I have a Marshall power amp that had dramatically high noise throughout the whole signal chain. I finally traced its origin to the plate resistor on the first tube stage. what confuses me is that the resistor measures right on. what am i missing? what would cause a resistor to exhibit so much noise but read properly?
noisey resistor
- Thread starter amorris
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Svart
Well-known member
do you mean that you changed it and the noise went away?
I think I read about this somewhere - something to do with high voltages across low-watt-ish carbon comps stressing the carbon crystals and increasing shot noise or contact noise or whatever over time...
I sooo don't know what I'm taking about
...
Peace,
Al.
I sooo don't know what I'm taking about
...Peace,
Al.
Viitalahde
Well-known member
I once puzzled with a monitor controller I was building for a client. It was oscillating (but still passed audio) and I tried changing every active component from the way, all caps, even a relay.
Then I finally changed a feedback loop resistor to an identical resistor out of the same pack. The oscillation stopped and everything was fine & has been ever since. The resistor measured OK in ohms, but was obviously still faulty.
The only explanation I could think of is that it was highly inductive at HF, increasing the gain of the amplifier at the hundreds of kHz range.
Then I finally changed a feedback loop resistor to an identical resistor out of the same pack. The oscillation stopped and everything was fine & has been ever since. The resistor measured OK in ohms, but was obviously still faulty.
The only explanation I could think of is that it was highly inductive at HF, increasing the gain of the amplifier at the hundreds of kHz range.
dabo
Active member
Be advised that there is debate about the value of using carbon composition resistors in amplifiers. The thermal noise generated by your failing plate load resistor is present in all resistors at some level. It's an unavoidable artifact of the physics of resistance.
Many believe that part of the magic sound of old amplifiers is because of the use of carbon composition resistors. There are modern lower noise parts that, in my opinion, have no negative impact on the character of an amplifier and are noticeably quieter, especially in the first gain stages of a high gain, multistage guitar amplifier.
I'd recommend the use of metal film resistors.
Many believe that part of the magic sound of old amplifiers is because of the use of carbon composition resistors. There are modern lower noise parts that, in my opinion, have no negative impact on the character of an amplifier and are noticeably quieter, especially in the first gain stages of a high gain, multistage guitar amplifier.
I'd recommend the use of metal film resistors.
Mbira
Well-known member
I was working on this old amp from Japan called an Elk. It was a sort of Fender Bandmaster-type copy. Every resistor in there is a wirewound. It was a trip. I actually bought it to build my own amp in the chassis. The inductance potential seems like it'd really affect the tone, no?
Joel
Joel
> just replace all plate resistors for lowest noise?
Unless you have a sick resistor, 90% of noise comes from the first stage. Nomrally that is the only stage where noise is critical.
Guitar signal is usually strong enough that you should not hear noise while playing. If you do, there is something wrong. Yes, a resistor can measure spot-on when cold and unpowered, yet go bad when hot with current.
> "a factor of 3 difference between a 1/2W and a 2W carbon comp resistor"
That seems extreme, maybe a worst-case study. And when a guitar amp tube or resistor goes bad, it is usually much more than "factor of 3" more hiss.
2W is a good value for carbon-comp resistors for reliability reasons; lower noise is a bonus. Carbon-comps DO fail. Maybe in a year, maybe in a century, depending on abuse and bad luck. Any hot-running carbon-comp in an UN-happy stage should be considered with suspicion. OTOH I'm not in favor of blind-replacing everything in sight on an amp that isn't seriously ill.
> Many believe that part of the magic sound of old amplifiers is because of the use of carbon composition resistors. There are modern lower noise parts that, in my opinion, have no negative impact on the character of an amplifier
Some guitar work is all about "negative impact". There's probably a band using that name.
I'm inclined to believe that the very small flaws in a healthy carbon-comp resistor may be part of the "tone" for some amps and some players. I would not blindly replace failed carbon-comp with carbon-film or metal without a lot of ear-testing. I might however new-design with modern resistors, and save the carbon-comp for when the player comes back complaining "too clean, needs dirt".
> The inductance potential seems like it'd really affect the tone, no?
What inductance??? The "core" is pretty nearly air. Find the formula for air-core coil inductance. It will be microHenries. Figure the impedance of a lot of microHenries at the top of the (guitar) audio band: it is just a few ohms. This is totally meaningless in series with a 100K plate resistor, and normally meaningless in a 1K cathode resistor.
You CAN get in trouble. I had 10 ohm wirewounds in a power-tube cathode, and the plate lead a little too close, it oscillated at radio frequency. Keeping plate output away from cathode (an input) was one fix, using non-wound resistors would be another.
And a low-price wirewound may not be wound so tight, may have internal vibrations, excited either by signal current or by speaker shake. The good wirewounds won't have this problem.
Unless you have a sick resistor, 90% of noise comes from the first stage. Nomrally that is the only stage where noise is critical.
Guitar signal is usually strong enough that you should not hear noise while playing. If you do, there is something wrong. Yes, a resistor can measure spot-on when cold and unpowered, yet go bad when hot with current.
> "a factor of 3 difference between a 1/2W and a 2W carbon comp resistor"
That seems extreme, maybe a worst-case study. And when a guitar amp tube or resistor goes bad, it is usually much more than "factor of 3" more hiss.
2W is a good value for carbon-comp resistors for reliability reasons; lower noise is a bonus. Carbon-comps DO fail. Maybe in a year, maybe in a century, depending on abuse and bad luck. Any hot-running carbon-comp in an UN-happy stage should be considered with suspicion. OTOH I'm not in favor of blind-replacing everything in sight on an amp that isn't seriously ill.
> Many believe that part of the magic sound of old amplifiers is because of the use of carbon composition resistors. There are modern lower noise parts that, in my opinion, have no negative impact on the character of an amplifier
Some guitar work is all about "negative impact". There's probably a band using that name.
I'm inclined to believe that the very small flaws in a healthy carbon-comp resistor may be part of the "tone" for some amps and some players. I would not blindly replace failed carbon-comp with carbon-film or metal without a lot of ear-testing. I might however new-design with modern resistors, and save the carbon-comp for when the player comes back complaining "too clean, needs dirt".
> The inductance potential seems like it'd really affect the tone, no?
What inductance??? The "core" is pretty nearly air. Find the formula for air-core coil inductance. It will be microHenries. Figure the impedance of a lot of microHenries at the top of the (guitar) audio band: it is just a few ohms. This is totally meaningless in series with a 100K plate resistor, and normally meaningless in a 1K cathode resistor.
You CAN get in trouble. I had 10 ohm wirewounds in a power-tube cathode, and the plate lead a little too close, it oscillated at radio frequency. Keeping plate output away from cathode (an input) was one fix, using non-wound resistors would be another.
And a low-price wirewound may not be wound so tight, may have internal vibrations, excited either by signal current or by speaker shake. The good wirewounds won't have this problem.
