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Thread: temporary intense glow/dull flash on filament when heaters are turned on

  1. #1
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    temporary intense glow/dull flash on filament when heaters are turned on

    in my old holden/wasp amp 1 of the 12ax7's filament will have an intense glow as the heater is turned on, and settle to the normal glow after a second or 2. all other tubes slowly start to glow over a couple seconds, but this one is instantaneous and much brighter than usual (but only for a short period). i repeat, the intensity is only for a moment, and it returns to normal over the next second. it is only one tube

    any explanation for this? is it a bad thing? is it maybe a different material used for the filament which reaches the operating resistance once heated (as resistance rises with heat)?

  2. #2
    Senior Member Enzo's Avatar
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    SOme heaters do this.
    Education is what you're left with after you have forgotten what you have learned.

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    thanks, i was a bit worried, as there are 3 of the same tube type in the amp, and only one does this.

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    This comes up every once in a while here. Some tubes were designed for a faster turn on in TV's, radios, etc. Those are the ones that 'flash'. Usually there's no clue on the box or tube if it is fast or slow, just plug in and watch.

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    black_labb: I don't suppose this was with an Aus made tube was it? I was having the same thing happen with an old AWA Radiotron 12AX7 today and it freaked me out!

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    Noodle of Reality Steve Conner's Avatar
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    I've seen quite a few preamp tubes that do this. I don't know why it happens, but it doesn't seem to cause any harm.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by andrew_k View Post
    black_labb: I don't suppose this was with an Aus made tube was it? I was having the same thing happen with an old AWA Radiotron 12AX7 today and it freaked me out!
    was a miniwatt, another aus tube but not a radiotron.

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    Senior Member TD_Madden's Avatar
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    all my Phillips 12AX7s do this.

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    Quote Originally Posted by black_labb View Post
    in my old holden/wasp amp 1 of the 12ax7's filament will have an intense glow as the heater is turned on, and settle to the normal glow after a second or 2. all other tubes slowly start to glow over a couple seconds, but this one is instantaneous and much brighter than usual (but only for a short period). i repeat, the intensity is only for a moment, and it returns to normal over the next second. it is only one tube

    any explanation for this? is it a bad thing? is it maybe a different material used for the filament which reaches the operating resistance once heated (as resistance rises with heat)?
    Lots of info on your amp on (Google) " anz amps".
    Your heater glow is quite normal.
    Does your Wasp still have 4 preamp valves?
    Quite a few of them were severely modded to only use two valves, I have just completed a complete restoration on a 200 Wasp.

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    Quote Originally Posted by unparalleled View Post
    Lots of info on your amp on (Google) " anz amps".
    Your heater glow is quite normal.
    Does your Wasp still have 4 preamp valves?
    Quite a few of them were severely modded to only use two valves, I have just completed a complete restoration on a 200 Wasp.
    i've been on the anz amp site a fair bit, its a great resource. was going to send in a schematic of the wasp amps, but i lost the diagram i was doing and need to redo it. i've got 2 200w wasps, one XL and one VBL. both have 4 6550's. great amps. i changed a coupling cap and cathode bypass cap on the xl as they were pretty small, making the amp a bit too bright.

    which one did you restore? any pics anywhere? did you change the electrolytics? i've been meaning to do that, but they seem to be fine.

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    Quote Originally Posted by black_labb View Post
    i've been on the anz amp site a fair bit, its a great resource. was going to send in a schematic of the wasp amps, but i lost the diagram i was doing and need to redo it. i've got 2 200w wasps, one XL and one VBL. both have 4 6550's. great amps. i changed a coupling cap and cathode bypass cap on the xl as they were pretty small, making the amp a bit too bright.

    which one did you restore? any pics anywhere? did you change the electrolytics? i've been meaning to do that, but they seem to be fine.
    200 XL - 6550s, it was ready for the dump when I got it!
    Replaced everything except the preamp pcb and the transformers.
    Made a new power amp PCB, original board was charred.
    restored the cabinet also.
    I used 3x 33uF 450v electrolytics to replace the ones on the PCBs, I have plenty of these NOS caps if you want some. (they are small axial lead type and fit perfectly)
    Some of the diagrams on anz site are missing cap values I can fill in the gaps there.
    Roly the web site owner wants me to send him pics which I will send him when I have written some blurb to go with them.

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    Quote Originally Posted by unparalleled View Post
    200 XL - 6550s, it was ready for the dump when I got it!
    Replaced everything except the preamp pcb and the transformers.
    Made a new power amp PCB, original board was charred.
    restored the cabinet also.
    I used 3x 33uF 450v electrolytics to replace the ones on the PCBs, I have plenty of these NOS caps if you want some. (they are small axial lead type and fit perfectly)
    Some of the diagrams on anz site are missing cap values I can fill in the gaps there.
    Roly the web site owner wants me to send him pics which I will send him when I have written some blurb to go with them.
    There are some photos of my work online: Google "guitar amplifier repairs howick"
    You will find my local Trade and Exchange (online newspaper) work wanted adverts.

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    Quote Originally Posted by TD_Madden View Post
    all my Phillips 12AX7s do this.
    Yep Philips and other Euro pre amp tubes will flash. Totally normal.
    "Reality is an illusion albeit a very persistant one " Albert Einstein

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    My old teacher explained this that the valve wasn´t 100% completly sealed and burned air.

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    Quote Originally Posted by SteikBacon View Post
    My old teacher explained this that the valve wasn´t 100% completly sealed and burned air.
    Interesting theory, my understanding is that air does not burn.
    Wish I could get my old V8 to burn air!
    If there was air in the tube the silver coating on the inside from the getter would oxidize and the tube would turn white.
    The filaments are designed to handle the inrush current on switch on.

    My old employer used to fill out service sheets " Replace rectumfire valve and tune for maximum smoke"

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    Quote Originally Posted by unparalleled View Post
    200 XL - 6550s, it was ready for the dump when I got it!
    Replaced everything except the preamp pcb and the transformers.
    Made a new power amp PCB, original board was charred.
    restored the cabinet also.
    I used 3x 33uF 450v electrolytics to replace the ones on the PCBs, I have plenty of these NOS caps if you want some. (they are small axial lead type and fit perfectly)
    Some of the diagrams on anz site are missing cap values I can fill in the gaps there.
    Roly the web site owner wants me to send him pics which I will send him when I have written some blurb to go with them.
    There is now a feature on ANZ amps about my rebuild, although it describes making a new PCB he has put up a Photo of the wrong board. I have advised him and he will no doubt change the photo in due course.

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    Quote Originally Posted by unparalleled View Post
    There is now a feature on ANZ amps about my rebuild, although it describes making a new PCB he has put up a Photo of the wrong board. I have advised him and he will no doubt change the photo in due course.
    just had a look, very well done.

  18. #18
    Noodle of Reality Steve Conner's Avatar
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    I don't buy the "burning air" thing either.

    (To be pedantic, your V8 burns air already, or rather burns the oxygen in it by combining it with gasoline. If you don't believe me, try bunging up the carburettor so no air can get in. This really improves the gas mileage because the engine won't work and you can't drive anywhere.)

    If a tube flashes, it's because there is a relatively long piece of heater wire left uncovered by the white stuff that forms the heater-cathode insulation.

    The resistance of tungsten increases with temperature, so if one part warms up quicker than the rest, it will drop more voltage and heat even more, in a vicious circle until it's dropping nearly the full 6.3V and glowing white hot. This is what the uncovered lengths of heater do, because the lack of white stuff reduces their thermal mass. As the rest of the heater begins to warm, the white hot ends dim down and eventually go out.

    Some people say that this is deliberate, as it slows the warm-up of the rest of the heater, protecting the white stuff from cracking due to thermal shock. I suspect that Philips just found it difficult to coat the heater all the way to the ends.
    Last edited by Steve Conner; 03-25-2009 at 01:44 PM.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    I don't buy the "burning air" thing either.

    (To be pedantic, your V8 burns air already, or rather burns the oxygen in it by combining it with gasoline. If you don't believe me, try bunging up the carburettor so no air can get in. This really improves the gas mileage because the engine won't work and you can't drive anywhere.)

    If a tube flashes, it's because there is a relatively long piece of heater wire left uncovered by the white stuff that forms the heater-cathode insulation.

    The resistance of tungsten increases with temperature, so if one part warms up quicker than the rest, it will drop more voltage and heat even more, in a vicious circle until it's dropping nearly the full 6.3V and glowing white hot. This is what the uncovered lengths of heater do, because the lack of white stuff reduces their thermal mass. As the rest of the heater begins to warm, the white hot ends dim down and eventually go out.

    Some people say that this is deliberate, as it slows the warm-up of the rest of the heater, protecting the white stuff from cracking due to thermal shock. I suspect that Philips just found it difficult to coat the heater all the way to the ends.
    No,nothing to do with burning air. European manufacturers didnt feel the need to worry about the phenomenom. US manufacters did and built in a slower thermal warm delay time into the design. Bob
    "Reality is an illusion albeit a very persistant one " Albert Einstein

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    Senior Member Enzo's Avatar
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    Just don't crack the glass. if the burning air inside the tube manages to get out, it could catch the earth's atmosphere on fire, and then we'd be screwed.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Just don't crack the glass. if the burning air inside the tube manages to get out, it could catch the earth's atmosphere on fire, and then we'd be screwed.
    You could avert that kind of disaster, just use your Goniometer to track the radioactive "white stuff" from the burning filaments.

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    I read somewhere that 12AX7As and 7025s don't do this, it's just 12AX7 (early US) and ECC83 (euro) valves, something to do with 12AX7A being designed for series string operation. It's most definitely the low initial resistance of the heater. For what it's worth, most of my mullard 12dw7/7247s do this too. Sure makes startup look dramatic !

  23. #23
    Supporting Member tubeswell's Avatar
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    Sure is dramatic - its a little fireworks party when you flick the switch on.

    Hey unparalleled, where d'ya get them gonzo meters?
    Building a better world (one tube amp at a time)

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    Quote Originally Posted by tubeswell View Post
    Sure is dramatic - its a little fireworks party when you flick the switch on.

    Hey unparalleled, where d'ya get them gonzo meters?
    How about DC filament supply with slow switch on? Should be simple enough to arrange a 3 pin reg with a capacitor.

    Goniometer, google it ( direction finding device. )
    (Not for detecting severity of VD)

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    Talking Filament flash or Bright glow

    Most Euro made ECC83 tubes do this on power up.

    No harm is being done, the filaments just flash or glow bright for a few seconds.
    My friend has a few ECC83 tubes in his Hi Fi amp.
    They have been inservice and doing this for years in his amps.

    I believe they do the same thing in regulated D.C. filament supplies.

    NO worries

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