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Hum pot, or not, with 300B ?

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Posted on October 24, 2009 at 23:34:55
Audiodyssey
Dealer

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 I've decided on building the JE Labs SE 300B deluxe. I noticed that most designs use a 880R 50w resistor bypassed with 100uf on the cathodes, and MOST employ a hum pot of 50R-100R. In the Sound Practices Model 91, Joe Roberts suggests using a center tapped 5V transformer and running the cathode resistor to ground, bypassed with a small oil cap, and running the bias voltage through the filament tranny via the CT. I'm having a pair of 5VCT trannies wound to give exactly 5V at 120VAC (which mine is fairly consistently) line voltage. Has anyone played with both configurations that can comment on results? Just to be sure, hum really is a HUGE buzz kill for me, so what's best? Also, if the hum pot is indicated, what value, type, wattage etc, is best?

Thanks as always...
"I KNOW you can hear it, but are you REALLY listening?"
RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 08:27:11
zarniwoop
Audiophile

Posts: 538
Location: NY
Joined: November 28, 2002
 I agree with Paul's observations. I would just recommend to use a good DC supply instead of AC for DHTs.

People seem to always get confused about the hum pot's ability to quiet things. The hum pot will null the fundamental heating frequency and odd harmonics only. The even harmonics (product of the triode's non-linearity) still remain, leaving a fair amount of 2xF and 4xF noise. The exact amount of which depends on the heating voltage and triode used.

So choke supplies (choke input) seem to be a good way to go. Does anyone have a source for filament DC chokes? I would like small easy to find ones,, not huge custom wound ones. hammond 155b is avail,, only 6mH at 2A, but it would work in CLC. Any other < 0.5lb ones that are stocked items?
RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 08:41:59
Audiodyssey
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 I'm going with the Bottlehead FC-1 and the circuit Paul posted. I'm not that experienced so the layout, parts availability, and the decision to go DC cinched it for me. Plus, I like the idea that this may well bring hum down to a "commercially" acceptable level. Hum is a total buzzkill for me, I can't have it. Now, I just need to stick with a design. I keep finding different things that interest me, and someone also suggested that with sensitive speakers, the 2A3 might not be a bad idea, either. It's worth considering, since I haven't bought 300B's yet.
"I KNOW you can hear it, but are you REALLY listening?"
RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 00:25:28
twystd
Audiophile

Posts: 1663
Location: Austin,Texas
Joined: December 9, 2000
 I like Paul's take on it, although I haven't heard it, I'm a less is more kind of guy, at least with audio!;-) However, about 6-7 years ago I decided to explore the AC vs DC filament issue directly.

I built a 2nd AC supply on my Ultimate Laurel 300B SETs (the Laurels come with a DC supply), with a double pole double throw switch, so I could go back and forth between the 2 supplies very quickly. The hum pot on The AC supply was sufficient to cancel any hum, so hum wasn't an issue.

The AC supply sounded a lot better than the stock DC supply, which was a standard diode bridge> 10,000uf cheapo cap> resistor> 10,000uf cheapo cap. The DC supply sounded dry and lifeless compared to the AC filaments. I then rebuilt the DC supply with a Schottky bridge> 3,300uf Cerafine cap> very nice vintage potted Chicago 75mh choke> 3,300uf Cerafine cap. Wow, what a difference! Now the DC filaments were way more dynamic, and just sounded all around better than the AC filaments.

Over the next several days I took my amps to 2 of my audio bud's houses and hooked them up in their systems. They were both surprised, and concurred that the DC filaments were MUCH better. I feel that the use of a choke in the DC supply is very important.

I never thought about Paul's suggestion of a common mode choke and a hum pot, seems like that would have to work, and like he says who wants to deal with electrolytics. Sure wish I could have A/Bd that with my DC rig. Maybe I'll feel ambitious one day, try it out, and report back.

twystd

This is interesting..., posted on October 26, 2009 at 04:45:33
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004
 and it follows along with what I read at the link below. It's a DC Filament supply test by Clarisonus. It does a great comparison of different topologies for DC Fil Supplies and seems pretty thorough.

I like the simplicity of what Paul suggests too. Well, I am going to take my time with this build and try some different methods.
"I KNOW you can hear it, but are you REALLY listening?"
RE: This is interesting..., posted on October 26, 2009 at 05:31:50
KlEo
 A-Odissey,
I think Atwood test is not enough for you, because he did not introduce in PSU test Bias voltage 65-70V and 65-70mA cathode current for 300B.

Keep an eye on Serenas and Cyclopes :-)
RE: This is interesting..., posted on October 26, 2009 at 06:08:23
Audiodyssey
Dealer

Posts: 512
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 "Keep an eye on Serenas and Cyclopes :-)"

I am sorry, but I don't understand what this means???

"I KNOW you can hear it, but are you REALLY listening?"
RE: This is interesting..., posted on October 26, 2009 at 06:15:20
KlEo
 :-) Nothing, just a joke.
Do you know who was Odyssey? On his return from Troy, Odyssey met with Serenas, Cyclops, Poseidon, etc.

RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 04:01:13
KlEo
 twystd,
3300u-75mH-3300u with dynamic load, ringing like crazy. Check it with Spice (ground point is only after Rk-Ck).
I am satisfied with my 10mF-5mH-10mF filter and low noise NOS carbon fixed resistors in hum-pot position. Type and quality of resistors for hum-pot are very important for sound quality.

Common mode choke brings very ugly voltage peak areas during load changes and I do not use CMC between 5V PSU and heating pins.

cheers

RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 13:53:13
twystd
Audiophile

Posts: 1663
Location: Austin,Texas
Joined: December 9, 2000
 "low noise NOS carbon fixed resistors in hum-pot position. Type and quality of resistors for hum-pot are very important for sound quality."

Are you talking about a couple of resistors forming a virtual center tap rather than a hum pot? If so what value are you using? Thinking of my current fave Takman REX carbon film resistors.

I don't have spice to model with, if it's not too much trouble, could you be so kind as to model 6.3V PT filament winding> Schottky bridge> 10uf cap> 60mh 1.3DCR> 10uf cap. I looked at my chokes, and they are actually 60mh and 1.3 ohm DCR. I have some nice 10uf 300V PIO Russian MBGOs to use instead of the electrolytics. Bet that would sound good if I don't get too much hum.

twystd

RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 13:59:40
KlEo
You say "10uF cap". Are you sure? You think 10mF (10mF = 10.000uF) perhaps.

RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 18:55:15
twystd
Audiophile

Posts: 1663
Location: Austin,Texas
Joined: December 9, 2000
 Sorry I saw 10mf, but thought 10uf. Wishful thinking I guess. The bottom line is it sounds PDG, any suggestions on how I could use that choke without ringing would be appreciated. Any clarification about what you are doing with the resistors would be helpful as well.

twystd
RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 03:09:43
KlEo
 Im sorry twystd. Choke with 1.3 ohm RDC requires 7.7V secundary voltage.W ith 6.3V you should have choke with 0.3 ohm RDC.
7.7V secondary-schottky-10.000uF-60mH (1.3ohm)-10.000uF, with active load, has a nice voltage chart.
I have not tried Takman carbon resistors. But should be good for the hum-pot position. I like sound of carbon resistors. Values from 22 to 50 ohms are good (I tried). Use one where the value of the voltage on the 300B heater pins does not fall below 4.9V. Measure heater voltage when the amplifier is turn-on and 300B is well heated.

RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 12:55:48
twystd
Audiophile

Posts: 1663
Location: Austin,Texas
Joined: December 9, 2000
 Thanks for your effort, but I guess there really is a difference between theory and reality. My supply is already built, and it does put out about 5.4V (a little higher than I like) with the 6.3V winding. I'll get some 10,000uf caps and try that, thanks for the suggestions.

twystd

RE: Hum pot, or not, with 300B ?, posted on October 28, 2009 at 03:11:26
KlEo
 Of course. As long as you are satisfied with the sound of your amplifier, your guess is true.

best diying
RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 14:42:22
Paul Joppa
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Joined: April 23, 2001
Contributor
  Since: July 1, 1999
 I did a certain amount of experimenting back when we were developing our current 300B design. At the time I really wanted to keep the cost as low as possible, and hoped to use AC heating. Here's what I found:

Using a center-tapped 5v filament winding, most tubes would result in 20-50mV of 60Hz hum at the speaker terminals. The best individual tube I tried was 12mV if I recall correctly. This is too much hum for almost everybody.

Using a hum pot, I could get the hum down to 5-8mV of 120Hz (and little or no 60Hz hum) reliably with almost any 300B. I believe that with a better power supply, a little care in tube selection, a roomier chassis, and more extensive care with the layout you might get down to the 4mV level, though I never pursued that. In my experience, anything over 2-3mV will produce complaints from at least some customers.

In a previous amp some 10 years ago we tried to implement a snazzy 120Hz hum cancellation circuit in addition to a hum pot. In addition to being hard to adjust two controls that interacted with each other, it only worked on some tubes, and made at best a small difference. I tried 2 or 3 variants of the circuit before giving up.

(For what it's worth, the lower filament voltage of a 2A3 will give you 2-3mV with a hum pot.)

I eventually gave up and went to a DC supply, using only a choke, and retaining the hum pot. I don't remember exactly, but I believe it is running about 0.8mV hum at 240Hz this way. The filament can be made quieter by adding a capacitor, but as far as I know nobody has actually done that - or felt the need.

If you use a conventional DC supply, there are a few things I believe to be worthwhile. I have not done careful experimenting with these, I merely observe that they have strong technical arguments, many have reported good results this way, and we have not seen the problems most often associated with failure to observe these precautions:

1) Shield the filament power winding inside the transformer

2) Use Schottky diodes

3) Use some common-mode inductance between the power supply and the filament

RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 21:27:32
Donald North
Manufacturer

Posts: 910
Joined: February 8, 2001
 Here's an interesting thread where Dave Slagle did a lot of modeling of filament supplies for the 300B. It shows a hum pot is necessary and an output common mode choke unnecessary to minimize current through the final filament cap.

Donald North

RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 22:23:40
Audiodyssey
Dealer

Posts: 512
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 Great link, thanks!
"I KNOW you can hear it, but are you REALLY listening?"
RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 16:34:12
Audiodyssey
Dealer

Posts: 512
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 "I eventually gave up and went to a DC supply, using only a choke, and retaining the hum pot. I don't remember exactly, but I believe it is running about 0.8mV hum at 240Hz this way. The filament can be made quieter by adding a capacitor, but as far as I know nobody has actually done that - or felt the need."

Is it possible you could expand on this a bit, when you say "using only a choke? What size and type was the hum pot? Thanks for detailing your experience, Paul. Doesn't sound too promising. Also, at what level does hum become a non-issue with speakers rated at, say, 98db?
"I KNOW you can hear it, but are you REALLY listening?"
RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 22:12:50
Paul Joppa
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Location: Seattle, WA
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Contributor
  Since: July 1, 1999
 The hum pot is 50 ohms - actually, 54 ohms - we use a 10 ohm pot with 22 ohm resistors off each end, so that adjustment is less critical.

The filter is a balanced choke with dual windings, one in each leg. You might call it an LR filter, with the filament being the "R". About 30mH total inductance. The windings are on the two sections of a dual bobbin so that the leakage inductance is not trivial - it is common-mode which has some advantages in this application. The choke is available - ask me at the new Bottlehead forum for details (http://www.bottlehead.com/smf/), this is not a place for us to sell stuff!

One could put a capacitor across the filament, making it a conventional LC filter and reducing the ripple at the filament substantially. We did not, since the hum was reduced adequately and nobody likes electrolytic caps anyhow. The point made by Tre' below (arguing that with a correctly adjusted hum pot the capacitor becomes less important) is interesting; I've not heard that before but it does make sense.

RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 22:51:32
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 How big of a choke?

Thanks, Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
About 30mH total inductance (nt), posted on October 26, 2009 at 12:46:46
Paul Joppa
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 nt

Thanks (nt), posted on October 26, 2009 at 17:16:01
Tre'
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Posts: 4845
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"
RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 22:39:22
dave slagle
Manufacturer

Posts: 3199
Location: NYC
Joined: April 27, 2001
 Hey paul,

No need to be humble or skirt the issue. Your idea of the "L" only filter with a humpot for DC is brilliant.

dave

RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 13:20:17
Paul Joppa
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Thanks, Dave, much appreciated!

Donald, I'm not clear what your question is so I've just shown the circuit - let me know if I missed anything you wanted to know more about.

The choke is wound on a split bobbin with two identical sections. The core has an air gap. Wired in series, the two sections provide about 30mH inductance at a maximum current of 1.25 amps, with plenty of magnetic headroom to support its use in a choke-input filter. The leakage inductance is about 1.2uH, but is common-mode so it provides a bit of RFI blocking in case there is any, e.g. capacitively coupled from the power transformer primary winding.

RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 12:40:44
Kyle K
Audiophile

Posts: 127
Location: Mililani
Joined: November 6, 2000
 Would this common mode line filter work?

http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=11220
RE: Hum pot, or not, with 300B ?, posted on October 27, 2009 at 15:49:58
Paul Joppa
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Location: Seattle, WA
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Contributor
  Since: July 1, 1999
 It would provide common-mode RFI protection, but no filtering action which is differential mode.

I should mention that a pair of Hammond 155B chokes can work in this circuit. They are 6mH, rated 2 amps, and 0.3 ohms DCR. One in each leg gives 12mH total. They make larger chokes as well, but with more DCR.

Thank you!, posted on October 26, 2009 at 14:59:07
Audiodyssey
Dealer

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 Well, I'm going for it! I got the chokes ordered already and I'll let everyone know how it goes...
"I KNOW you can hear it, but are you REALLY listening?"
RE: Hum pot, or not, with 300B ?, posted on October 26, 2009 at 14:44:20
Donald North
Manufacturer

Posts: 910
Joined: February 8, 2001
 Thanks for posting this schematic. Now I see what's going on.

Donald North

RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 23:14:18
Donald North
Manufacturer

Posts: 910
Joined: February 8, 2001
 Hi Dave and Paul,

Can you explain this design?


Donald North

So is AC heating DEFINITELY going to cause hum?, posted on October 25, 2009 at 13:32:26
Audiodyssey
Dealer

Posts: 512
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 Well, I want to have a crack at direct heating. I've been doing a lot of reading and it seems AC is probably going to cause some hum, no matter what. As I stated, I am having a pair of filament transformers wound that will keep the AC at 5.0 with a 120VAC line. I guess I am going to try some different things and see what works best. I really think this time I am going to breadboard one of the pair first.

I am looking more for people with experience that are using AC heating of the 300B that may have some helpful ideas on hum reduction.
"I KNOW you can hear it, but are you REALLY listening?"
RE: So is AC heating DEFINITELY going to cause hum?, posted on October 25, 2009 at 13:39:59
Plinko
Audiophile

Posts: 1359
Location: N East, USA
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 I cannot hear hum at the seating position from my AC heated 300B amp into 98-100db Altecs. The pots work nicely and the hum cancellation is superb. Depends on the amp.

RE: So is AC heating DEFINITELY going to cause hum?, posted on October 25, 2009 at 13:53:51
Audiodyssey
Dealer

Posts: 512
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 I'm not sure of the efficiency of my speakers, Speakerlab K horns, but it's at that range. What amp(s) do you have?
"I KNOW you can hear it, but are you REALLY listening?"
RE: So is AC heating DEFINITELY going to cause hum?, posted on October 26, 2009 at 18:36:00
Caucasian Blackplate
Industry Professional

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 K-horns are very, very sensitive. You will want DC heating or a 2.5V 300B (beefy 2A3).

RE: So is AC heating DEFINITELY going to cause hum?, posted on October 25, 2009 at 16:42:12
Plinko
Audiophile

Posts: 1359
Location: N East, USA
Joined: July 21, 2005
 Hi...custom build. I sit about 10 feet away. There is hum if you're right in front of the speakers, though.

RE: So is AC heating DEFINITELY going to cause hum?, posted on October 25, 2009 at 14:16:43
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 10 turn hum pots help.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No pot, a good DC fil supply, posted on October 25, 2009 at 06:20:52
eyes
Audiophile

Posts: 98
Location: Oxford Mills, Ontario
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 If your speakers are even remotely efficient you will get audible hum with an AC supply and a pot on a 300B. If you have the room, go DC. Lots of schematics on the web for good DC filament supplies. If you have the room and the cash, get a pair of the TentLabs 2.5A filament supplies (~$200) and never worry about hum again.

/eyes
A good DC fil supply and a pot or a pair of resistors, posted on October 25, 2009 at 13:21:40
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 When you ground one side of the DC supply, you are inviting the last cap of the supply into the signal path.

If you have a pot, or two resistors making a center tap, the last cap is mostly excluded from the signal path.

I run a 6b4g (6.3 volt filament) setup this way with a LCRC filter and there is no hum and it sounds great.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
RE: Hum pot, or not, with 300B ?, posted on October 25, 2009 at 03:23:48
lovetube
Dealer

Posts: 2334
Location: Melbourne Australia
Joined: June 8, 2003
 I think 300b need a hum pots to really quite them down unlike the 2a3 where you can leave it out.
normal good value for hum pot is 50ohms , bot 70 or 100ohms are also fine.
LT

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