Pilot Light Burnout

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MARTIN MARRIS:
I have a rebuilt Heathkit EK-2B receiver. The pilot lamp that lights the tuning dial (#47) always burns out after a few hours -- I've now gone through half a dozen of them. The measured voltage checks out as correct (6.3v). Suggestions?

73 de Martin, KB1WSY

Gregory J. Beat:
The Heathkit EK-2B is the 1950s - 1960s educational kit for Basic Radio course.
As I remember, this is an All-American Five style design ---
Tube filaments and pilot lamp are wired in series with the AC Line Voltage (no isolation transformer).

When this kit was designed (1950s), USA line voltage of 110 to 115 Volts was common,
today 120 to 125 Volts is more common.
For example, my residential line voltage, this evening, is 125 Volts !

The #755 is a well-known replacement (longer life) for the #47 lamp.
Very inexpensive at $1.35 for 2 lamps.
http://www.2bradioparts.com/page22.html

(Longer Life)
#755 Bulb /Specifications
Size  T-3  1/4 Lamp Size
Volts 6.3V
Current .150 Amp
M.S.C.P. 330
Average Life 20,000 Hours
===
Factory Original Bulb
#47 Bulb Specifications
Size  T-3 1/4
Voltage 6.3V
Current .150 Amp
M.S.C.P .500
Average Life 3,000 Hours

Dieter Kuespert:
The dial lamp uses the same voltage as the heating coming from a transformer. Put a resistor in series with the lamp thus reducing the voltage.
A 3 Ohm resistor would reduce the current by 10 mA. That should be sufficient. A more radical approach would be a LED in place of the bulb. This also would require a resistor and a diode due to AC being used.

Peter Chadwick:
I've used 47 ohms in series with a#47 in my Hy Gain beam rotator boxes. Still on the original bulbs from 1985.....

MARTIN MARRIS:
GB: it is a fairly ancient kit but there is a heater secondary on the transformer, and as I said the voltage checks out at 6.3 although I agree that with today's line voltages it is probably pushing the envelope. When I measure the line voltage here it is usually around 122-125.

POL, RZP: I had considered installing a resistor and will do so now; s/b a 1W I presume (6.3 x 0.15).

GB:... and if that doesn't work I'll then try the #755; sounds like it would be worth getting in a stock of those anyway.

POL: I'd also thought of using a LED but I try to maintain authenticity in these sets if possible ... call me weird. It is true that the pilot lamp is awkwardly located (right at the top of the front panel, almost touching the underside of the cheesy wooden cabinet) and gets very hot both because of the accumulated heat from the tubes and the proximity of the cabinet. Using a LED would solve that issue as I assume they are not very heat sensitive and give out almost no heat.

I've set up an ultra-low power AM broadcast band transmitter (this is legal here) so that I can transmit Internet radio stations throughout the house and listen to them on my ancient AM radios. The Heathkit EK-2B is next to the desk where I run my home business, so that pilot lamp is getting a lot of use! This "mucking about with radios" is standing in as a proxy for later, when I will have more time to do "real radio." The stash of parts for my very first homebrew RX and TX is sitting forlorn in the shack, if you can call it that, since it has not originated a ham signal yet. Sigh.

Thank you everyone for the advice.

Martin, KB1WSY

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