keithostertag wrote:
Hey Les- In trying to understand how this circuit works I am comparing it to a conventional common cathode triode circuit. But this one is different since it uses a direct heater/filament. And, there is no cathode resistor, AFAIK. What is biasing this circuit?
No bias resistor on cathode. I know it seems odd because g1 cannot go below k, which means 0-sig is above k. It is operating as, what I call, grounded cathode. I've seen only one other circuit operate in this mode. You can see the AF amp diagram from a circuit I traced from a Sears SilverTone AM/FM/Phono that uses a similar set-up in the driver (6SQ7) circuit:
In the 272A, you notice that V1 H/k is in series with V2 H/k, thus V2 H/k is biased above V1, and therefore the reason the VR2 is referenced to the connection between p5 of V1 and p3 of V2. I'm not sure if C3 (capacitor off p5 of V2) is there for power stability or if it is acting as bypass.
keithostertag wrote:
Also, the datasheet says "max cathode current" = .5mA. How is that different from the heater current, which they recommend to run at about 10mA?
Cathode current (k
(I)) refers to the current flowing between plate and cathode, where as heater current (H
(I)) is just that. That means, however, you need to be aware of H
(I) while designing the cathode bias to be sure you are not also putting too much voltage on the htr circuit.
The best approach for a DHT, IMHO, is to use a heater supply that is floated, that is, no ground reference at all. The method being applied in the 272 is not the best method because the heater and cathode are using a common ground. I am considering redesigning the 272 from an approach of an isolated heater supply. I'm not sure the ringing problem with these little jewels is totally mechanical, but rather possibly aggravated by biasing and loading issues. I'm sure the tubes have some inherent microphonics, but it should not be more than any other "traditional" tube. I can't see these being used in any high end product if the inherent microphonics are truly that serious, and supposedly they are used in high end foil type studio microphones and were used in military field radios. I really have hard time accepting it's purely mechanical based also on the fact my K301, which is well damped mechanically, will self oscillate from time to time even during no signal. I have seen references on other forums that support my suspicion that there is a loading issue that is contributing.
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