Tube flashes upon turn on, why? - diyAudio
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Old 29th October 2004, 02:11 AM   #1
arnoldc is offline arnoldc  Philippines
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Default Tube flashes upon turn on, why?

I have a Matsushita 12AX7 that every time I power on the circuit where it is connected will cause a flash inside the tube as if it got toom much voltage/current and the filament glows really bright and goes back to normal.

I have isolated it to be a filament thing, because once I give it the filament voltage it flashes briefly.

It does sound normal after that. Is there something wrong with the tube?
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Old 29th October 2004, 03:00 AM   #2
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Thats normal.. I've had tubes do that before too,and it kinda wierded me out..But it's Not a problem.. (I think alot of Mullard and Tele tubes "flash" like that..)
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Old 29th October 2004, 03:03 AM   #3
mrstone is offline mrstone  Hong Kong
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Some Mullard and Philips tubes got same "flash" phenomenon, it seems like it's their characterisitc??

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Old 29th October 2004, 03:10 AM   #4
Colt45 is offline Colt45  Serbia
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normal. lots of western european tubes do that.
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Old 29th October 2004, 03:15 AM   #5
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Default Excessive voltage ?

Hi Guys,
Did you consider that the tube must be having excessive voltage at the anode ?
On power up , before the tubes turn on , the voltage drops in the power supply components are practically zero and the full dc would appear at the anode . This could be higher than the tube can handle. As soon as it heats up the current will start flowing and the voltage will fall.
You should measure HT voltage at power on and after a while.
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Old 29th October 2004, 03:49 AM   #6
Colt45 is offline Colt45  Serbia
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anode voltage has nothing to do with this.

they'll do it with no circuit except heater power..
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Old 29th October 2004, 04:03 AM   #7
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Thanks for the info Colt45.
Never seen this happen yet. Strange how it flashes at such low voltage. I assumed it was a white flash like a HV spark that you were talking about.

However if you are refering to the bright orangish flash from the heater , I have seen that in some ECC series tubes.
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Old 29th October 2004, 04:17 AM   #8
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Nichrome has a positive temperature coefficient, so if you have a heater wire with a few small sections of smaller diameter, they will hog all of the current for a very short time as they heat up and the resistance increases.
This could be wrong but it sounds as good a hypothesis as any.
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Old 29th October 2004, 05:20 AM   #9
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Tube heaters have a positive temperature coefficient, meaning that they have a lower resistance when cold that they do when hot. This causes a current surge to occur on turn-on, which can make the heaters glow white-hot for a short time. Some claim this shortens tghe lifetuime of the tube.

The "inrush" current can be limited by using a device with a negative temperature coefficient, such as a thermistor, in series with the heater transformer primary winding. The thermistor's resistance is a few hundred ohms when cold, falling to a very low resistance when hot (the opposite behaviour tio a tube's heater).
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Old 29th October 2004, 09:32 AM   #10
arnoldc is offline arnoldc  Philippines
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thanks guys. admittedly i freaked out since this is the first time i've experienced one.

on a similar note, i've owned a NOS RCA 5U4G that after first power on in a circuit gave some "lightning effect" on the top part (ST bottle). i though it was a "welcome, i'm alive" thing it was never repeated.
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