Resistors. Is it 1/8th W carbon or 0.4 W metal? Is the tolerance the clue?

[anotherjim]

The subject says it. I found a faulty (open circuit) 10k resistor in a Marshall SS amp (MG30DFX) but it has 4 value bands with a gold tolerance band. I can't say I've noticed this combination before, I replaced it with an 0.4 W 1% metal film. I'm not worried about the repair, but just curious to know if it's possible to have 5% metal or 1% carbon. Usually, the carbon 5% only has 3 value bands but I do have some metal 1% with only 3 value bands. Generally, 1/8 W carbon and 0.4 W metal look the same.
For the curious, the failed resistor is R1 and is the series input protection from the input jack. It hadn't broken or burned, just failed. Such a little part with a big job.

[antonis]

I do have some metal 1% with only 3 value bands.

The only 400mW metal film 1% resistors I have are 4-band colour code (and are about 3 x 600mW respective price..)

Can you post some pics, Jim..??

[Rob Strand]

10k resistor in a Marshall SS amp (MG30DFX) but it has 4 value bands with a gold tolerance band.

It's worth checking if it's really a resistor.   In certain eras there's caps that look like resistors!   I know you got it from position R1 but sometimes manufacturers do weird things.  Trace the input path through to the opamp and see if you can find an input cap,  that will tell you it's really a resistor - I suspect it is from the 10k value.

(Not relevant here but you can get fireproof resistors in equipment, which could be mistaken as metal films.)

If you create a matrix of three resistor types (carbon film, metal film, metal glaze) and tolerances 1%, don't forget 2%, 5% and number of *value* bands 3, 4, you end-up with a lot of combinations.   We could even throw in body color.  Some combinations are common, some uncommon and some non-existent. 

I used to keep images in my head of funky part examples from my part stock and pieces of equipment.   It's so long since I've looked at them so I no longer have a clear recollection.  What I will say is nothing would surprise me.  I've seen all sorts of combinations, especially from telecommunications equipment - very off the norm parts.

5% metal

I'm very certain of 3-value band 5% metal films.
I'm quite sure of 4-value band 2% metal films.
I'm a bit iffy of 4-value band 5% metal films.

For 2% I've probably seen all combinations except maybe carbon 2% with 4-value bands.

For carbon 5%, I recall E24 series markings but I wouldn't stick my neck out to say I've seen 4 value bands.

So I guess the hanging question is do we ever see:
- 5% and 4-value bands
- carbon film and 4-value bands  ; without measuring the tempco it's hard to know what type it *really* is.
- carbon film and 1%                ; I give that a maybe

Some some carbon resistors have an extra band *after* the tolerance band.  They might have 5 bands but they still only have 3 value bands.

[puretube]

According to this schemo:
https://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_mg-30dfx_sch.pdf_1.png
it really is a 10k resistor.
IMHO you can replace it with any wattage or tolerance.
Metalfilm gives lower noise. To some people carbon gives more mojo ...
While you`re at it, you can check C1 (that comes before it) for non-leakiness, i.e. no DC on the tip of the input-jack, while a plug is plugged in.

[anotherjim]

At first glance they look like metal film. Blue body with a total 5 bands. I have a schematic so the 10k resistor for "R1" was never in doubt. However, in some places, there might be an impact if you mix up 0.4W with 0.125W?
I did wonder if I was mistaken and the gold was brown, but it definitely looks metallic compared to the brown value band. All of the low power resistors in this amp were the same type.
I have the same problem as Rob, I can't find those metal films with only 3 value bands - yet.

[puretube]

5%, 5 bands, 0.4W metalfilm, blue:
https://www.mouser.de/ProductDetail/Vishay-BC-Components/SFR2500001002JA500?qs=ClR5yMXwuUc6XTOkUC9ejw%3D%3D

[anotherjim]

That settles that then, there are 5% metal film out there.

[puretube]

1% carbon is possible too, but only 1W (or more).

[anotherjim]

I'll have to try and take a photo, but I found some 47k 1% 1/4W mf I bought fairly recently and they are blue body Yellow, Violet, Orange, Brown.

Secondary pondering.
1/8W carbon resistors are smaller than 1/4W carbon.
How come 0.4W metal film resistors are smaller than 0.25W metal film?

[PRR]

> How come 0.4W metal film resistors are smaller than 0.25W metal film?

Newer design/spec-sheet with more aggressive numbers?

We rarely want to be anywhere near the rating in normal operation. That's a show-off number. Life is short. And while more-refractory materials can run hotter, the heat still has to go somewhere, generally via air, and it takes space for the heat to go away. Otherwise you are just toasting neighboring Rs and Cs and Qs.

[GGBB]

How come 0.4W metal film resistors are smaller than 0.25W metal film?

There are also 1/4W MF that are smaller than the usual 1/4W MF. I have to be careful now when searching on Digikey etc. because I don't want the shrimpy ones!

https://www.seielect.com/catalog/sei-rnf_rnmf.pdf
https://www.digikey.ca/en/products/detail/stackpole-electronics-inc/RNF14FTD1K00/1706678
https://www.digikey.ca/en/products/detail/stackpole-electronics-inc/RNMF14FTC1K00/2617302