[R-390] Resistors

[Barry Hauser]

Hi Roy

Thanks for the explanation.  Of course, now I'll be listening for oboes and 
clarinets ;-).

It's difficult to say what the audio builders think as a lot of that stuff 
is now mixed with lore, urban legend, and maybe a large dose of hype (as in 
monster cables, oxygen, etc.)

I think part of the reason for favoring carbon comps is avoidance of 
inductance or capacitance introduced by metal or carbon film resistors --  
whether real or imagined.

Some audiophile restorers seem to be looking to preserve whatever component 
characteristics inherent in the old tube designs -- good, bad or 
indifferent -- to maintain that warm, fuzzy 50's feeling or whatever.  This 
ranges into the fringe area where used Black Beauties fetch high prices.

This is not to poke fun -- I really don't know -- but merely suggest that 
the reasons for favoring this or that type of component do not necessarily 
fall along familiar parametric lines, known elements of the laws of physics 
and chemistry, electricity, etc.  I suppose if your objective is to retain 
authentic performance, including distortion and noise, it would make sense 
to stay with components of identical or similar construction -- providing 
functional components of the type can be found today.

As I recall, there was a thread some years ago about whether or not a modern 
carbon film resistor might cause a problem somewhere in the R-390A if used 
as a replacement for a carbon comp.  I think the conclusion -- at least the 
one I drew from the thread -- was that it might affect one or two areas, but 
generally no problem and not worth worrying about..

Barry






----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: "bernie nicholson" <vk2abn at bigpond.net.au>; <r-390 at mailman.qth.net>
Sent: Tuesday, January 03, 2006 9:37 AM
Subject: Re: [R-390] Resistors


> 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!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Cell 301-928-7794
> Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
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