   #4
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Like you say, it can't do any harm and if it's in one leg of the power tranny primary, where it's normally placed, it can also help protect the PS smoothing caps from inrush if you have an SS rectifier.
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Old 31st January 2009, 09:41 AM   #5
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Hi Folks,

I have had a look at these IRCL devices (but never used one) and they appear to be simple and cheap.

Question: Why do many amps have soft start circuits which are far more complicated when compared with an IRCL device. I presume that there are limitations to IRCL's or some advantages to a soft start circuit? Can anyone shed any light on this?

Regards,

Rob
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Old 31st January 2009, 02:40 PM   #6
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Soft start is a vague term but you're probably thinking of delayed start. The problem with thermistors, which maintain a high resistance when cold but have a low resitance when hot, is that they are only cold for a short time after the amp is switched on - a few seconds at most.

The short-term high resistance of the thermistor helps soften the inrush current but, with SS rectification, it is not enough delay to hold off the B+ until the tube heaters are properly warmed up. A separate delay is sometimes employed for that purpose in the B+ line. With tube rectifiers, of course, it's not an issue.
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Old 31st January 2009, 03:08 PM   #7
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Quote:
when was the last time you saw a heater filament incandesce as brightly as a common household lightbulb?
A friend of mine just recently had a Chinese 12AX7 stop working on him. Only half of it glows. Does it happen often enough to worry about? Probably not, but it sure doesn't hurt to limit inrush current. I did a constant current DC filament supply. This limits current at turnon as well. It's also a lot simpler than a constant voltage regulator.
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Old 1st February 2009, 10:25 AM   #8
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Hi Ray_moth

Here is a schematic of the 'soft start' that I have built.

It does stop the inrush current to the caps but you are correct, it does not delay the B+.

I am wondering if I should have bothered, it seems that a simple IRCL thermister may have been all that I needed?

Rob
Attached Images
File Type: jpg soft start.jpg (58.8 KB, 453 views)
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Old 1st February 2009, 10:46 AM   #9
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I think a thermistor is OK for what it does and I wouldn't bother to model it with other circuitry. It belongs in the primary of the power tranny, to protect all heaters from inrush. I know some engineers who regard thermistors as unreliable in service.

The reason some people like to delay SS-rectified B+ is mainlly to prevent damaging cold cathodes through the premature application of plate voltage (cathode-stripping). I have read that cathode-stripping doesn't occur below 1100 v but I don't know if that's true.
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Old 1st February 2009, 11:47 AM   #10
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Default Re: Inrush current limiting heater filaments

Quote:
Originally posted by boywonder
Would limiting the inrush current of heater filaments extend their life?

No.


Emission drops off long before the heaters ability to heat does.

Cheers!
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