[R-390] B+ current

[Tisha Hayes]

Don,
That was a great technical write-up of choke input filters. You should
write that section up for a university book as many EE's have a
problem understanding the concepts you detailed out. It also speaks to
one of the reasons on why not to use the standby mode on the radio as
the B+ voltage rises into risky areas. Radios that still have the
"killer cap" on the IF filters should be particularly aware of that
risk as well.

Tubes are forgiving of minor excursions but the idea that the input
windings of an IF filter could handle the several hundred mA of B+
current from a shorted coupling capacitor can cause you to lose sleep.
As the radio sits in standby for hour or days all of those B+ caps
pick up a nice charge that is probably well in excess of the 250 mA or
so normally supplied by B+. Even if you have the B+ 250 mA fuse
retrofit I bet that the IF filter wire will burn out before the fuse
wire even gets slightly soft.

You could imagine some sort of current limited zener diode scheme to
regulate the B+ to prevent it from floating upwards into danger-land
(even in standby mode) for the caps but normally the radio does not
care too much about normal variations (within reason) on the B+
voltage. I do not plan on going that far as 99.5% of the original
engineering on the radio is optimal (except for the AGC and audio
response).

To me the CL-80 is more of a transformer and microswitch inrush
current limiter and to be a little nice on the tube filaments and the
ballast tube. Chronic high line voltages are something that I believe
we will see becoming less of an issue over the next few years as more
electric utilities implement voltage reduction schemes on distribution
power. If someone is adverse to adding one to the internals of the
radio there are ways of putting one into a tiny project-box that
exists outside of the radio.

-- 
Ms. Tisha Hayes/ AA4HA
-
"Life isn't about how to survive the storm, but how to dance in the rain"