[quote author="buschfsu"][quote author="Minion"]Well try adjusting the resistor Values so you can get as much voltage to the Plate(Up to 30V), and adjust the Values to you get a lower Voltage at the Capsule.... This should hopefully Improve the performance some....
Cheers[/quote]
adjust the 47k resistor to maxmize plate voltage? what about reducing capsule voltage? which resistors are going to take care of this? the 4K2's?[/quote]
Hey guys. I few comments here. This schematic was from my prototype, but the production versions are different. Create a separate plate supply from pin 2 and pin 3 with two 100K resistors. That will get you to 17V plate, which is plenty. Plate voltage is not the limitation here--if you look at the tube datasheet, 15V is actually in a sweet spot of low distortion--and low gain, but none is needed here. Also, if you increase the plate resistor value, it changes the gain of the circuit, but the voltage at pin 1 of the tube drives the filament current, so you can't move too far outside these values or the circuit falls apart.
Change the filament resistor (100R) to a 200R trimpot, and the circuit will be much more successful. Otherwise it can be hit or miss with the combination of tube, capsule, and transistor variation. Generally, set tube pin 3 voltage = pin 4, but play around, you get some interesting results on either side.
It doesn't matter too much what capsule you use. It so happens that the FET electret capsules spit out just about the right voltage if you hit them with 12V or so. I have tried many, and all of them work, if you set their operating points by changing their drain resistor, or the trimpot setting. Shoot for around 2V at tube pin 4.
