[R-390] Need Resistor info

[Tim Shoppa]

"Bill Hawkins" <bill at iaxs.net> wrote:
> Hello, got your shotgun loaded, do you?
>
> Vishay? Wasn't their specialty high resistance precision resistors?
>
> I suppose each of us has got to do what the voices tell us to do, but
> this seems extreme. It's a communications receiver, not a precision
> measurement device. Do not expect precision resistors (or capacitors)
> to improve the performance of a radio that depends on its mixers,
> crystals and the Q of its tuned circuits. Or perhaps you are one of
> those audiophiles who looks for perfection in all the wrong places?
>
> Apologies if I've missed the mark . . .

Bill - 
  My experience is that for 50's/60's vintage gear, many of the
carbon comp resistors (often 10% or 20% parts originally) are now
off by HUNDREDS of percent. This is enough that the behavior/
performance is probably different than the original design. But
often the radio still mostly works (although not necessarily
as good as new.)

  I don't think Perry's intention was to drop 0.1% resistors in,
but at the same time modern cheapo metal and carbon films are so
much better than the carbon composition ones of old that I think
the Xicon/Vishay distinction is probably meaningless.

  As a practical matter with modern metal or carbon film resistors
there is no tolerance band that is wider than 5%.

  I have been mostly using Mouser/Xicon 1W and 2W metal films with
great success in tube gear (old and new). Note that the Xicon 2W 
units are about the same size as the original 1/2W carbon comps. 

One of my opinions: The Xicon 1/2W carbon films look
like cheap crap (but are probably still way better than the original
carbon compositions.) The metal films look way classier (but still
of course look very little like the original carbon comps.)

I am not ENTIRELY sure that I believe the Xicon 2W rating. It's
possible that this optimistic number comes from mounting to a thick
PCB with very short leads, and this is not how they're typically
mounted in tube gear. At the same time metal films can get so hot
that they are glowing dull red and still be within their original
spec after you clear the fault and let them cool down :-).

Others may worry about putting a metal film in place of a carbon comp
and what inductance may do to RF performance, but in my primitive
measurements of replacing 300%-out-of-tolerance carbon comps
with metal films the performance always increases (probably having
everything to do with getting the DC bias point back to where it
was originally designed to be and nothing to do with a little bit
extra inductance.)

  I do not have ambitions as large as Perry's to think about
doing hundreds of resistors at a time. Generally I restrict myself
to resistors that ohm out way out of spec or have obviously suffered
great abuse (charring, swelling). Certainly in my 390A's nearly all
the 2.2K's usually into this category, and there were a couple of
original resistors on my audio decks that were consistently off too.

  What I am jealous of is that OBVIOUSLY Perry has much more time
than me to play with old radios!!!

Tim.