[Collins] Re: 75S-1

[Dr. Gerald N. Johnson, electrical engineer]

There are three probably sources of hum in the 75S-1.

One is heater-cathode leakage in a tube. When it doesn't vary with the
volume control setting, it can only be the 6AT6 or the 6BF5.

Second is plate supply filter capacitor. Shunting it may help, but it
best to remove an old electrolytic from the circuit. Sometimes they get
leaky when they get old and that causes them to heat. The heat makes the
leakage worse and they heat more. Sometimes they expand more rapidly
than the vents will control and then they fill the chassis with shredded
foil, and conductive electrolyte and crepe paper filled with that
conductive electrolyte. Not a picnic to clean up.

Third is the bias supply filter capacitor injecting ripple into the
output stage grid.

They can be discriminated apart by a critical ear, though an
oscilloscope on the speaker make that discrimination easier. The first
will be 60 Hz sinewave, no harmonics. The second will be a 120 Hz
sawtooth because the plate supply uses a full wave rectifier. The third
will be a 60 Hz sawtooth because the bias supply uses a half wave
rectifier. So by ear you have to pick the octave (60 Hz is close to B an
octave above the bottom of the standard 88 key piano), and the
harmonics. Or just replace the potentially suspect parts, the two audio
tubes and the filter capacitors.

Unless the audio coupling capacitors are already orange drops, its of
benefit to replace them because their leakage will make the audio tubes
draw extra current which can add hum to the output and also will shorten
the life of those tubes and help the 6BF5 scorch the cabinet lid.

73, Jerry, K0CQ, Technical Advisor to the CRA.
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.