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pi filters and choke filters for tube filaments

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Posted on May 14, 2023 at 16:34:14
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 I understand that using a choke filter in the power supply affords greater regulation of the B+ and therefore a quieter B+. Because that is true, is there any reason why we do not use this design of filter on the filaments supply? Would it not make it quieter especially for directly heated SET?
Thanks in advance.

 

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A critical inductance choke input filter...., posted on May 14, 2023 at 19:02:42
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 will insure that whatever ripple is left at the end of the filter is a sine wave. A cap input filter produces saw tooth ripple and that is very audible and hard to listen to.

For my 300b SET amp filament I use a LCRCL filter and the ripple is a very low and is a sine wave.

I think DC filament heating gets a bad rap because it is normally not done right. Many designs just have a bridge followed by a big cap. Not good!

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

 

My experience as well!, posted on May 22, 2023 at 15:29:52
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 Dirty DC for filaments is worse than AC. If I use DC filaments I always regulate, not do much for regulation bit to kill any ripple.

 

"Dirty DC for filaments is worse than AC." Absolutely. nt, posted on May 23, 2023 at 09:39:25
Tre'
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Posts: 17304
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 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

What size choke are you .. . , posted on May 15, 2023 at 11:59:32
John PA
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Posts: 4019
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 using for this? I like using chokes and have used LCRCL etc. in a preamp I worked on for a long time, for the main voltage and it was wonderful. I don't remember now what I did with the heater but is was pretty much free of ripple.


iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.

 

RE: What size choke are you .. . , posted on May 15, 2023 at 13:18:05
Tre'
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Location: So. Cal.
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 The input choke is 70mH. Critical inductance with the load the 300b filament is drawing (1.2 amps) is only 11mH

Choke value in Henrys = ((voltage/current) + dcr of choke)/1000

FYI Critical inductance is the amount of inductance that it takes, for a particular circuit, to keep the diodes on, each in turn, for their entire half cycle. That keeps the charging currents to the first cap (and the load) just a little bit higher than the load current. With a cap input filter the charging currents can be many many times the load current because the diodes are only on for a short time and those current spikes cause (according to Lynn Olson) RF to be generated and then it's just everywhere. You can't filter is out, you have to try to shield it.

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

 

So what you are saying is. . . , posted on May 17, 2023 at 12:20:31
John PA
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 To get the current just above what is needed to not have the issue with RF?



iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.

 

RE: So what you are saying is. . . , posted on May 17, 2023 at 15:32:29
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
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 No, you keep the diodes on for their entire half cycle so that current is always flowing to the filter so the charging currents will remain low.

With a cap input supply filter the diodes are off most of the time and only on for a very short time. The same amount of "power" has to be delivered to the filter (so it can be delivered to the load circuit) so the power supply has to deliver a huge amount of current in that short amount of on time (to make up for the longer time when no current is being delivered).

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

 

Agree with Tre, posted on May 15, 2023 at 09:20:50
garymuffley
Dealer

Posts: 445
Location: Maryland
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 Proper choke filtering of the filament supply is well worth the effort.

 

RE: A critical inductance choke input filter...., posted on May 14, 2023 at 20:36:59
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 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.

 

RE: A critical inductance choke input filter...., posted on May 15, 2023 at 07:03:00
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: A critical inductance choke input filter...., posted on July 10, 2023 at 10:58:20
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 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?

 

RE: A critical inductance choke input filter...., posted on July 16, 2023 at 06:59:45
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: A critical inductance choke input filter...., posted on July 16, 2023 at 09:26:09
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 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?

 

RE: A critical inductance choke input filter...., posted on July 25, 2023 at 16:09:01
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: A critical inductance choke input filter...., posted on July 25, 2023 at 16:15:31
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 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?

 

RE: A critical inductance choke input filter...., posted on July 25, 2023 at 16:20:06
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 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"

 

RE: A critical inductance choke input filter...., posted on July 25, 2023 at 17:02:24
lotsmorebetter1
Audiophile

Posts: 21
Location: North Carolina
Joined: July 31, 2016
 got it, thanks.

 

RE: A critical inductance choke input filter...., posted on May 15, 2023 at 14:55:29
Ian L
Audiophile

Posts: 1366
Joined: July 5, 2002
 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.

 

Have to Disagree, posted on May 15, 2023 at 11:17:16
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 "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:














 

UltraSonic AC, posted on July 10, 2023 at 12:43:35
banpuku
Audiophile

Posts: 1008
Joined: January 19, 2006
 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.

 

Very interesting!, posted on May 18, 2023 at 02:06:54
Salectric
Audiophile

Posts: 1358
Location: East Coast
Joined: February 23, 2003
 Cool report TK. Did the ultrasonic AC solve the hum issue?

 

RE: Very interesting!, posted on May 19, 2023 at 08:15:41
Triode_Kingdom
Audiophile

Posts: 10048
Location: Central Texas
Joined: September 24, 2006
 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!




 

RE: Have to Disagree, posted on May 15, 2023 at 13:41:55
Tre'
Industry Professional

Posts: 17304
Location: So. Cal.
Joined: February 9, 2002
 "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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