Metal Film or Carbon Film Resistors in Marcof PPA-1h ??

[Ron AKA]

Here is a picture of some resistors on this 1979 vintage moving coil head amp from Marcof. Do you think these are carbon film or metal film design? Since this is a high gain (34 dB), very low signal device, I was thinking it could benefit from a lower noise resistor. Noise here is further amplified by the phono section amp, then the pre-amp, and finally the power amp. Metal foil would seem to be the best but is not available in correct values, and metal film would appear to be the next best choice.

I'm thinking the beige body color and the vintage would suggest carbon film??

An externally hosted image should be here but it was not working when we last tested it.

 

[Rodeodave]

5-band 1% carbon film resistors? I could be wrong, but I don't think they make 'em like that.

 

[ticknpop]

They are 1% , likey metal film

 

[simon7000]

The double width last band is the give away. Blue body color is usually meant to mean flame proof. As modern metal film resistors do not contain carbon they are flame proof. Double width tolerance band means metal.

 

[john dozier]

Texas Components will make Vishay foil resistors in any value and tolerance that you need. Price may be higher than you are willing to pay however. Regards

 

[Ron AKA]

Thanks for the quick responses. On a check it appears 1% carbon film resistors are now about as common as hen's teeth. Anyone know if it was the same 30 years ago? If so along with the other clues it would seem virtually 100% certain these are metal film. Thought there might have been a cheap upgrade to reduce noise, but it appears not.

 

[simon7000]

Thanks for the quick responses. On a check it appears 1% carbon film resistors are now about as common as hen's teeth. Anyone know if it was the same 30 years ago? If so along with the other clues it would seem virtually 100% certain these are metal film. Thought there might have been a cheap upgrade to reduce noise, but it appears not.

Change the loading resistor to a dale RN65 type. That will be an improvement.

 

[Ron AKA]

Change the loading resistor to a dale RN65 type. That will be an improvement.

By the loading resistor do you mean the loading resistor for the amp (R7 in the circuit drawing at the link below)? Or the loading resistor for the moving coil (R1)?

Marcof PPA-1h Circuit Drawing

Yes, there appears to be differences in the types of metal films. See the figures near the end of the article below. However, finding the good ones in small quantities seems to be another matter. Sources for the RN65? Is that just a military designation for the Dale CMF?

Resistor Current Noise Measurements

 

[Steve Dunlap]

Change the loading resistor to a dale RN65 type. That will be an improvement.

I agree with this.

They used carbon film resistors.

 

[mickeymoose]

Why would anyone do carbon-film in 1%? Take out one R and get a metal-film of the same resistance. Attach wires, measure at room temp. Put in oven to about 100 Deg.C, measure again and compare resistance. The mf should have less than 10% drift of the cf. E

 

[john dozier]

I still suggest the Vishay "nude" type. Very low temp coefficiency. Available from Texas Components direct, or Parts Connexion or Michael Percy.

 

[Ron AKA]

I still suggest the Vishay "nude" type. Very low temp coefficiency. Available from Texas Components direct, or Parts Connexion or Michael Percy.

They would be a significant improvement in noise. However I had checked them out already at Parts Connexion. Values are not what I need, and they start at about $16 each.

 

[john dozier]

Texas Components can supply the values you need. I warned you they would be costly, but nothing else even comes close. Come on , wallet up. This is the first electronic component in the entire chain. Everything else depends on what you do here. Regards

 

[Steve Dunlap]

Why would anyone do carbon-film in 1%?

They had a price point to make. Carbon film was cheaper and easier to obtain.

 

[Ron AKA]

I'm thinking of using the Welwyn RC55 metal film? They seem to test fairly well. See figures 7 & 11 in this document. Need to convince myself they are OK in 1/4 watt as that is all I can get. The Vishay Dale CMF come in 1/2 watt, but don't seem to test as well... Can also get some Yageo, but in poorer qualities than those tested. And the ones tested were poor.

 

[simon7000]

The Dale's are stock at Mouser. The military version in 25 PPM tests well by measurement and listening equal to the bulk metal Vishay's at a much lower price. Use it as a cartridge load resistor and get a few values to see which one you like best.

See Linear Audio Vol 1 for details.