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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: 15
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: 16796
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"

 

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

Posts: 15
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: 16796
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"

 

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

Posts: 426
Location: Maryland
Joined: March 29, 2006
 Proper choke filtering of the filament supply is well worth the effort.

 

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

Posts: 9806
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:














 

What size choke are you .. . , posted on May 15, 2023 at 11:59:32
John PA
Audiophile

Posts: 3998
Location: The lower 48.
Joined: August 27, 2000
 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'
Industry Professional

Posts: 16796
Location: So. Cal.
Joined: February 9, 2002
 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"

 

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

Posts: 16796
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"

 

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

Posts: 1310
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.

 

So what you are saying is. . . , posted on May 17, 2023 at 12:20:31
John PA
Audiophile

Posts: 3998
Location: The lower 48.
Joined: August 27, 2000
 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: 16796
Location: So. Cal.
Joined: February 9, 2002
 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"

 

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

Posts: 1347
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: 9806
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!




 

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

Posts: 3485
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'
Industry Professional

Posts: 16796
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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