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In Reply to: RE: A critical inductance choke input filter.... posted by Tre' on May 14, 2023 at 19:02:42
Please explain: the bad rap of dc filament heating;
why is it not done right;
why a bridge followed by a big cap is no good, etc.
Thanks in advance.
Follow Ups:
Some people say that only AC heating of direct heated triodes sounds good and that DC heating does not sound good.
I think the people that say that have listened to amplifiers where the direct heated triodes are heated with poorly filtered DC.
A bridge followed by a large cap will have saw tooth ripple. Saw tooth ripple will cause the sound to be raspy.
Good filter chokes are expensive (the main reason why you don't see them used much) but worth it in terms of sound.
In general, the ripple from a power supply that uses a input choke will be much lower and what there is of it will be shaped like a sine wave (smooth and round) instead like a saw tooth (hard and raspy).
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I would like to return to the question of choke filtration, specifically, the point of using this design "properly" for filament supplies.
Given the level of filtration/regulation afforded by the LCLC configuration and that afforded by a VERY quiet voltage regulator, is using them both on a filament supply redundant? And if so, which part, the regulator? or the LCLC filter?
The filament or heater of a tube doesn't change it's current draw so regulation is not needed except for eliminating ripple but my filament supplies don't have much ripple to start with.
My B+ supplies for the voltage gain tubes in my system are LCLC plus constant current source feed VR tube shunt regulated.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thank you for your response, it was helpful. You said that your filament supply's current doesn't change; what filter configuration do you use? Pi filter or choke? If you use a pi filter with less regulation, don't you also get more noise? So would it not be helpful for noise to use a very quiet regulator?
My filament supplies for the direct heated tubes are LCRCL and my heater supplies for the indirect heated tubes are LCRC with critical inductance input chokes and a lot of capacitance. In both cases the ripple is almost non-existent.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/25/23
Thank you. That is very helpful. IN both of your situations, does the resistor do anything to remove some of the noise, or is it only to reduce some of the voltage?
Both. The resistor value is chosen to dial in the voltage I need and a resistor before a cap will allow the cap to better filter the ripple.Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/25/23
got it, thanks.
Hello Tre'I remember that in the early 2000's a lot of the discussions of tube cathode heating were about the differences between regulated and non, as well as differences between voltage regulated and current regulated.
If a regulator is used the supply that feeds it might have a lot less impact on the sound.
Rod Coleman's design idea (as I understand it) of putting high impedances at both ends of the cathode in a DHT does seem to have merit as his regulator's do make for good results. I guess that might line up with your use of the choke on the output of your supply. I don't remember who it was back then but there was another Asylum member who talked about his supplies having a choke in series with each end of the filament to good effect.
Bottlehead had a design that used the leakage inductance of a (Paul Joppa designed?) dual bay choke (search "Bottlehead FC-1") to act as a common mode choke.
P.J. also posted a circuit in the post linked below that shows a dual choke design like the one mentioned above.
Edits: 05/15/23
"Some people say that only AC heating of direct heated triodes sounds good and that DC heating does not sound good. I think the people that say that have listened to amplifiers where the direct heated triodes are heated with poorly filtered DC."
The prototype for my 211 SETs initially used 60Hz AC to heat the filament. The sound was wonderful, but I couldn't reduce the hum to a reasonable level. I even tried a reverse-phase injection scheme to cancel it, but the waveform included energy that was complex and non-repetitive. Eventually, I came to the conclusion that the long filament structure was actually vibrating and creating AC anomalies that could not be predicted or cancelled.
At that point, I switched to DC. The first attempt was the usual rectifier/filter approach. It was quiet, but the life seemed to be gone from the music. Thinking this might be due to residual AC energy or harmonics, I added a regulator. That produced pure DC, but it didn't help sound quality. Finally, I used a switching supply to isolate the filament from the supply for common mode effects. Still not good.
In the end, I resolved the issue by heating the filament with ultrasonic AC. That brought the life back to the music. This is all very subjective, of course. I also can't claim it applies to all DHTs, but other builders have reported similar AC VS DC results with other tubes.
Here are the filament supplies in their final form. The output is a quasi-square wave at roughly 80 kHz:
Triode_Kingdom - would you mind sharing where you purchased the Ultrasonic Supply? Also, does it support 4A or more? Any other specs you can share?
Finally, have you compared the Ultrasonic AC vs. battery filament supply? I am currently using battery (un-regulated) but the battery charging is a PIA.
Cool report TK. Did the ultrasonic AC solve the hum issue?
Yes, the amp is very quiet with the ultrasonic supply. The project is essentially complete now, just need to finish a few cosmetic items. Yay!
"Eventually, I came to the conclusion that the long filament structure was actually vibrating and creating AC anomalies that could not be predicted or cancelled."
I didn't see that coming! Interesting.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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