Quote from a previous unrelated thread...
"BTW an old timer on another forum suggests placing the dropping
resistor between the transformer and diode, NOT placing the dropping
resistor between the rectifier and smoothing capacitor. I don't agree!"
. . . .
Often what people are calling a dropping resistor is not there for the
purpose of dropping voltage. The classic app is "I'm replacing a
selenium rectifier with a silicon diode - what value resistor do I need
to add?." The typical 50s-60s ac/dc table radio with a selenium drops
about 4 or 5 volts DC as compared to 0.7 volts DC with the silicon
diode. This really doesn't need any compensation but there DOES need to
be a surge limiting resistor to protect the rectifier. Odds are that
there was one in place already and usually on the input side of the diode.
The same type of set utilizing a power xfmr really doesn't need much if
any limiting because the power xfmr does this inherently. It doesn't
hurt to have one though.
The resistor commonly found in such an application is typically in the
22-47 ohm range, sometimes as much as 100. Do the math. For a 40ma
load, a 100 ohm resistor is only dropping 4 volts. This is hardly of
consequence on the B+ line which may vary that much given incoming ac
If I want to do massive voltage dropping I'd personally tend do it on
the DC side of the rectifier. The is the situation often found in TV
sets, suitcase radio filament strings like the TO, etc. I suppose it
could be done equally as well at the rectifier input.
I'm not sure why this convention is in place. Maybe its just an
engineering 'habit' to do the AC-related chore of protecting the diode
on the AC-side and to do the DC-related task of voltage dropping on the
"DC side". Maybe there are issues with a resistor's ratings on the AC
side? Maybe the resistor AFTER the diode is more effective at limiting
outgoing DC voltage during the first couple of cycles while the filter
cap charges? Maybe it really doesn't matter at all?