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 Post subject: Deosited carbon resistors
PostPosted: Jan Sat 13, 2018 2:09 am 
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What is the best resistor to replace a deposited carbon resistor in a tube amp with?. I dont seem to find much available in these to be able to replace with an original style. But have read that they used them in amps for sound quality and that is what is originally in this Fisher Amp. Thanks


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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Sat 13, 2018 2:20 am 
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Location: Columbus Ohio
where is the resistor in the circuit? some prefer carbon just on the cathode and metal film on all the others.
I doubt if I could tell the difference, you need a really "good ear" for that precision

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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Sat 13, 2018 2:42 am 
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Its on two pins of a 12ax7 which is designated as the tone control amplifier. R28 on v3 between the cathode and grid.


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Last edited by Arizona Sam on Jan Sat 13, 2018 2:47 am, edited 1 time in total.
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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Sat 13, 2018 2:43 am 
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Not to clear but its th 2700 tied in with a ceramic cap.


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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Sun 14, 2018 7:04 am 
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Arizona Sam wrote:
What is the best resistor to replace a deposited carbon resistor in a tube amp with?. I dont seem to find much available in these to be able to replace with an original style. But have read that they used them in amps for sound quality and that is what is originally in this Fisher Amp. Thanks


You can replace them with any resistor of the correct resistance and rating. Very occasionally, in some applications, the method of construction matters, in that the resistor has a very small parasitic capacitance or inductance, but that makes no difference at all for audio applications.

Brett


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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Sun 14, 2018 7:44 am 
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Joined: Aug Fri 28, 2015 9:26 am
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Location: Fort Mohave Arizona 86426
Thanks Brett


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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Mon 15, 2018 10:42 pm 
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Joined: Apr Thu 14, 2016 8:25 pm
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Location: pensacola fl
hi all. The type of resistor in low level stages have an impact on noise. Many tube amplifiers to get low noise figures use 2W carbon types in the plate circuit to keep thermal noise down. Mic, phono, and tape playback preamps used this technique as well as starved heater to get it as quiet possible.


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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Mon 15, 2018 11:23 pm 
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Voltage coefficient matters too, at least in stages that handle large signals. Carbon composition resistors frequently have (relatively) large voltage coefficients. Since their value changes as the voltage across them changes (at a constant temperature so power is not the issue), they can cause distortion. Any resistor with low voltage coefficient would be a good choice in these locations. Consult the manufacturer's data sheet to find out what this value is.

Wire wound resistors, although they have low voltage coefficient may not be the best choice because of their inductance.

Voltage coefficient is the reason that manufacturers specify the voltage to be used in testing resistors to determine if they are in tolerance.

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 Post subject: Re: Deosited carbon resistors
PostPosted: Jan Tue 16, 2018 1:19 am 
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Location: Fort Mohave Arizona 86426
Thanks Audioman and Jim Mueller for your responses.


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