With cathode bias, 470K grid resistor can be used, which the 12AX7 will be happier driving... the 12AX7 has about a 60K plate resistance, so with 100K typical 'AX7 plate load, about a third of its drive capability is lost driving 100K grid resistor, vs. about 10% loss with 470K.
As pointed out in the thread:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=68001
A 12AX7 is commonly used in guitar amp applications to drive a 6L6. This was probably done originally for cost reasons and blindly copied for decades. If you are designing a HiFi amplifier, and flat frequency response without serious phase shift is important a tube with a lower plate resistance may be a better choice (12AU7, 12BH7, 12AT7, 6CG7, etc). Most guitar amps have little response above 10KHz.
The input resistance of a 6L6 (or most tubes) is very high until it is overdriven, then it drops like a rock. The overload characteristics of the driver in this case is one of the attributes that determine the sound character of a guitar amp when it is "cranked to eleven." The input of any tube is also capacitive, and the drivers ability to drive this capacitance is one of the factors that determine the frequency response of the amp.
http://www.diyaudio.com/forums/showthread.php?s=&threadid=68001
A 12AX7 is commonly used in guitar amp applications to drive a 6L6. This was probably done originally for cost reasons and blindly copied for decades. If you are designing a HiFi amplifier, and flat frequency response without serious phase shift is important a tube with a lower plate resistance may be a better choice (12AU7, 12BH7, 12AT7, 6CG7, etc). Most guitar amps have little response above 10KHz.
The input resistance of a 6L6 (or most tubes) is very high until it is overdriven, then it drops like a rock. The overload characteristics of the driver in this case is one of the attributes that determine the sound character of a guitar amp when it is "cranked to eleven." The input of any tube is also capacitive, and the drivers ability to drive this capacitance is one of the factors that determine the frequency response of the amp.
Hi mrhotmark,
It's more a question of capacitance instead of resistance. In class AB2 you drive the grids positive and the impedance drops like a stone.
So you need to qualify the circuit, single ended, push pull AB1 or AB2 (or A even). In class AB1 you can go near the grid bias voltage as a peak value. So for a bias of -40V, the drive might be +/- 40V peak.
A tube manual is a help here. It'll give an approximate capacitance as well.
-Chris
(I've been lost in a vortex - miss me?)
It's more a question of capacitance instead of resistance. In class AB2 you drive the grids positive and the impedance drops like a stone.
So you need to qualify the circuit, single ended, push pull AB1 or AB2 (or A even). In class AB1 you can go near the grid bias voltage as a peak value. So for a bias of -40V, the drive might be +/- 40V peak.
A tube manual is a help here. It'll give an approximate capacitance as well.
-Chris
(I've been lost in a vortex - miss me?)
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