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    Flash in filaments.

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    Pillo69

    Posts : 158
    Join date : 2012-04-11
    Location : Granada (España)

    Flash in filaments.

    Post by Pillo69 on Wed Oct 24, 2018 2:50 pm

    I have used in the PCB of the ST70 tubes 6GH8A, now I have gotten ECF82 tubes of the brand "TESLA " and have the flash when turning on the amplifier.

    This generates a problem to the tubes?.
    can be solved?.
    Thank you.

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    Peter W.

    Posts : 965
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Flash in filaments.

    Post by Peter W. on Wed Oct 24, 2018 3:15 pm

    Pillo69 wrote:I have used in the PCB of the ST70 tubes 6GH8A, now I have gotten ECF82 tubes of the brand "TESLA " and have the flash when turning on the amplifier.

    This generates a problem to the tubes?.
    can be solved?.
    Thank you.


    It depends much on the tube, the design of the amp, and how much 'headroom' (capacity) in the filament winding. What happens is this:

    a) Cold filaments have very low resistance, so the filament winding is very nearly dead-shorted at turn-on.
    b) All that current hitting small-signal-tube filaments can cause a momentary overheat until the filaments warm and resistance increases.
    c) Which results in a *FLASH*.
    d) Back in the day, certain brands of audio equipment that tended to use oversize transformers would be more prone than others, Eico comes immediately to mind that way.
    e) Generally, this is not a problem. But, I will not opine specifically to post-blight tubes - perhaps Mr. McShane can help that way.

    Now, some brands of tubes are far more prone to this than others. I have a set of Telefunkens that flash when dead-cold, but not if they are turned off for less than an hour or so.
    A thyristor will solve the problem nicely, as it happens.
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    peterh

    Posts : 1054
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Flash in filaments.

    Post by peterh on Wed Oct 24, 2018 3:16 pm

    Pillo69 wrote:I have used in the PCB of the ST70 tubes 6GH8A, now I have gotten ECF82 tubes of the brand "TESLA " and have the flash when turning on the amplifier.

    This generates a problem to the tubes?.
    can be solved?.
    Thank you.

    No, this is a normal behaviour on some tubes. Mullard/phillips are the most common but others exist.
    It does not harm the tubes in any way.
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    Pillo69

    Posts : 158
    Join date : 2012-04-11
    Location : Granada (España)

    Re: Flash in filaments.

    Post by Pillo69 on Wed Oct 24, 2018 3:45 pm

    Thank you for your input.

    nmchiefsfan

    Posts : 47
    Join date : 2012-03-21

    Re: Flash in filaments.

    Post by nmchiefsfan on Wed Oct 24, 2018 9:26 pm

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    cci1492

    Posts : 203
    Join date : 2016-05-09
    Age : 58
    Location : Bergen County NJ

    Re: Flash in filaments.

    Post by cci1492 on Wed Oct 24, 2018 9:36 pm

    I have 1 out of 8 cv4003 that does flash like that. The seller said it's normal for a triode but wouldn't be normal for a power output like a 6550 or 6L6 type. So I live with it, my guess it will die sooner than the rest.

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