Low Current C4S RC Resistor Substitution

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Offline Loading

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on: July 05, 2023, 04:55:27 AM
When building the low current C4S board, I accidentally broke both of my 220 ohm resistors that belong in the RC position (page 57 of the manual). Is it critical that I use 220 ohm, specific resistor material type, or certain brand?

I ask because I have some extra resistors that I was wondering if might work:

  • 220 ohm vishay dale metal film
  • 270 ohm carbon film
  • generic 220 ohm carbon film (questionable source and quality due to bulk resistor box)

If not, which resistors do you recommend I purchase for replacing the broken ones?



Offline Paul Birkeland

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Reply #1 on: July 05, 2023, 05:14:48 AM
The Dale 220 and carbon film 220 would be acceptable substitutes.  Just be sure to put a meter on them to double check the value.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


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Reply #2 on: July 05, 2023, 09:23:16 AM
Would there be any downsides with using the dale resistor?
I'd imagine the reason it was carbon film in the first place was to minimize unnecessary cost.



Offline Paul Birkeland

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Reply #3 on: July 05, 2023, 11:44:16 AM
Technically carbon composition is the best choice for that spot, but those suckers like to break if you aren't careful with them, so we changed over to carbon film.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


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Reply #4 on: July 06, 2023, 05:46:37 AM
Could you please clarify why the resistor composition matters for a location?



Offline Paul Birkeland

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Reply #5 on: July 06, 2023, 05:51:12 AM
The TL431 is relying on that 220 ohms being 220 ohms at high frequencies, so resistor inductance isn't desirable (carbon composition resistors are non-inductive by design).  In practice, the tiny bit of inductance from a 220 ohm carbon film resistor is inconsequential for proper circuit operation.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Doc B.

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Reply #6 on: July 06, 2023, 08:02:17 AM
That is a pretty broad topic. Carbon composition (CC) resistors are less affected by RFI so we traditionally used them as grid and plate stoppers, which stop RF. But as PB mentions the CCs we were able to get were fairly fragile where the lead entered the resistor body and they would often get broken there. So we switched to carbon film which is also pretty good at RF rejection and is non-inductive as well.

For some apps, say like an RIAA filter in a phono preamp, precision is important. In these cases metal film resistors, which can be trimmed to a tight tolerance like 0.1%, are ideal. Some feel this type of resistor construction may color the sound less than other types and it is often spec'd for situations where the audio signal passes thru the resistor like an attenuator.

In applications like power supplies and plate and cathode resistors there can be a lot of current going thru the resistor and/or a large voltage drop. For these apps we need a resistor that can handle a lot of power and thus a lot of heat. Something like a wirewound ceramic or a metal oxide resistor often is appropriate here.

All of this said, sometimes we use a resistor value or type because it is a part we already stock for other kits and it is a good fit for the new application as well. We go through thousands of 220 ohm CF resistors.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.