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!
Carbon Comp resistor drift
- Thread starter Neuronal
- Start date
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.
In a position where they experience significant thermal dissipation, things can get a bit different.
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.
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
Tim
