Carbon Comp resistor drift

Neuronal · 836

0 Members and 1 Guest are viewing this topic.

Offline Neuronal

  • Full Member
  • ***
    • Posts: 120
    • Datta Lab Website
on: March 01, 2021, 06:14:11 AM
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!

Bob D
Rega P2, Cosecant DAC, Reduction, EFP III, Paramount V1.0 + soft start, Omega CAMs + DeepHemp 8s


Offline Paul Birkeland

  • Global Moderator
  • Hero Member
  • *****
    • Posts: 19694
Reply #1 on: March 01, 2021, 06:39:15 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline galyons

  • Sr. Member
  • ****
    • Posts: 626
  • Geary Lyons
Reply #2 on: March 01, 2021, 07:28:51 AM
Just curious! Did you measure them before processing?

Cheers,
Geary

VPI TNT IV/JMW 3D 12+Benz LP-S>  Eros + Auralic Aries + ANK Dac 4.1 >Eros TH+ Otari MX5050 IIIB2 > BeePre >Paramount 300B 7N7 > EV Sentry IV-A

Thorens TD124/Ortofon RMG-212/SPU >Seduction > Smash^Up> Paramour 45 MQ >K12's


Offline Neuronal

  • Full Member
  • ***
    • Posts: 120
    • Datta Lab Website
Reply #3 on: March 01, 2021, 07:48:56 AM
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.

Bob D
Rega P2, Cosecant DAC, Reduction, EFP III, Paramount V1.0 + soft start, Omega CAMs + DeepHemp 8s


Offline Tim G.

  • Jr. Member
  • **
    • Posts: 48
Reply #4 on: March 01, 2021, 08:07:22 AM
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



Offline Neuronal

  • Full Member
  • ***
    • Posts: 120
    • Datta Lab Website
Reply #5 on: March 01, 2021, 09:27:22 AM

Bob D
Rega P2, Cosecant DAC, Reduction, EFP III, Paramount V1.0 + soft start, Omega CAMs + DeepHemp 8s