Carbon Comp resistor drift

[Neuronal]

I am doing the soft start upgrade for the Paramounts, and because I have read good things about them I went ahead and bought a bunch of Allen Bradley 220 ohm carbon comp (NOS) resistors from Federal Connectors to use as grid stoppers. I did the thing where you heat them gently for a day in an oven to blow off any water and then seal with shellac, and to my surprise after that all (20 resistors) read 230-240 ohms. So I ordered another 20 and before heating them took resistances - again all are between 230-240 ohms. This isn't just the 5% error because it is systematic - they all are too high by about the same amount. I know carbon comps drift over time - is this basically a sign that these are going to be noisy and I shouldn't use them? I have a bunch of new Arcol carbon comps as well which measure perfect on my ohm meter (a positive control!), so I could just use those instead, but I was wondering whether this is par for the course for the ABs and doesn't predict noise levels in a circuit, or whether I should just use the Arcols and call it a day instead. Thanks for any advice!

[Paul Birkeland]

For a grid stopper, you don't really need to do anything like this.  They could drift 100% in either direction and you'll never hear anything, and I wouldn't expect them to ever really get noisy as grid stoppers. 

In a position where they experience significant thermal dissipation, things can get a bit different.

[galyons]

Just curious! Did you measure them before processing?

Cheers,
Geary

[Neuronal]

for the first batch I didn't, which was a mistake. After the first set of high readings I thought the heat might have caused some sort of systematic drift upwards (or, since I used pre-dissolved shellac, that maybe some aqueous component of that might have hydrated the resistors), which is why I ordered a second set. But the second set of resistors reads high before I do anything, so this is probably about the resistors themselves. These are all NOS of unknown age - I wonder whether this is just a general feature of old carbon comps that they tend to rise over time.

[Tim G.]

They definitely tend to drift, but like Paul mentioned it tends to be driven more by they heat they are exposed to over time.  So NOS ones I would expect if they were stored properly should be ok.  And wouldn't those be 20% tolerance anyway, so anything between 200-250 would be within their standard readings I would think.

Tim

[Neuronal]

these were mil-spec ABs, so 5%: https://federalconnectors.com/RCR07G221JS-Allen-Bradley