sampleaccurate wrote:
I read a website (can't find it right now) that made big claims about the impedance of the filament supply for directly heated cathodes. Basically this guy was saying that the ends of the heater (cathode) filament in DHTs should NOT be AC bypassed with a normal DC voltage supply or capacitors. In other words, even for grounded cathodes, the heater supply should provide power (in this case DC) to the filament while at the same time looking as close to an open circuit as possilbe on BOTH sides of the filament supply to any AC signal.
A CCS as a heater supply would do no good (in his opinion) since only the end of the heater with the CCS would "look" like a high impedance to the signal. The other end would offer a low impedance to ground. He came up with a circuit using a CCS on one side and some circuitry on the other side that effectively allowed DC to pass and blocked AC, without chokes and very inexpensively. He gave some reasons why this was desireable but I can't remember what they were. It was something to do with the signal potential that develops across the cathode and that shorting it (so the cathode has no AC voltage across it whatsoever - a "unipotential cathode" DHT?) causes a decline in sound quality for some reason, AGAIN that I can't remember.
If his "theory" is correct it would indicate the best place for chokes would be on the heater side of the caps with no caps directly across the heater to short the AC. I wish I could find the article - I'm only giving sketchy details. He may be full of it but I found it interesting.
Thinking about it, it makes some sense. If the cathode is powered by DC, one end will hog a little more signal current than the other. NEGLECTING the DC completely, it seems to me there would be a slight imbalance in the signal voltage that is seen at each end of the cathode. A cap or low impedance DC supply across this will create a small circulating current composed of the signal being amplified, would it not? How this translates into the sound of the amp I have NO IDEA. I just found it interesting.
I believe you are talking about Rod Colemans CCS-Gyrator filament supplies, I agree that the caps across the heater supply should be avoided, but that is probably a topic for another thread.
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