hey CJ,
What exactly is the supply voltage?? 100V or 200V? and your bias current is 11mA!?? :shock: hmmm...... I would love to see the output on the CRO with a signal running through it, also the noise figure with closed circuit.
What is the gain of this tube?? I would not think it will be too high if the current is at 11mA.
I don't have transformer in my circuit input but I have a coupling resistor connected to the headphone output of my CD player, this resistor reduce the noise but also the input signal. I wondering if that the input transformer works the same way, otherwise by connecting the input directly to the grid makes horrendous noise but may be because I have a grid shunting resistor.... I will go and try that without the resistor.....
Same case at the output, I have a resistor connected in series with the output coupling cap. This resistor kills both noise and gain, so its a compromise. Without it and with no FB for the tube, I get a sine wave like sort of noise at the output and its loud. I forgot to measure its frequency.....
How did you work out the bias, using load curve? Interested in replacing the plate resistor with a 500K ohm pot then run a 1khz signal through the tube, probe the plate coupling cap and tweak the plate resistor pot. Watch it on the CRO and see what value of the pot you arrive at to get optimum gain and minimum noise, you might want to add another 500K pot for FB as well, since it will help to flatten the noise you'll see it on the CRO!
I don't have cathode decoupling cap on any of my tubes because I found that it added noise, may be I didn't wait long enough until the noise settle down a bit after turning on the circuit. I might try that again.....
I also tried all different coupling caps and could hear NO difference, the only thing I find is that the biasing point is absolute critical to gain, noise and distortion which to me pretty much defines the sound. There is a optimum region for the value of plate R and FB R and screen R(in case of the Pentode), where by combining these 3 variables you can achieve maximum gain with minimal noise in comparison to every other combination of the 3 variables. I only tweak for max gain and min noise because there isn't much I can do to alter the distortion plus its really minimal, can not really see it on the CRO also to me it defines the personality of the tube as well as the characteristic of the sound.