[R-390] Audio resistors?

[Bill Hawkins]

Resistors make noise because their atomic particles are dancing
in the heat above absolute zero. The noise voltage is in the
range of 1 to 100 microvolts. Looking in Henney's "Radio
Engineering Handbook" under "Resistor Noise Characteristics"
it says that carbon composition resistors also have microphonic
noise because they are made from grains of carbon suspended in
a binder so that only points of the grains touch. "The noise
level increases with increasing voltage, increasing resistance,
and decreasing [physical] size." The section ends with:

"Most composition-resistor manufacturers [ca. 1959] are reducing
the noise level of their resistors, and most are so low that the
microphonic noise cannot be detected in any but highly specialized
amplifiers."

Also noted for variable carbon track resistors is that the
microphonic noise is greater, and worse if the track carries DC.

It seems unlikely that you would ever hear any fixed resistor
noise in the audio section of an R-390 class receiver, unless
the resistor was faulty. Tube noise seems a lot more likely, 
but the only way to reduce it is to lower the heater temperature.

It's fascinating what you can learn when you turn away from the
shared ignorance on the 'net and open a book. Better yet, do some
experiments to locate the noise, measure it, and see how it changes
(or not) with the price of the resistor.

Happy New Year,
Bill Hawkins