Can i use multiple Resistors for G7
- Thread starter Minguta
- Start date
Help Support GroupDIY Audio Forum:
> Can i use 4 x 250M 1/4W or 10 x 100M 1/8W ? Is that work?
It should work.
But.... this is a vacuum tube amp, right? Try just using 250Meg both places. I suspect 1Gig is "too high" for the very best performance.
1G is appropriate for FET, and a super-high resistance injects less thermal noise into the capsule capacitance.
But tubes have much higher grid current. Shot-effect noise increases with grid resistance, and 1G with most tubes makes more hiss than the thermal noise. Optimum for tubes is more like 100Meg-200Meg. So 250Meg bias and 250Meg grid may work very well.
It will work with 100Meg both places but noise will be a little higher and bass response not so extended.
It should work.
But.... this is a vacuum tube amp, right? Try just using 250Meg both places. I suspect 1Gig is "too high" for the very best performance.
1G is appropriate for FET, and a super-high resistance injects less thermal noise into the capsule capacitance.
But tubes have much higher grid current. Shot-effect noise increases with grid resistance, and 1G with most tubes makes more hiss than the thermal noise. Optimum for tubes is more like 100Meg-200Meg. So 250Meg bias and 250Meg grid may work very well.
It will work with 100Meg both places but noise will be a little higher and bass response not so extended.
> noise seems to lower with the higher value resistors in tube microphones.
It's complicated. A lot of interaction. Over most of the audio band, the capsule capacitance is lower-Z than the grid resistance, so it shorts-out the resistor noise, and a larger resistor is shorted-out better and the result is lower noise.
But that happens when resistor noise is fixed-power thermal noise. Double the resistance means 1.4 times the noise voltage, but 2:1 better attenuation from capacitance, so 1.4 times less noise voltage.
When the noise source is a pure current, like grid shot-noise, double the resistance is double the noise voltage. Also double the attenuation from capacitance. So with a tube, increasing the resistance may not change the midband noise much. (Noise current in the grid/gate of a small FET is 10X to 100X smaller than tube grid current, so FETs can use ultra-high grid resistors.)
But the situation changes in the bass, where capacitive impedance is no longer much-lower than resistance. The curves bend. And what is best in midrange is worst in deep bass. And the curves generally slope somewhat like Fletcher-Munson, so we can't just say that bass noise is inaudible. So the "offensiveness" of the total noise versus resistance may be very much a matter of taste (and how bass-noisy your studio is).
It's complicated. A lot of interaction. Over most of the audio band, the capsule capacitance is lower-Z than the grid resistance, so it shorts-out the resistor noise, and a larger resistor is shorted-out better and the result is lower noise.
But that happens when resistor noise is fixed-power thermal noise. Double the resistance means 1.4 times the noise voltage, but 2:1 better attenuation from capacitance, so 1.4 times less noise voltage.
When the noise source is a pure current, like grid shot-noise, double the resistance is double the noise voltage. Also double the attenuation from capacitance. So with a tube, increasing the resistance may not change the midband noise much. (Noise current in the grid/gate of a small FET is 10X to 100X smaller than tube grid current, so FETs can use ultra-high grid resistors.)
But the situation changes in the bass, where capacitive impedance is no longer much-lower than resistance. The curves bend. And what is best in midrange is worst in deep bass. And the curves generally slope somewhat like Fletcher-Munson, so we can't just say that bass noise is inaudible. So the "offensiveness" of the total noise versus resistance may be very much a matter of taste (and how bass-noisy your studio is).
On a similar theme, some of the tube mic designs don't even have a grid resistor. I'm thinking of the circuit in the 'Royer' mods which relies on grid leakage at the tube IIRC.
So how significant is this 'leak'? Is it of the order of gigaohms or hundreds of megaohms, and should we be considering this as effectively 2 parallele resistors when we have a very large grid resistor?
:?:
stewart
So how significant is this 'leak'? Is it of the order of gigaohms or hundreds of megaohms, and should we be considering this as effectively 2 parallele resistors when we have a very large grid resistor?
:?:
stewart
Similar threads
