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RE: Would You Buy a SET Amp Using DC Heating on the Output Tubes?
Posted by Tre' on July 21, 2011 at 12:12:41:
I think DC gets a bum rap.
DC done right does not cause bad sound in my experience.
First the DC filament power supply filter should be a critical choke input with plenty of capacitance to really drop the ripple.
I use LCRC.
Second the filament voltage should be floating to the filament, not grounded.
I have seen many DC filament supplies that have the negative side grounded.
That's a big mistake. It guarantees that the last cap in the filter WILL be in the signal path.
By floating the filament, the audio signal should flow equally through the last cap in the supply in opposite directions which should be the same as no flow at all, taking that cap out of the signal path.
This is what I do with my cathode biased DHT output tubes.
Using AC, I null out the hum using a 10 turn wire wound pot then without changing the hum pot, I replace the AC with DC.
Doing this, the only difference I hear between AC and DC heating is the lack of hum.