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Low cathode resistor bias

Hi chaps

I've recently been intrigued by the concept of using a very low value of cathode resistor for biasing. Such a low value doesn't need bypassing. The current for the resistor is obtained from a secondary supply. Here's a basic description from the tnt review of the European Triode Festival 2004 where it won a shootout of biasing methods:

"Manfred Huber (Germany) elaborated on the various methods of providing bias voltage to a triode. In the following shootout the audience rated LOW-R bias best (that is using a low value resistor in the cathode and having additional current from an auxiliary supply - typically the dc heater supply - flowing to it, such that the desired bias would settle), then surprisingly Miss Piggy got a second price (capacitor bypassed cathode resistor), all the other methods (battery in cathode, battery in grid, LED bias, fixed voltage for grid and cathode to ground, non-bypassed cathode resistor) proved to have more or less uncomfortable sounding shortcomings."

A basic schematic is on the tnt website at the enclosed URL if you are interested.

I built a circuit (just like the one shown on the tnt site) today to see how it works. From what I can tell, it's just basically a voltage divider (between R1 and Rk). Maximum cathode voltage is limited to the secondary voltage supply (eg. DC heater supply). Isn't this just the same as using fixed bias, but applied to the cathode instead of the grid? Is it really a good idea to share the heater voltage with the cathode?

Perhaps I'm doing it wrong? Anyone else out there used this?

Cheers. Doug





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Topic - Low cathode resistor bias - Doug Flynn 21:47:01 07/16/05 (43)


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