Series wiring for heaters - diyAudio
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Old 12th December 2005, 09:03 PM   #1
XZur is offline XZur  Philippines
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Default Series wiring for heaters

How should I wire 2 tubes in series, in way to supply 12ax7 with 12v (shouldn't be using H tap pin), and one 6dj8 with 6v? Am not sure if simple series wiring will work for a 19v suppy. I am worried it will apply 9v to each equally.
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Old 12th December 2005, 11:50 PM   #2
ilimzn is offline ilimzn  Croatia
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12AX7 at 12V needs 150mA, 6DJ8 needs about 330, so no, this will not work. But, you could use 12AX7 strapped to 6.3V, that would be 300mA and a PCC88 equivalent (series heater equivalent of 6DJ8) also at 300mA in series connection, and that would require about 113.5-14V. Best would be to use a higher voltage and a series resistor to get less dependance on mains.
You could also use a resistor in parallel with the 12AX7 to match current of the 6DJ8 but then you have the problem of different heater characteristics, you will get varying currents during heat-up. A current source for heaters would help a little. The resistor in parallel with 12AX7 heater would be about 70 ohms, 68 is the closest 5% standard value but should work OK. The only thing is you are wasting power by conversion into heat on it, about 2.3W, so you need a 5W resistor at least. You may need to trim it a bit to get the right voltages.
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Old 14th December 2005, 02:33 AM   #3
XZur is offline XZur  Philippines
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Thanks.... crystal!
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Old 14th December 2005, 03:01 AM   #4
ALBQ is offline ALBQ  United States
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Default 6DJ8 - 12AX7 Filaments

xZur & ilimzn,
I am not sure what I am missing here but, why not tie pins 4/5 together on the 12AX7 and run the tube at 6.3 volts? A 12AX7 is designed to operate at either 12.6V (pins4-5) or 6.3V (4/5-9). In either case the tube requires ~300mA to operate both halves of the filament. In this case you would simply run the 6DJ8 and 12AX7 filaments in parallel - provided your supply could provide 600ma-700ma required for both tubes
-ALBQ

6DJ8 - pins 4-5 (6.3V only)
12AX7 - pins 4/5-9 (6.3V) pins4-5 (12.6V)
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Old 14th December 2005, 11:45 AM   #5
ilimzn is offline ilimzn  Croatia
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If I understood correctly, XZur has a 19V supply available, but not a 6.3V
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Old 19th December 2005, 02:43 AM   #6
XZur is offline XZur  Philippines
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That's correct, exploring to use 19v for these tubes.
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