Top resistors

I'm a newbee when it comes to circuit tweaking, but I recognize the importance of low noise resistors in critical positions like input stages. As I said above, the folks at Audio Asylum steered me toward carbon comps. I took their advice and used them in a position that connects the coupling caps of my amp's input stage to the power stage. This is obviously in the signal path. For those that like details, here's the circuit diagram and resistors in question: R18, R19, R20, R21.

After reading the article: Resistor Types - Does It Matter?, and hearing what others have said, I realize these are the worst possible resistors I could be using... especially with a 1/2 watt power rating.

The article states:

Contact noise is dependent on both average DC current and resistor material/size. The most significant contributor to noise in guitar amplifiers is the use of low-wattage carbon composition resistors. Since the noise is proportional to resistor size, the use of 2W carbon comp resistors will improve the performance over that of 1/2W resistors. Studies have shown a factor of 3 difference between a 1/2W and a 2W carbon comp resistor operating at the same conditions.

Another interesting quote:

Wirewound resistors are the quietest, having only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.

So I want to replace the carbon comps. My question is whether TX2575 or TX2352 are overkill for this situation. They aren't cheap at $10 and $7 a piece. I need a total of eight. Would Mills MRA-5 wirewound be a decent option? They're less than half the cost.

Noise from resistors is based on the amount of current flowing through them, and there's virtually none flowing through R18, R19, R20, and R21, especially at audio frequencies. These are "grid-stoppers", whose purpose is to create a high-frequency pole in the amplifier's open-loop response, in conjunction with the output tubes' grid (Miller) capacitance. Don't worry about these.

If your want to swap some resistors, I'd start with R2 - the voltage that is amplified by every other stage begins as a current through this resistor. Second would be R12, then R3 and R4, as this makes up the feedback ladder. If you change R12, you might put a high-quality polystyrene cap for C4.

For plate-load resistors like R2, you should keep an eye on resistor voltage ratings as well. IIRC, the Vishay S102 is rated at 300V, so if the 'naked' derivitives are rated the same, you should be fine here.

Many thanks Kirkus. I appreciate you taking the time to look at the schematic and offer such a thoughtful reply.

I have PRP's in the positions you mention. In your opinion, would it be worth swapping in low noise resistors or will the benefit be marginal? It's probably hard to know the results for sure, but thought I'd ask anyway. I'm willing to upgrade if it offers an improvement, but don't want to throw money at the situation when it doesn't do anything.

my opinion changing resistor doesn't make significant difference such we change tube or capacitor ,although they still have difference sonic character ,another good resistor that i recommend is Riken RMG ,i was ever been compare with vishay S102,the riken have better midrange reproduction & warmer sound than 102 ,but 102 have better tranparancy & clarity.i have never been compare to TX2575 yet

Mr Y- Any components(passive or active) that can be replaced in the signal path(or power supply) to provide a more transparent conduit (and gain) for the signal will have a cumulative effect. Many of us desire to provide as transparent a path, from the source to the speakers, for our music as possible. That entails replacing anything that adds or subtracts from the original signal. Some components(generally utilized in manufacture for the sake of economy) are known to induce noise, warmth or other colorations, and are targeted for elimination. What may seem insignificant or desirable to some, is found unacceptable to others.