OT: white LEDs (was: Re: Solderless breadboarding (and 68010 vs

IIRC, there were American valves (perhaps I should say tubes :-)) that contained a half-wave rectifier and an audio autput pentode in one envelope. The number 70L7 springs to mind, but I am not well up in US valve numbers. They were desinged for use (in the obvious way) in lo-fi record playes (a crystal pickup cartridge would give enough signal to drive the pentode section). I am suprised your conde practice oscillator didn't use one of those. Did it have haedphones as the audio output devices? If so, it only takes one little insulation breakdown to turn it into a fair version of the electric chair... Antohter trick used over here in hybrid TV sets (using transistors for the small-signal stages and valves in the output stages) was to put a diode in series iwth the heater string. This reduced the dropping resistor needed (and saved power). Often, the supply to the transistor stages was taken from the bottom end of the heater string, using the heaters as the dropping resistor for that supply. Getting back to LEDs, I bought a cheap wall-wart polarity tester. It conists of a couple of LEDs and limiting reistors wired to assorted power connectors. The LED/resistor pairs are wired in inverse paralel, so one lights for tip +ve, the other for tip -ve (and both light for an AC wall-wart). But of course the LED that's not on gets the full wall-wort output voltage applied to it backwards, which is not a good thing. I modified miue by removing one of the resistors and adding a jumper wire on the PCB so the LEDs were in inverse parallel, and thus each LED limited the reverse voltage applied ot the other one. This must be one of the few cheap products which can be improved by removing a component -tony