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Old 9th September 2006, 09:08 PM   #1
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Question Is this in-rush current?

Hi all,


I have an audio innovations 300mk2 integrated amp. It uses fairly standard 12AX7 and 12AU7 in the preamp but more unusual ECL86 tubes for the power amp end of things. When I turn on the amp, the tubes briefly light up quite brightly then settle down to their normal glow. The amp works AOK and beats many high price others I have heard.

Now to my question - what causes this bright glow on turn on? I have heard of a thing call in rush current, but am not sure of what exactly it is? Is this what is happening? Basically I'm worried about adversly affecting tube life/operation. Would it be easy to fix?

Schematic attached - which I got from Drtube website.



Many thanks in advance for any suggestions,
Fran
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Old 9th September 2006, 11:29 PM   #2
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Hi

the resistance of the filaments is lower when it is cold...lower resistance, but keeping applied voltage the same, means higher current (inrush current?) going through the tube. From what I have read here and there I get the impression this phenomen is not that bad, also, the tube is designed with this in mind.

but, in case...to prevent it there are two easy solutions. When using AC you can select a trafo with a higher voltage than the filaments. Knowing the current normally drawn you can determine the value of the resistor, that, in series with the filament, will drop the excessive voltage. When turning the amp on, the filaments will be low resistance, so the resistor will burn most voltage...as the filaments get warmer, resistance increases and you get more voltage across the tube. Another option, when using DC, is to fit in a CCS (using a LM317). Set it to the needed current for the tube..the CCS will pass just this current, so if the tube is cold and the resistance is low, the V=R*I equation determines that voltage will just increase when resistance increases.

Erik
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Old 9th September 2006, 11:47 PM   #3
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Inrush current is an abnormally high current that occurs at turn-on. It can be due to the low resistance of cold heaters and/or the combination of SS rectifiers and high smoothing capacitance in the B+ supply.

Some people feel that unrush current does more harm then good, so they place a thermistor in series with the power transformer primary. The negative temperature coefficient (NTC) thermistor has a resistance of typically around 200 ohms when cold and less than 1 ohm when hot and helps to reduce inrush current.

The downside of this is that the thermistor has to run hot, although you could arrange a timer relay to bypass it after a few seconds.
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Old 10th September 2006, 09:29 AM   #4
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Thanks fellas,

OK, so the jury is out on whether I need to be concerned about this at all....


So just to check, if I wanted to put in the in rush current limiter, I would put it on the mains side (primary) of the traffo? Also how much heat would one of these give off?

Fran
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Old 11th September 2006, 07:36 AM   #5
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It actually looks worse than it is - and some tubes look worse than others. ECL86 is one tube that it looks really bad, lights up like a torch globe and then settles back. I have never bothered to do anthing about it on any of my 3 ECL86 based amps andI've never had a heater failure on any of them.

For your possible interest:
http://www.rtie.com/ntc/surggard.htm

There are at least 5 or 6 other manufacturers but these guys stuff is typical. I use some from this company in a laser power supply in an aircraft to limit charging currents into a big electrolytic capacitor at power on.

The devices don't get too hot BUT then if you are putting one inseries with the mains supply you are not going to touch it when the amp is on anyway - RIGHT. I've never had any problems just mounting them in a screw terminal block.

Cheers,
Ian
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Old 11th September 2006, 07:21 PM   #6
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So,


seeing as its all fairly easy to do and the parts are cheap, what do you guys think?


Should I bother doing it?

The ones I've seen here are rated as 2 ohms at 35 deg C falling to 0.025ohm at maximum current (eg rating of limiter eg 15 or 18 amps)

link to item: in rush current limiter

thanks again,

Fran
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Old 11th September 2006, 11:58 PM   #7
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Fran,
Those devices do not have suitable ratings for your application. The maximum steady state currents are way too high.

For these sort of devices, if (for example) you want to limit inrush current in a line where the steady state current is 3 Amps then you should use a device specified for around 4 or 5 Amps BUT not more than 6 Amps otherwise it will not get up to temperature and reduce its resistance properly.

Also remember that impedances reflect across transformers according to the square of the turns ratio. A current limiter device in the primary will reflect as a much smaller impedance in the secondary low voltage heater winding and so you will need a much larger "cold resistance" in the primary to be effective at suppressing heater inrush currents. It will however also give some suppression of large High Voltage Capacitor charge currents at switch on.

If however you put the device in the heater winding itself, then you can use a device with a lower "cold resistance" and it becomes more important to make sure that its "hot resistance" is low enough and that you have not selected a device rated at such a high steady state current that it will not get up to temperature.

Farnell have some EPCOS devices which will suit.

Look here for application info
http://www.epcos.com/web/generator/W...ssurl=&shop=no

Farnell carry:
B57235S509M (Farnell 975-1866)
B57235S100M (Farnell 975-1874)

B57236S509M (Farnell 975-1882)
B57236S200M (Farnell 975-1890)
B57236S250M (Farnell 975-1904)

B57237S100M (Farnell 975-4199)

Look at the application notes and data.
Decide whether you want to put a device in the primary of the transformer (in which case you will want one of the larger cold resistance values)
OR
whether you are going to just put one in the heater winding (in which case you need to check its hot resistance carefully).

Cheers,
Ian
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