[AI2V]

In case anyone missed it in an earlier post, here's the schematic of the shadow meter to LED conversion mod. Please comment on success or failure but NOT if you used a 10mm LED or one brighter than 4K mcd- there is usually not enough current to drive those. Also, don't get fancy and expect much light through both a band mask and a dial together. None of the values are critical, not even the zener diode. The 8.2 volt zener works fine but so do other zeners within a volt or two.

This circuit has worked well in several manufactures' superhet radios, not just Philco's. If you attempt non-Philco's, you must separate out the B+ lines to the tuned RF amp and IF tubes only, not any oscillator, detector, mixer, or audio tubes. This circuit will not work in a radio with only a two section tuning cap.

[TA Forbes]

Thanks for the neat info. As I look at your schematic it occurred to me that a similar circuit could be devised using a zener diode to fire a conventional 6v lamp. I.E use the shadowmeter lamp without the meter.

[AI2V]

TA- I think you would find that an incandescent bulb would require much more current than the circuit would allow. That's the reason that I chose the 5mm LED's; even the 10mm LED's drew too much current to be usable.

A lamp or larger LED could be used if someone wants to do a design including an active circuit with a FET or transistor; that wouldn't be very difficult but would require many more parts and a low voltage supply.

[TA Forbes]

Yes, that's right. I often forget the low voltage and current requirements that modern components have. 99.99% of the stuff I work on is 1930-38, a whole different set of operating parameters.

[AI2V]

Some have asked for a quick explanation of how this circuit works: the zener sets a fixed voltage (e1) below the B+. As the receiver is tuned to a station, the voltage at the load end of the 8K (e2) rises and approaches the full B+. This difference between these two voltages (e2-e1) lights the LED.

If an LED were simply placed across the 8K, the LED would become dark when tuned to a station, just the opposite of what's desired.