[R-390] Resistors

[Roy Morgan]

At 12:07 AM 1/3/2006, bernie nicholson wrote:
>Guys I don't want to be PICKY   but WHAT is a LOW NOISE RESISTOR????????????

Bernie,

A perfectly good question!

It turns out that anything that is not very very very cold makes electrical 
noise. (That means near absolute zero - minus 273 degrees Centigrade or 
some such.)  The phenomenon is based on the fact that molecules move about 
if they are at any temperature above absolute zero.  This does not matter 
much to R-390A/URR users, though it does to folks doing radio astronomy and 
folks building detectors to sense submerged submarines from bumps in the 
earth's magnetic they produce.  But I digress.

If you put some current through a resistor, the material makes additional 
electrical noise. Some resistor materials make more than others, current 
and voltages being equal.  It appears that folks who build phonograph 
cartridge amplifiers and low noise VHF receivers can tell the difference 
between such resistors.

Almost all the noise from a properly aligned R-390A/URR receiver is 
generated in the first RF amplifier tube.  It's quite possible that if the 
resistors around that tube are particularly noisy, they will contribute a 
noticeable amount to the receiver noise.  If the IF gain is set very much 
too high, the IF amplifier will make noticeable noise. This is bad.  Any IF 
amplfier makes measurable noise, and Roger has been telling us how to 
measure that, and to select tubes to reduce that noise.

Additional noise is generated in the mixer tubes. Some tube substitutions 
suggested in modification articles make less noise than the ones normally 
used in the R-390A.  I have not tried these changes, but I hope to one day. 
I have a spare RF deck for the purpose.

It's the opinion of experienced radio users that very very few of us live 
in places where the received noise level is low enough so that the noise 
generated in a receiver is of much matter at all.  On HF, that is.  Above 
30 mc it's a different matter.

Faulty (that is leaky) RF and IF bypass caps definitely make noise.  After 
you have found this going on, and fixed it a number of times, the noise 
from this source is quite distinguishable from other noise.  It's sort of 
like telling the difference between an oboe and a clarinet.

As I understand it, carbon composition resistors are favored by the high fi 
builders for low noise and for other differences they hear in the 
sound.  And they also report that some other resistors are better for noise.

I would be glad to hear about any perceived or measured differences in 
noise in the R-390 receivers due to resistor changes..


Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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