[R-390] Replacing carbon comps with metal film resistors

[Bob kb8tq]

Hi

You need to measure the actual Q of the circuits to see the difference. 
That’s not as easy as it might sound. In some cases the added C
from a metal film is going to make a difference. 

There are formulas for all this.I’m not quite able to dig them up at the moment. 
For a given frequency, It’s more pronounced at higher values.

Bob


> On Jul 29, 2019, at 8:01 PM, Perry Sandeen <sandeenpa at yahoo.com> wrote:
> 
> Yo Bubba Dudes! and Bob
> 
> Bob showed that I was mistaken when I said there were no carbon comps in RF circuits in the A as some are used for Q swamping.
> 
> That said, IMNSHO at the frequencies we're dealing with, I don't think it will make any difference to use the MF resistors.  But, I could be wrong (again).
> 
> If someone could show that MF's made a difference when used for Q swamping in the I and the list would like to hear what the results were.
> 
> Regards
> 
> Perrier
> 
> On Monday, July 29, 2019, 4:39:10 PM PDT, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> 
> Hi
> 
> Indeed you will find resistors across coils in various radios to set the 
> Q of the circuit. That’s where the trouble comes in. A conventional carbon
> comp does not have the same RF characteristics (cap in parallel with the
> resistance) that other resistors have. Instead it’s resistance stays “pure R”
> and just drops as frequency goes up.
> 
> Bob
> 
> > On Jul 29, 2019, at 6:26 PM, Perry Sandeen via R-390 <r-390 at mailman.qth.net <mailto:r-390 at mailman.qth.net>> wrote:
> > 
> > Yo Bubba Dudes!,
> > IF you go the route of replacing the carbon comps, understand that theirRF characteristics are unique. No other resistor out there does quite the same thingat HF that a carbon comp does. There may be some fiddling involved..
> > That's very interesting. I never new that.
> > Fortunately all resistors in both the A and SP 600 are only used as voltage dividers or in audio for gain increase.
> > Before Dr. Jerry got hounded off this list by moron trouble makers, he did experiments to see if the spiral trimming would cause any inductance problems at the RF frequencies us B/A aficionados use.  He found no problems whatsoever.
> > About 12 years or so I make a bulk purchase of 4,000 .5W MF resistors and made resistor kits for both the A and SP 600.  I sold over 25 kits of each and no one ever said that they had a problem. 
> > 
> > So I guess from Bob's comment is that all the *chip* resistors we see now are made out of something besides cabon.
> > Regards,
> > Perrier
> 
> > 
> > 
> > 
> > 
> > 
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> > 
> > 
> > 
> > 
> > 
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