[R-390] Re: Parts Selection

[Mark Huss]

I had a similar discussion with an Army Instructor. He claimed that a 
Carbon resistor acts as a fuse, and always fails open. That it will open 
under overcurrent(I.E. Heat) faster. Finally he told me to put my money 
where my mouth is. We took a handful of carbon comp. resistors, a power 
supply, and an ammeter. Then put 1 Watt through the 1/2 watt resistors. 
Every time, current went up for about thirty seconds, climbing higher 
and higher, until the carbon resistor was nearly a short, then it 
opened. Don't remember the figures, but I got Beer money for a night in 
Leominster for the whole class out of it. Heat causes Carbon Comp. 
resistors to drop in value (crystalization?) until the current is so 
high the heat causes it to crisp open. Somehow, that does not strike me 
as a good fuse.

Drew Papanek wrote:
> Perry wrote:
>
>> I champion 1% metal film resistor replacement over carbon comps for 
>> the following reasons. First >they are decades more stable, have a 
>> far lower noise factor, are smaller than,and are one third the >price 
>> of carbon comps.
>
> I like the "smaller than" part. It makes repairs and restorations much 
> easier. Noise is a consideration only where the resistor is in the 
> signal path. In the R-390A RF amplifier stage, for example, the 
> cathode resistor is bypassed with a capacitor (two, actually) and 
> hence is not in the signal path. Carbon or metal film would make no 
> difference.
>
> There are very few R-390A signal path locations where carbon 
> composition resistors are found. One that comes to mind is in the 
> crystal filter circuit. Another is the plate load resistor for the 
> 12AU7 audio amp stage. Signal level at those points is high enough 
> that metal film would make no difference in noise performance.
>
> One caveat in any resistor replacement is to consider the voltage 
> flashover rating. It is not of much concern in today's solid state low 
> voltage circuits, but it is of great importance in tube equipment. 
> Mouser carries Vishay/Dale metal films; the CCF55 series is rated at 
> 300V maximum and the larger CCF60 series is good to 500V.
>
> Low level audio amp stage plate load resistors would benefit from 
> appropriately voltage-rated metal film resistor replacement. In that 
> application there is 100V or more across a resistor of a couple 
> hundred K or so and carbon comps fail frequently, drifting way up to 
> sometimes over a meg or even becoming intermittent, causing "crackles" 
> in the audio.
>
>> Also some "A"’s 2.2K resistors are underrated so with metal films you 
>> can double up in the same >space or opt for the 3/4 watt rated Vishay 
>> series.
>
> There is no harm in uprating in many circuits but isolation resistors 
> in plate circuit is not the place to do it. The 2.2K resistors in the 
> B+ feeds to various R-390A stages also serve as fuses. There always is 
> the possibility of a shorted tube which will burn out the isolation 
> resistor and limit current to a little less than 100 mA while doing 
> so. The radio's B+ fuse(s) might not blow fast enough to prevent 
> damage to other components.
>
> Metal film resisors will withstand much greater overload than carbon 
> comps. In most circuit locations that would be good, but not where the 
> resistor serves as a fuse. The aforementioned 2.2K resistors should be 
> replaced with carbon composition types. I do not know how well carbon 
> FILM types would work in a fuse role. Since carbon films are more 
> reliable, cheaper, and more readily available than carbon comps, I 
> think some experimentation would be in order. <Evil Grin> Time to get 
> out the suicide cord! Mine is the UL-rated version with 3 alligator 
> clips for grounding :-). Time to let the smoke out of a few resistors, 
> 110VAC style!
>
> As far as underrated 2.2K resistors goes, a fuse protects best when 
> operated close to its limit. Maybe some of the 1/2 watt 2.2K's should 
> actually be 1/4 watt? (I haven't taken time to calculate.)
>
> The resistance value of those 2.2K plate circuit isolation resistors 
> is not too critical. If used in a circuit where the current were, say, 
> 5mA the voltage drop would be about 10V. If the resistor drifted to 
> 3K, the drop would be 15V. That 5 volt difference is small compared to 
> changes due to tube characteristic variations and line voltage 
> changes. I say leave them alone unless they're WAY off... There is 
> always risk of damage (breaking a terminal off an irreplaceable coil, 
> for example) when reworking and I don't think the small gain justifies 
> the risk.
>
>> Barring tube or cap shorts 50 years of operation for metal films is 
>> just warm up time.
>
> I agree. Metal films everywhere except where burnout (fuse) 
> characteristics are a concern.
>
> Drew
>
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