Neon lamp question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ECS, Apr 2, 2015.

  1. ECS

    ECS Member

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    So I'm working on a Vox AC4 clone using parts from an Akai M8 tape machine. I'm trying to use as many of the original parts as possible, and there's a perfect neon indicator lamp which was originally the "record" indicator.

    I'm having a hard time finding out if this lamp will be happy to run off 120Vac as a pilot light for my new build. The lamp itself doesn't have any markings on it. If I'm reading the schematic correctly it's running off a DC voltage through inductor L2 in the original circuit with a 300k resistor in series. Will this lamp be alright to run off 120Vac with a suitable resistor in series, if so, how do I calculate the resistance and wattage? Schematic attached with part in question circled:

    [​IMG]
     
  2. Chrome Dinette

    Chrome Dinette Silver Supporting Member

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    The neon lamp would probably work just fine on 120ac even with no limiting resistor, but you could pick an arbitrary value to put in series with it, just in case. Just do the ohm's law math to see what the voltage drop will be and make sure you don't dissipate more watts than the resistor is rated for.

    Neon lamps are cheap, so if you blow it up, it is easily replaced.

    I have a really great PDF of the old GE neon lamp manual that can probably answer your question for certain. It also has some neat schematics, including one and two octave down circuits using neon lamps. It also describes using neon lamps to couple two tube stages, instead of caps.

    PM me if you want a copy.
     
  3. PushedGlass

    PushedGlass Member

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    I'm pretty sure that for 120VAC, any neon lamp needs a limiting resistor to make their dynamic effective resistance positive. I blew one up as a kid not knowing any better. :)
     
  4. Chrome Dinette

    Chrome Dinette Silver Supporting Member

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    Yeah, you are right.

    Data sheet for modern leon lamps, with series resistor values, which are probably much the same as the one in question in the Akai.

    http://www.mouser.com/ds/2/72/Neon Indicator Lamps_7___8-277248.pdf
     
  5. ECS

    ECS Member

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    Ended up using a 68k resistor in series and the lamp glows quite happily!
     
  6. J M Fahey

    J M Fahey Member

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    Yes, you always need a series resistor, a typical value for 120V is 100k 1/4w ;m68k is reasonable and will make it shine a little brighter.

    I made experiments, found that by increasing current a lot (by lowering series resistors) made them glow brighter until above a certain current the colur changed, becoming a beautiful pinkish glow instead of plain orange ... problem is they died in less than a week, and glass was blackened inside.

    Oh well.
     

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