[TenTec] pilot light bulbs

[Dr. Gerald N. Johnson]

On Mon, 2008-08-04 at 12:39 -0700, Jim Brown K9YC wrote:
> On Mon, 4 Aug 2008 15:20:41 -0400, Rick Denney wrote:
> 
> >I would think it would be quite easy to install a couple of rectifier
> >diodes inline in the 12-volt power line from the RCA power plug on the
> >back of the tuner. 
> 
> Yes, I've done that too. Works fine. You only need to verify the current 
> draw and diode rating. 
> 
> Jim K9YC
> 
I prefer a resistor to a gaggle of diodes. The status lites panel on an
821A-1 transmitter contained 41 or 42 lamps, all running 28 volts. Takes
at least 4 diodes to make a useful drop for each lamp, or 165 diodes per
radio. And in the 1964 time period we figured it cost a quarter to mount
a resistor or diode. And my experience then was that two diodes per
thousand of 1N645 were marked backwards and so didn't provide the
correct circuit function.

The other reason I prefer a resistor is that an incandescent lamp takes
a big current surge when full voltage is applied, generally 15 to 16
times the rated operating current. So a 14 volt 80 ma lamp has a hot
resistance of 175 ohms. Inserting 10%, or 18 ohms, drops the voltage
about 10%. But the lamp cold resistance is only 11 ohms, so the 18 ohm
series resistors makes the cold combination 29 ohms, limiting the cold
surge to .48 amp instead of 1.28 amps and that can contribute to longer
lamp life. A diode or two won't limit that surge current as much (only
by the percent voltage drop) and if the diode isn't big enough the diode
junction may melt and loose that 0.6 volt forward drop.

Besides a resistor still costs less than a diode and at Collins special
care with diodes made them more expensive to mount than resistors, the
diodes getting extra strain relief in the way the leads were bent.

73, Jerry, K0CQ