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Old 21st September 2013, 11:23 AM   #1
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Default About metal film resistor in a tube tuner

Hello, an a nice WE to everyone !
I'm about to service a Quad FM2 tuner.
On the quadmod page : Quad FM2 tuner service notes by Quad Mods USA
it is specified : "1. Replace all the resistors attached to the HT line with 1 watt carbon."
I don't understand the choice of carbon resistors. Is it only an "objective" choice, to keep the look and perhaps also the original sound of the tuner (in the HT line ?), or is it most seriously because metal film resistors, in a tuner, can bring inductance problems ?
I have already metal film resistors in my box, and I would like to know if I need to find carbon one, or if I can use those I have without problem (1W metal film resistors, 450 V max, HT line on a Quad FM2 is 175 V).
Thank you very much !!
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Old 21st September 2013, 12:14 PM   #2
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Solid carbon resistors only come in relatively low resistances.

Carbon Film and Metal Film both use a thin layer of metal on a non-conductive body, the resistance is made by etching a fine spiral (or coil) into the film coating. So both will exhibit a small inductance.
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Old 21st September 2013, 12:18 PM   #3
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OK !
So, for you, no special problem to use my metal film resistors on the HT line of this tuner ?
I don't understand why carbon resistors are specified by Quadmodsusa...
Thank you !
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Old 21st September 2013, 12:24 PM   #4
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In a word, "age". What was available when the equipment was designed.
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Old 21st September 2013, 03:00 PM   #5
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I have some solid carbon 80meg 1/8w Panasonic resistors
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Old 21st September 2013, 03:05 PM   #6
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Quad used carbon resistors because they had nothing else at this time, ok. But why NOW Quadmodsusa chooses it, that's what I don't understand...
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Old 21st September 2013, 03:13 PM   #7
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Possibly Nostalgia. Capt Grogg has proved me wrong by saying that he has 80M solid carbon. I don't know anywhere that sells solid carbon resistors at that value these days, most of them are carbon film.

Carbon is OK if there is little or negligible current flowing through it. At higher current they are considered noisy.

They are traditionally used as grid stopper resistors in valve amps but there is no audible difference to metal film as replacements.

All I can say is try them and see, as the metal film generally cost but a few pence each.

Last edited by KatieandDad; 21st September 2013 at 03:33 PM.
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Old 21st September 2013, 03:21 PM   #8
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Thank you for your reply. I will use my metal film resistors.I don't want to get a tuner with exactly the same components used in factory, my priority is stability and nice working. I love these small tubed and stereo tuners !
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Old 21st September 2013, 04:11 PM   #9
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Don't use metal films in ANY of the RF, IF or detector circuitry. (&Replace only those that are bad)
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Old 21st September 2013, 04:14 PM   #10
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Kevin, do you include also the HT line ?
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