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Thread: Valve amp question

  1. #21
    Join Date: Jun 2015

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    I'm Lawrence.

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    I had assumed that it was unlikely to be the valves given that the one in the same location did the same thing when I changed the pair to NOS. But I will make doubly sure and swap them around next time I use the amp. At the moment I'm using valve pre/SS power as my new Celestions seem to prefer that.

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  2. #22
    Join Date: Jun 2015

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    Quote Originally Posted by anthonyTD View Post
    Ok,
    If this only happens in that one position with any valve you put in there, then it is not the usual heater filament diffrences you get in diffrent types of valves that's causing the valve to come on bright, its posibly a heater circuit issue, especialy if this amp has DC heaters on the first valve, which many amps do!
    I am not saying for certain that your amp does indeed have an issue, but i just find it odd that it only seems to do this in one position. A look at the schematic for this amp would be a useful start.
    A...
    Ok I've changed back over to the new valve amp today and swapped the 12AX7s around before I switched on. The bright flare up at switch on followed the valve so it looks like it's not the amp at all!

    My assumption that I was highly unlikely to have two completely different valves that did the same thing, and put them in the same location, has been proved to be false! (Will strictly it hasn't, I may have just been very unlucky with a tail event but that's another question!).

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  3. #23
    Join Date: Feb 2008

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    Sounds Good!
    Always good to re-check things, and in this case it looks like it is indeed just the character of the valve heater.
    Anyway, Good news.
    A...
    Quote Originally Posted by Lawrence001 View Post
    Ok I've changed back over to the new valve amp today and swapped the 12AX7s around before I switched on. The bright flare up at switch on followed the valve so it looks like it's not the amp at all!

    My assumption that I was highly unlikely to have two completely different valves that did the same thing, and put them in the same location, has been proved to be false! (Will strictly it hasn't, I may have just been very unlucky with a tail event but that's another question!).

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  4. #24
    Join Date: Jun 2015

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    Thanks for your help sir, and everyone else.

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  5. #25
    Join Date: Jun 2015

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    I've been thinking about this some more as something bothers me intellectually. I can't believe that the variation in heater resistance/electrical characteristics in two otherwise good valves could be so different that the voltages across then would vary so much to cause the big difference in brightness on start up. So I've got a theory!

    I surmise that the behaviour of transient current may not behave in the same way during a brief voltage surge than in a steady state. So the flow of current will take "the path of least resistance" rather than distribute more evenly according to the relative resistances of the heaters.

    An analogy may be a sudden surge in flood waters down a river if, say, a dam burst. Suppose the river split and one part was straight ahead while the other turned off (say it went round an eyot). The momentum of the moving mass of water might take the vast majority down the straight path rather than distribute the flood water between the two paths as would normally happen.

    What do people think? Does anyone care?? It might have implications for circuit design and safety/longevity so I imagine it is of interest.

  6. #26
    Join Date: Oct 2012

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    Quote Originally Posted by Lawrence001 View Post
    I can't believe that the variation in heater resistance/electrical characteristics in two otherwise good valves could be so different that the voltages across then would vary so much to cause the big difference in brightness on start up.
    I think you have a wrong assumption there, I don't believe the voltage varies across the two valves. The heater supply will be fairly stiff as it will be rated at an amp or more (particularly if shared with the power valves).

    I think the phenomenon is due to thermal mass, the 'tail' of the heater wire not wound together or withing the heater/cathode sleeve will have a lower thermal mass and will heat up more quickly. Once the bulk of the heater heats up and the hot resistance increases the heater voltage is distributed evenly across the whole heater.

  7. #27
    Join Date: Jun 2015

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    Quote Originally Posted by Firebottle View Post
    I think you have a wrong assumption there, I don't believe the voltage varies across the two valves. The heater supply will be fairly stiff as it will be rated at an amp or more (particularly if shared with the power valves).

    I think the phenomenon is due to thermal mass, the 'tail' of the heater wire not wound together or withing the heater/cathode sleeve will have a lower thermal mass and will heat up more quickly. Once the bulk of the heater heats up and the hot resistance increases the heater voltage is distributed evenly across the whole heater.
    Sorry you're right I was thinking of them in parallel so the same voltage but power dissipation varies ie. I squared R as the current flows mainly through the slightly lower resistance heater.

    Still doesn't agree with your idea though.

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