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I have 6GK5 as CF, and now the resistor value is 19.6K, what value of plate choke should I use to replace the 19.6K resistor?
As a general rule, the inductive reactance needs to be five to 10 times the operating impedance of the circuit at the lowest frequency of interest. In the case of a CF, operating impedance is determined by the output Z of the tube. As an example, if the output Z of the tube is 300 ohms, XL should be roughly 3K ohms at 20 Hz. That can be accomplished with a 25H choke. In addition to plate chokes, it's possible to use the primary of inexpensive SE transformers for this. You only need to ascertain the inductance of the winding. As for the total resistance needed to bias the circuit, bypass the cold end of the choke to ground with a good quality capacitor, then add enough resistance in series with the cold end of the choke to equal the desired value.
Buy Chinese. Bury freedom.
first guess would be that R_choke, + Some-Other-R = 19k6 Ohms. It is likely the operating point of the 6GK5 is set by this resistance, so what ever choke you use should have some series resistance added or the operating point will change.
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Doug is correct about tending to the operating point.
It is necessary for the AC impedance at 20 Hz. to be at least equal to that of the simple resistor load. Applying the formula for inductive reactance (X L = 2ΠFL) to the provided 19.6 Kohms yields 156 H. Loading chokes are always costly.
JMO, a bipolar PSU plus CCS loading of CFs is a "better" way to obtain similar results.
My friend has just brought up this circuit, I guess this is a Conrad Johnson ART.
Instead of having so many resistors 10 x 19.6K 1W (1.96K 20W). Why not custom wound one single plate choke to handle the task????
nice to see after all these years. I remember reading martian colloms review of this preamp declaring it a new perfect preamp. I thought about building it back then. I would of liked this schematic back then. I still don't get it though. I imagined at the time it would have at least had active loads. I thought mc alluded to the fact that it had computer bias managing each tube independently. That would make sense to me being the biggest problem keeping them singing together. I just don't get why a bigger single triode would be better but maybe not. It makes me want to try it. how about 4 12b4 aside hmm. but maybe fixed bias to keep them adjusted. bipolar power supply o remove output cap you think cj could of done that and used a computer to manage the offset after all the preamp cost like 20k or something.
I do remember that it was filled with custom order film foil polystyrene caps.
What about lundahl ll1667?
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