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 Post subject: Thermistor sizing question
PostPosted: Jan Mon 09, 2017 4:02 pm 
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Joined: Oct Thu 02, 2014 5:57 am
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Location: Memphis, TN
I've been reading some on thermistors for inrush current limiting to help prolong the lives of these tubes. Rather than hijack another ongoing thread, I wanted to ask my questions separately. It seems like most people suggest anything between 120-220 ohms cold, and usually want the 'hot' resistance to be 2 ohms or less. From what I can gather, there's a 2-3 amp inrush on an AA5, but the radio only draws about 1/4 of an amp or less when it's warmed up. I'm looking to outfit everything from an AA5 and AA6 that are nearly daily players to some larger consoles.

1: Is there any practical difference between 120 and 220 ohms cold? It would make sense to me that the higher cold resistance should be 'safer', but it comes down to whether that's needed. Basically I'm curious if nearly doubling the cold resistance nearly doubles the protective effect, or if a 1 amp inrush essentially moves the tubes likely failure mode from an open fillament to cathode depletion.

2: Simmilar to #1, I'm curious if running a 2 amp one on a small radio would cause problems. I'd guess it would, but I don't know for certain. Is this a case where as far as current calability is concerned, you dont really want a lot of headroom? It seems to me that if I ran a 2 amp one on an AA5 it wouldn't ever get hot enough to drop down to 1 or 2 ohms, and therefore the radio would be running at a much lower line voltage, which would shorten the cathode's life. I can see a bit of an oscillation starting too, where the thermistor gets cold, the tubes draw more, then it warms up, the tubes warm up and draw less, the thermistor cools off, and so on. I have 125-128 in my house every time I measure, so I run a bucking transformer/switch/fuse box that drops the voltage down to 112, and I have one made up for each radio, so I wouldn't need anything to drop the voltage further. Plus, I don't think the thermistor would regulate very consistently.

3: How hot do these things actually get? I read that they need to be shielded from air flow so that their temperature stays fairly constant, but I don't know if mounting them in a plastic junction box would cause things to get too toasty. I'm thinking of mounting them in my bucking transformer boxes. Ill post a picture.


Last edited by jack.estes on Jan Mon 09, 2017 4:11 pm, edited 1 time in total.

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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Mon 09, 2017 4:06 pm 
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Location: Memphis, TN
Here's the transformer box in question. Inside there's a fuse, switch, and the bucking transformer. I use these to drop my house voltage down, protect the radio, and use the switch on the box to turn things on and off instead of using an 80 year old volume control knob...


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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Mon 09, 2017 6:10 pm 
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The bucking transformer should be fine for about any transformer set, it's the AC/DC sets that see a huge inrush of current... Unlike a transformer, the voltage/current supplied to tube heaters doesn't "sag" in a series string set at turn on... A bucking transformer would help but should be sized for current of radio, one capable of several amps will hit the tubes with approx same current as the AC line... For these radios a permanent thermistor install inside would be my choice... As far as operating resistance, 20-30 ohms hot isn't going to cause any issue...

Current is voltage divided by resistance so 120v divided by 120 ohm(approx resistance of a AA6 string), equals 1 amp... I consider this as excessive but apparently engineers didn't(maybe the toob mfgr's were anticipating increased sales?)... If you add 120 ohms, the current drops to half amp, add 220 to the base 120 ohms, start up current is around .35A...


Tom


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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Tue 10, 2017 5:06 pm 
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Thanks Tom! So, do you think the thermistors are going to get too hot for installation inside of the bucking transformer box? I'm thinking of basically having some transformer intended boxes, and series set boxes.


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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Tue 10, 2017 9:50 pm 
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I don't know about having thermistor in a enclosed plastic box, though I'd think if it has a couple inches clearance It'd probably be fine...

I have some Ametherm SL 12 22101 that measure around 280 ohms @ 72*F(rated 220 ohm cold but I'm not sure of temp)... In free air, connected to a AA6 I've not seen temp over 130*F with approx 5.5v drop... Enclosed where it could build additional heat I suspect voltage drop would be less...


Tom


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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Wed 11, 2017 3:27 am 
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For best results, inrush limiters should not be used at less than 1/4 of their maximum rating. Below this current, the resistance changes drastically with changes in load current.

They can get plenty hot but they don't dissipate a lot of power, so if you give them some room, they will be fine. I have put them in plastic enclosures, Sometimes they scorch the enclosure, sometimes not. It depends on the type of plastic and how big it is.

The higher the cold resistance, the better the protection. But the amount of protection also depends on what the cold resistance of the load is. You can use ohms law on the set itself and then on the set plus the limiter to see how much effect it has.

And AA5s don't draw 2 - 3 amp inrush; it's more like 1.5 amp peak.

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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Thu 12, 2017 1:26 pm 
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Thanks guys! I guess I've got some parts to order. That does make it easier that I won't need one for the transformer sets, and now I've got a better idea of what current ratings I need to look into too.


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 Post subject: Re: Thermistor sizing question
PostPosted: Jan Fri 13, 2017 7:22 pm 
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I just got these in yesterday, seem to be OK, dropping approx 3v in a AA6... They measure between 100 & 130 ohms cold, the one that's run for maybe 15 minutes settled back to 150 ohms once it cooled... But note ambient is approx 65* vs 72* when I checked them all...

http://www.ebay.com/itm/20pcs-120-Ohm-1 ... 1664960360?


Tom


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