I've read with great interest the sticky post about heater wiring. Seems much of it revolves around noise cancellation using AC heaters. My preamp (Paragon Series E) uses DC heaters for its 12AX7As. I'm building a new supply to replace the one with 40 year old electrolytics and can use this as a time to address the heaters as well. A couple of questions:
- Twisting: is it critical to twist heater wiring even with DC filaments?
- Polarity: I remember that one of the drawbacks of DC heaters is uneven "wear" of the filaments over time, due to some electrical influence. Would it make sense to wire one channel's filaments at opposite polarity to the other, so tubes can be swapped periodically and overcome this?
Twisting is done to get the electromagnetic fields to cancel each other out. That's only important with AC, since the fields are pulsing.
There are some ways that AC is nicer to filaments than DC but i'm not sure we've seen evidence that it actually becomes a problem.
There are some ways that AC is nicer to filaments than DC but i'm not sure we've seen evidence that it actually becomes a problem.
With DHT's, one end of the filament (or cathode) is at different potential than the other end with DC. So, the current load is higher on the positive end than on the negative end. In principle this could shorten the tube life.
With indirectly heated tubes, the heater is isolated from the cathode, which is heated by heat radiating from the heater, and the cathode has a uniform charge over it's length. That's why it's called a "unipotential" cathode. So there wouldn't be any wear difference, AC vs. DC, across the cathode. Also the temperature of the cathode stays quite constant, even with AC on the heater, and generally DC heating is not needed.
Sheldon
Last edited:
Thanks! Sheldon, your explanation makes sense. I had failed to differentiate between the two. I'm going ahead with DC filaments because that's how the preamp was originally designed and when I first got it many years ago, it had a problem with mains-frequency hum that I only fully got rid of by carefully filtering the filament. I will stay with what worked before, I think.
I have had trouble with hum in valve amps in the past.
Changing to dc heaters made little or no difference.
Changing things like decoupling and improving layout and grounding made a much bigger difference.
Keep valves away from transformers.
Keep high VAC away from audio signals.
Keep input circuit very short and close to valve.
Keep grid stoppers close to valve grid.
Changing to dc heaters made little or no difference.
Changing things like decoupling and improving layout and grounding made a much bigger difference.
Keep valves away from transformers.
Keep high VAC away from audio signals.
Keep input circuit very short and close to valve.
Keep grid stoppers close to valve grid.
The OP's preamp came with DC- powered heaters, so it's probably best to stay with that now. Otherwise, I totally agree with you.
Sent from my phone with Tapatalk. Please excuse any typpos.
Sent from my phone with Tapatalk. Please excuse any typpos.
- Status
- Not open for further replies.