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  • #16
    Originally posted by Helmholtz View Post
    The series lamp acts as a compressor. Its resistance increases with applied voltage.
    With 12V across the lamp, its nominal resistance is 66R. At low voltage its resistance might be only 10R.
    So total load impedance may roughly vary between 6R and 7R.

    Problem is that at 100W output the 12V lamp voltage will be considerably exceeded.
    Maybe two 12V lamps in series?
    Just realised, the drawing states 12volt, I have a 24volt lamp installed; https://uk.rs-online.com/web/p/incan...-bulbs/3607755
    Never had to replace it and forgot!
    The monitor speaker I use is from a scrap flat screen television.
    The resistors are mounted on a piece of aluminium kick plate. 1/2" thick and 12" X 12". Gets warm when used at high levels!
    As Helmholtz rightly states, there is a nominal impedance of 7 - 8 Ohms ish. It is a dummy load after all and the final impedance is not important.
    The lamp acts like a PTC resistor, the hotter the element, the higher the resistance giving compression and expansion to avoid damage in the monitor, neighbours and ears.
    Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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    • #17
      Originally posted by Jon Snell View Post

      It is a dummy load after all and the final impedance is not important.
      So I can put this gadget in any speaker output 4, 8 or 16 Ohms? It does not matter?

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      • #18
        Originally posted by Jon Snell View Post
        Just realised, the drawing states 12volt, I have a 24volt lamp installed; https://uk.rs-online.com/web/p/incan...-bulbs/3607755
        .
        Now this lamp has much higher nominal resistance (480R), so total impedance will be closer to 8 Ohm.

        - Own Opinions Only -

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        • #19
          Originally posted by Emetal View Post

          So I can put this gadget in any speaker output 4, 8 or 16 Ohms? It does not matter?
          For tube amps it is best to match the load properly. So you would use an 8 ohm load resistor on the amp's 8 ohm output.
          For solid state amps you do not want the load to be lower than the amps rated minimum load.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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