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RE: Would You Buy a SET Amp Using DC Heating on the Output Tubes?

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Tre'

In Reply to: RE: Would You Buy a SET Amp Using DC Heating on the Output Tubes? posted by Gerry E. on July 21, 2011 at 10:56:10

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.

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

Follow Ups:

RE: Would You Buy a SET Amp Using DC Heating on the Output Tubes? - opus64 16:06:58 07/24/11 (1)
Hi,

Is the resistor simply a Rdrop value for purpose of dialing in exact heater voltage or a larger value?

TIA
Opus
- RE: Would You Buy a SET Amp Using DC Heating on the Output Tubes? - Tre' 10:16:29 07/26/11 (0)
  Yes, it's there to drop (adjust) the voltage and it helps with filtering.
  Without the resistor it would be an LC filter. With the resistor it's a LCRC filter.
  Without that resistor, but with the same amount of capacitance and adjusting the AC voltage feeding the bridge down to end up with the same amount of dcv, the ripple would be .04 volts peak to peak instead of .0012 volts peak to peak.
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  
  Edits: 07/26/11 07/26/11 07/26/11 07/26/11 07/26/11 07/26/11
Agree on choke input - andy evans 01:16:46 07/22/11 (0)
DC works well with a choke input supply - that's the important bit. I use a 280mH choke, 30,000uF or so, and then follow that with a Rod Coleman designed filament supply with a gyrator and a current source.

Few if any commercial SET amps would use a choke input, but that's the way to go if you are building and are serious about a clean supply.

Andy

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