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12AX7 heater filament resistance
12AX7 heater filament resistance
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Old 24th August 2019, 08:48 PM   #31
6A3sUMMER is offline 6A3sUMMER  United States
diyAudio Member
Join Date: Jun 2016

I love your text and picture of the Martian (Bugs Bunny cartoon?).

Sorry, back to old filaments, and other circuits. The amp was a stereo amp. Using a tube header, two 56k 2W resistors, and a 20 Ohm 5W resistor, I built a substitute for the top tube in the SRPP stages (or whichever circuit that took the output from the bottom tube plate, not the top tube cathode). It had formerly been a stacked 12AU7. The reason for the 20 Ohm 5W resistor was that the filament supply was 6.9V instead of 6.3V. There was a dropping resistor in series with the 6.9V to the filaments. With 1 filament now missing, I would have had to change the dropping resistor. But I wanted to be able to insert either the top 12AU7 or the substitute, and do so quickly, without re-wiring the dropping resistor. I actually liked the sound of the substitute better. Plus I did not need to elevate the filament supply anymore.

Last edited by 6A3sUMMER; 24th August 2019 at 08:57 PM.
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