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previous: Trace q{Why do you think the input resist... -- 2/13/2000 6:10 PM View Thread

Re: I learned this trick from Trace..So I have to agree! Works in my Marshall

2/13/2000 9:20 PM
Randall Aiken
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Re: I learned this trick from Trace..So I have to agree! Works in my Marshall
Trace, your empirical findings agree with the theoretical differences between resistors with regards to noise.  
 
First of all, noise (thermal, shot, and contact noise) in a resistor is proportional to the resistance value, the geometry (size) of the resistor, the material of the resistor, and the current through the resistor.  
 
What this means in the practical world, is that resistor values should be kept as low as practical (a 100K resistor is quieter than a 1Meg resistor), the power ratings should be as high as possible (a 1W resistor is quieter than a 1/2W resistor, all other factors being equal, likewise, a 2W resistor is quieter than the 1W), the currents should be kept as low as possible (1mA flowing through a resistor creates less noise than 2mA), and as far as material vs. noise goes, ressistors are ranked as follows, from quietest to noisiest: wirewound, metal film, metal oxide, carbon film, carbon comp.  
 
From this information, a couple of general rules can be determined. Since resistor value is proportional to noise, the 1Meg to ground resistor will create much more noise than the 68K grid resistor, because the value is 14.7 times larger.  
 
This means that carbon comp vs. metal film is more noticeable and important for the 1Meg resistor than the 68K input resistor.  
 
Also, since resistor noise is proportional to current flow, the 68K grid resistor is going to be quieter than a 68K use as a plate resistor. There is around a mA or two of current flow in a typical plate circuit, but the grid current flow is practically negligible. This means that it is better to use metal film for plate resistors. The exception to this rule comes when two resistors are used as a voltage divider from the plate of one tube to the grid of the other. There is no grid current flowing, but there is current flowing in the voltage divider string, so metal films should be used in these positions for lowest noise.  
 
Lastly, the noise contribution is greatest at low-level stages, such as input stages, reverb recovery stages, and effects loop recovery stages, so the plate resistors, grid-to-ground resistors, and grid divider attenuation resistors in these locations should be metal films for lowest noise, while locations where there is little gain from that point to the output can use noisier resistors without adding too much to the overall noise level of the amplifier, because the signal leval at that point is many times greater than the noise level produced by the resistors.  
 
Don't forget that pots are resistors as well, and the typical 1Meg volume pot can contribute a lot of noise in a low-level stage. Conductive plastic pots should be used for lowest noise (if you can find them!).  
 
For what it is worth, it is my opinion that higher wattage metal films should be used everywhere in an amplifier. I don't want the tone of my amps to be determined by something as variable as a noisy carbon comp resistor, which will drift and change with time and temperature. I'd rather tailor the tonal response with known, repeatable circuit design rather than depend on the variable lot quality of carbon comps.  
 
One more consideration for resistors: it is sometimes overlooked that resistors have a max voltage rating. The 1/2 watters and some 1 watters usually are only rated for 250-350V. Be sure to get a resistor rated for the appropriate voltage in the amplifier. I use only 500V min (continuous, 1000V surge) rated resistors.  
 
Randall Aiken  

 
Replies:
Mark Knapp A Toast -- 2/14/2000 4:28 AM
Trace Re: I learned this trick from Trace..So I have to agree! Works in my Marshall -- 2/14/2000 5:08 AM
GFR I'll second that toast too!... -- 2/14/2000 11:35 AM
Joe L q{The 1/2 watters and some 1 watter... -- 2/14/2000 4:35 PM
Bob How can one tell what kind of resis... -- 2/18/2000 12:34 AM