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Old 23rd February 2005, 01:13 PM   #1
jwatts is offline jwatts  United States
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Default Filemant Voltage

Does the filament voltages need to be a regulated dc voltage or can I just run it straight from the transformer. The transformer taps are set at the rated voltage for the tube. The transformer filament tap voltages are a volt higher, but I suspect they will drop to the correct voltage when under load. The tubes are ELL80 and 12AX7. I am new at this by the way.
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Old 23rd February 2005, 01:51 PM   #2
Fuling is offline Fuling  Sweden
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ELL80 and 12AX7 should work just fine with AC on the filaments.
If you get hum problems there are several ways to deal with them.
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Old 23rd February 2005, 02:13 PM   #3
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typically, power tubes are heated with AC, though this is your choice. preamp/driver/phase splitters etc can be either, but tend to have less noise with DC, at least in my experience. for phono preamps (because of their high gain), DC is nearly imperative. whether you regulate or just smooth the DC is up to you.
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Old 23rd February 2005, 02:23 PM   #4
rdf is offline rdf  Canada
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A third option is to DC bias the centre tap of the AC filament supply secondary to a voltage greater than the highest cathode potential. More practical for low power amps and preamps than something requiring a couple of hundred volts of drive to the grid.

http://www.rru.com/~meo/Guitar/Amps/...um.html#dcbias
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Old 23rd February 2005, 07:12 PM   #5
amperex is offline amperex  United States
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Another 'hum' reduction option is to ground the filament CT. No center tap? Use a .01 200 volt cap from one transformer filament lead with other cap lead to ground.
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Old 25th February 2005, 01:52 PM   #6
garbage is offline garbage  Singapore
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Quote:
Originally posted by amperex
No center tap? Use a .01 200 volt cap from one transformer filament lead with other cap lead to ground.
will this technique work for dc filaments?
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Old 25th February 2005, 06:56 PM   #7
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Hi,

Quote:
will this technique work for dc filaments?
There would be no point to that.

The idea of the grounded CT of the heater xformer is to equalize AC currents through each half of the xformer.
When no CT is present a pair of equalizing resistors would create the same function as that CT when the midpoint of the two resistors is connected to ground.

Once you're working with DC there's no AC current present, or at least there shouldn't be if the filtering is properly executed.

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