On 2019/05/18 11:22 a.m., Cursitor Doom wrote:
On Fri, 17 May 2019 21:19:07 +0100, Mike Coon wrote:

I can hardly wait!

Then I shall keep you in suspense no longer.

Here's what I found from a random selection of old components I snipped
out. Firstly, pretty much *all* the capacitors were fine. The 350VDC
Hunts capacitors could easily have been new. An Erie plate ceramic of
0.01uF likewise. A Dubilier type SM22 50pf cap, however, had gone up to
62pF. That one was one of the ones used for tuning. The biggest changes
were as expected in the carbon resistors, all of which aged to higher
values like so:

27k became 38.6k (bad)

another 27k ---- 29k (within 10%)

100k ---- 107k (within 10%)

10 ---- 10.7 (ditto)

3.3k ---- 4.2k (bad)

4.1k ---- 5.2k (bad)

15k ---- 20.7k (bad)

220k ---- 246k (barely bad - just above 10%)

8.2k ---- 9.9k (bad)

400k ---- 509k (bad)


These were all marked with a silver tolerance band, so clearly Taylor
back then at least not *that* bothered about accuracy.


No, those resistors have drifted since their original construction.
People did have ohm-meters back then and would verify values on
resistors particularly if they were colour deficient or colour blind as
I did, testing Rs when building kits - red/green deficient vision. I
still check almost all resistors I come across with a meter as I don't
trust my green, dark red, and brown differentiation.

If they are in power circuits - plate or cathode or voltage dropping
then those do drift as they were only compressed carbon and heat/cooling
cycles would cause them to change value - usually upwards as you have
discovered. I consider your values typical of tube gear that is 40 or
more years old.

1920s resistors were a chunk of carbon rod with a wire wrapped around
each end then dipped in a sealant - how long do you think that value
would last within X%? Wire wound resistors external connections were
pressure bonded to the resistance wire, not uncommon for that joint to
fail over time...

Like I said earlier you HAVE to verify all the resistors as they wander
over time. Modern Rs are much more reliable when operated under their
rated wattage.

John :-#)#

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