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This is a power supply to a small audio amplifier in John Lenk's book Handbook of Simplified Solid-State Circuit Design (p. 82). It appears highlighted below in parallel with the transformer and fuse, and in series with an unlabeled resistor. It is not described in the accompanying text. What is it, and what does it do?

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This is a neon glow lamp and its ballast resistor.

It is there to indicate to the operator that the circuit is powered on.

As of today, neon indicator lamps are considered vintage (or obsolete, depending on the context).

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  • \$\begingroup\$ I was wondering if it might be some kind of an indicator but couldn't find the symbol anywhere. I suppose an LED would suffice nowadays? \$\endgroup\$
    – nuggethead
    Commented Aug 14 at 10:53
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    \$\begingroup\$ Yes, with some caveats noted here: Is there any reason nowadays to use a neon indicator lamp instead of an LED? \$\endgroup\$
    – Dave Tweed
    Commented Aug 14 at 11:21
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    \$\begingroup\$ LED will be better placed at the secondary side after the rectifier. \$\endgroup\$
    – fraxinus
    Commented Aug 14 at 12:50
  • \$\begingroup\$ Another alternative would be to use a lighted switch. There are also LED panel lamps made specifically for use with AC line voltage, those would be a direct replacement for the neon lamp, many are even advertised as "neon indicator lamps" despite being LEDs. \$\endgroup\$
    – GodJihyo
    Commented Aug 14 at 17:03
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    \$\begingroup\$ Please don't try and tell me that my Nixie tubes are (sob) obsolete! \$\endgroup\$
    – Jon Custer
    Commented Aug 14 at 18:54

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