K515 (the Owl) Ch= 5K03

• alternative name: Chicago Radio Lab

Model K515 (walnut) is essentially identical to the model K518 series with the following exceptions.

• The K518 has the Zenith crest and "Zenith DeLuxe" on the front.  The K515 does not have the crest and only says "Zenith"
• The K518 has three knobs on the clock, whereas the K515 only has two knobs.

The following models are identical except for cabinet color.

The K515, Chassis 5KO3 uses the Zenith "Tube Life Saver" which is basically an NTC (negative temperature coefficient) resistor in a red metal box at the rear of the chassis. It  initially measured at 1580 ohms cold resistance, dropping to about 40-50 ohms when heated. Note that those resistances are temperature dependent and will vary. To compensate for the reduced line voltage for the series filaments caused by insertion of that resistor into the series string, Zenith replaced the 12BA6 with a plug-compatible 6BJ6 which uses a filament of 6 volts at 150 mA in place of the 12 volts at 150 mA for the more typical 12BA6.

Note that model K518 does not have the "Tube Life Saver".

More details in "The 'All-American Five' - Two Zenith clock radios and Hallicrafters S-38EM" in the July 2023 issue of The Spectrum Monitor magazine.

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