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.
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.
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.
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.
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.
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.
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.
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
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
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.
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.
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.
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
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
Attachments
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.
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.
Re: Re: Inrush current limiting heater filaments
Even when the tubes are power cycled several times per week?
It appears that sizing these ICL critters is a little challenging at least for a newbie. I cannot find any at Digikey that are "light duty" enough for my heater transformer.
I used V1*A1=V2*A2 across the heater transformer giving me
A1=V2*A2/V1 or 6.3V*600ma/120V=.031ma of current on the primary side of the heater transformer. It seems like all of the current limiters listed operate at higher average currents, so they would not heat sufficiently to reduce their resistance during normal operation.
Am I missing something here? I suppose that I could put one on the primary of both my mains and heater transformer to get the primary current up.
Or not worry about it since most tubes die of natural causes before the filament pops.
Geek said:
Even when the tubes are power cycled several times per week?
It appears that sizing these ICL critters is a little challenging at least for a newbie. I cannot find any at Digikey that are "light duty" enough for my heater transformer.
I used V1*A1=V2*A2 across the heater transformer giving me
A1=V2*A2/V1 or 6.3V*600ma/120V=.031ma of current on the primary side of the heater transformer. It seems like all of the current limiters listed operate at higher average currents, so they would not heat sufficiently to reduce their resistance during normal operation.
Am I missing something here? I suppose that I could put one on the primary of both my mains and heater transformer to get the primary current up.
Or not worry about it since most tubes die of natural causes before the filament pops.
Re: Re: Re: Inrush current limiting heater filaments
Even when they are cycled several times per day.
They were designed to do this, it's their job and they do it quite well 🙂
Even the "flashing" 12AT7's are OK.
Gotta admire those old slide-rule engineers 😎
Exactly.
Cheers!
boywonder said:
Even when they are cycled several times per day.
They were designed to do this, it's their job and they do it quite well 🙂
Even the "flashing" 12AT7's are OK.
Gotta admire those old slide-rule engineers 😎
Exactly.
Cheers!
AC or Dc
Hey
If you are running ac to the heaters then a 25 W ww resistor on each leg might help.. you could also reduce the voltage to the heater to say 6V for a 6.3 volt.. it would be in the order of .1 or .15R
Hope it helps
Nick
Hey
If you are running ac to the heaters then a 25 W ww resistor on each leg might help.. you could also reduce the voltage to the heater to say 6V for a 6.3 volt.. it would be in the order of .1 or .15R
Hope it helps
Nick
SpreadSpectrum wrote :
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.
Hi boy wonder ,
If you really want to do any thing about inrush current ,
IMHO this is the most reliable solution and is very simple to implement it
the traditional and cheap LM 317 , wired as a constant current source can
do the job quite well .
Ray moth wrote :
I know some engineers who regard thermistors as unreliable in service.
Hi RAY ,
I am among them . I have had many experiences with thermistors , and
most of them ,was not necessarily “ a pleasure “ .
I think that the thermistor’s main usefulness is on switched mode P.S. and
on electronic ballast for compact / fluorescent lamps .
Geek wrote :
Emission drops off long before the heaters ability to heat does.
Even when they are cycled several times per day.
They were designed to do this, it's their job and they do it quite well
Hi Geek ,
I agree entirely with you if we are talking about low power tubes , BUT
I have to disagree from you ( and then I think that you will agree with
me ) if we are talking about high power tubes like transmission tubes
or even the M.P. direct heated tubes like 211 , 811 , 813 , 845 , etc.
Regards for all ,
Carlos
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.
Hi boy wonder ,
If you really want to do any thing about inrush current ,
IMHO this is the most reliable solution and is very simple to implement it
the traditional and cheap LM 317 , wired as a constant current source can
do the job quite well .
Ray moth wrote :
I know some engineers who regard thermistors as unreliable in service.
Hi RAY ,
I am among them . I have had many experiences with thermistors , and
most of them ,was not necessarily “ a pleasure “ .
I think that the thermistor’s main usefulness is on switched mode P.S. and
on electronic ballast for compact / fluorescent lamps .
Geek wrote :
Emission drops off long before the heaters ability to heat does.
Even when they are cycled several times per day.
They were designed to do this, it's their job and they do it quite well
Hi Geek ,
I agree entirely with you if we are talking about low power tubes , BUT
I have to disagree from you ( and then I think that you will agree with
me ) if we are talking about high power tubes like transmission tubes
or even the M.P. direct heated tubes like 211 , 811 , 813 , 845 , etc.
Regards for all ,
Carlos
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