81 "All American Five" 50L6 radio
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
81 "All American Five" 50L6 radio
I've got an "All American Five" 50L6 radio that has new filter caps,
but the hum that comes out of the speaker is really out of sight. I can
hear it in the next room when the volume is so low I can't really hear
the station it's tuned to. I know these sets hum, but should it be that
bad? All the tubes test good on a mutual conductance tube tester.
No---you've probably got a very common tube fault that a
tube tester doesn't detect, heater-cathode leakage, probably in the
50L6. In these sets, the low end of the 50L6 heater is about 38 VAC
above ground, and the high end, up at 88 volts. What you are getting is
AC on the cathode, and the only real solution is a 50L6 that doesn't
have heater-cathode leakage. 12SQ7's can also have this problem,
although they are always wired at the ground end of the heater string.
The only real diagnostic is to scope the cathodes of both tubes.
One item that aggravates this situation is that many "All
American Five" sets had no bypass capacitor across the power amplifier
cathode bias resistor. Hanging a 50 mfd. 50 volt cap here often will
improve set performance and reduce hum, although it won't solve a
serious case of leakage.
Before trying to diagnose hum problems, particularly in a series
string set, try turning the plug to the wall socket around the other
way, to reverse the polarity of the chassis. Many of the older 300 ma.
series string sets were very sensitive to primary power polarity, and
would have very loud hum if the power plug were connected the wrong
way.
Continue to:
My Books
