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RE: using cathode bias voltage for preamp tube heaters

Just curious, but how could a 6550 (biased at, say 100mA) or even 2 in parallel passing 200mA at the extreme, ever pass the 600mA required to light a 6SN7? (who's heater is effectively a 10 ohm resistor). You must supply the tube heater with 600mA or it will not function right, there is no free lunch of magic current being created.

I have seen this using a 12AX7 with the heater in series passing 150 mA which will light a 12AX7 with about 12.6 volts. It is effectively a 84 Ohm resistor. Are you sure about the tubes he used?


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