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Input tube, tube bias, and hum (Read 2669 times)
schebb
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Input tube, tube bias, and hum
07/19/13 at 05:07:38
 
Good day folks,

I built a Zkit1 with the latest board and recommended transformers. Decided on 6922 tube and bought one from Decware for build.
Had very slight hum, more on one channel than the other. Bias voltages were 3.2 VDC and 3.35 VDC.. Recommended is 2.6 VDC +/- 0.5 VDC??? Hum measured at speakers was 3.5 and 5.1 mV AC.
Purchased 6922 from a different source with matched triodes. now bias voltages match at 3.3 and 3.31 VDC, still high. Hum is greatly increased and measures 11.1 mV AC on both speakers with no input and volume at minimum. If the volume is turned to maximum with no input, the hum totally disappears when the volume hits the 9:00 o'clock position although speaker voltage climbs to around 38 mV AC.
Could someone shed some light on the high bias voltages and possible reasons for increased hum with different tube and hum disappearing with no input and high volume. The hum does not disappear with input connected and high volume.
The sound itself is wonderful but hum is audible from listening position during quiet passages. Thanks.

Steve
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« Last Edit: 07/19/13 at 14:57:11 by schebb »  
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mboxler
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Re: Input tube, tube bias, and hum
Reply #1 - 07/19/13 at 17:52:34
 
Hi Steve

I had similar bias voltages with the 6922's (3.25 IIRC).  When I switched to 6n1p's, the bias dropped to around 2.85.  I assume each tube represents a different resistance to the circuit.  I'm using the 6922's.

Did you go through this thread?

http://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1257041114/0

I didn't have any hum problems, but perhaps you will find something there.

Good luck!

Mike
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schebb
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Re: Input tube, tube bias, and hum
Reply #2 - 07/19/13 at 19:06:18
 
Thanks Mike,

I have gone through all of the threads pertaining to hum and did not find a specific issue. I cannot get ground loops as the chassis is wood with a single connection from the IEC socket to the metal fuse/switch/power bracket and to the board. I have a 6N1P coming today and will give that a try. I will attempt to attach pictures as a there may be a layout issue which I am not aware of?

Steve

Can't seem to attach pictures from mi iPad. I'll work it out and get some pictures uploaded.
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schebb
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Re: Input tube, tube bias, and hum
Reply #3 - 07/21/13 at 15:39:16
 
Folks,

Still working on pictures as I need a certain number of posts to insert a link to the pictures. I am pleased to say the 6N1P tube cleared up all of my issues, except the sound. I really do prefer the sound of the 6922 but am very happy now that my bias voltages are in the 2.8 to 2.85 VDC range, the voltage at the speakers measures 2.0 mV AC at both speakers, and there is only a very slight hum with one's ear to the speaker and inaudible from the listening position.

Steve
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mboxler
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Re: Input tube, tube bias, and hum
Reply #4 - 07/22/13 at 02:48:30
 
Hi Steve

Forgot to ask...what output tubes are you using?  Also, some day I'm going to play around with the input tube's cathode resistors.  Crazy Bill had these suggestions in another thread.

http://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1272591393/7#7

Mike


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schebb
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Re: Input tube, tube bias, and hum
Reply #5 - 07/22/13 at 04:46:29
 
Mike,

I am using 6P15P-EV output tubes supplied by Decware. May be worth mentioning that I am also not using 1N4007 rectifier diodes but opted to use UF4007 diodes as talked about in a very early thread. Am building a second Zkit and will use 1N4007 diodes in that one? Read the post you linked and although interesting, at the moment I am satisfied that the amp is built and operating as per Steve's (Decware) parameters. It sounds great to me and being an aging victim of aural abuse and mild tinitis, I'm not sure I could hear the difference.........although there was something about the 6922 that I prefer over the 6N1P? Maybe not as aurally challenged as I think.  May play/experiment in future. Thanks.

Steve
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schebb
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Re: Input tube, tube bias, and hum
Reply #6 - 07/22/13 at 04:48:51
 
Photos of amp. They are a bit "fish-eyed" but give good idea of layout. If you see issues (other than xmer grounding) let me know. Xmer grounding is a whole other issue probably best served in a new thread........one which I will probably start shortly!

http://imgur.com/a/0NKV9

Hope this works! Thanks.

Steve
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« Last Edit: 07/22/13 at 06:05:36 by schebb »  
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mboxler
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Re: Input tube, tube bias, and hum
Reply #7 - 07/22/13 at 14:42:09
 
Very nice, Steve!

I still get a kick seeing the different ways the kit is used.  I used the "stock" coupling caps, but recently bought some Jupiter Beeswax caps to replace them.  Haven't installed them, but perhaps that will improve the sound (?).

One thing I'd try is connecting the speaker wires from the OT's directly to the binding posts.  Not an expert, but they may be interfering with each other looped that close together.  But hey...I'm still new to this stuff.  

Good luck!

Mike
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Lon
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Re: Input tube, tube bias, and hum
Reply #8 - 07/22/13 at 14:50:22
 
I had the Jupiter caps installed in my Torii Mk III and they really improved the amp in all sonic ways. I'm sure the same will happen to your amp. Get 'em in there! Smiley
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Decware:ERR,HR-1,ZP3,CSP2+,Torii Mk III,PS Audio PWT+DS,PowerBases,PPP,AC-12 pcs, Denon DVD-A1UDCI, Rega RP3 +TTPSU,white belt+Exact2, VooDoo Cable:Stradivarius, Ultra Linear+Evo ICs, Iso-Pods,Mapleshade:Double Heiix Plus,Samsonv2+v3 +4" platforms,Herbie's Iso-Cup
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Lon
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Re: Input tube, tube bias, and hum
Reply #9 - 07/22/13 at 14:51:59
 
Steve, NICE WORK! Good looking amp!
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Decware:ERR,HR-1,ZP3,CSP2+,Torii Mk III,PS Audio PWT+DS,PowerBases,PPP,AC-12 pcs, Denon DVD-A1UDCI, Rega RP3 +TTPSU,white belt+Exact2, VooDoo Cable:Stradivarius, Ultra Linear+Evo ICs, Iso-Pods,Mapleshade:Double Heiix Plus,Samsonv2+v3 +4" platforms,Herbie's Iso-Cup
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schebb
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Posts: 11
Re: Input tube, tube bias, and hum
Reply #10 - 07/22/13 at 15:35:37
 
Thanks for the remarks guys. This amp has the Auricaps for coupling as they seem to be considered relatively good bang for the buck and boutique caps can be very expensive. As I do have some tinitis I am not sure I could hear the sonic differences to make expensive caps worth while. Mike, I to am not sure that the speaker wires coiled together would have any effect. The next amp will be built on the same block of hardwood but the speaker terminals will be mounted on the back of the wood block into a recess I will mill into the block. This will allow for direct connections. Being old school I always have a problem cutting wires shorter on transformers and the like since I am always thinking reuse, and with component leads, " it's better looking at it than looking for it"! Will look into the Jupiter caps. Thanks.

Steve
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clowkoy
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Re: Input tube, tube bias, and hum
Reply #11 - 07/27/13 at 09:32:27
 
A few things you can do to lower the hum:
1. make an artificial center tap on your heater wiring using 2-100 ohm
resistors.
2) increase C2 to 80-100 uf.

Install hazen mod.

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schebb
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Re: Input tube, tube bias, and hum
Reply #12 - 07/27/13 at 18:52:03
 
Thanks Clowkoy,

My amp already has the Hazen mod and the artificial CT on the heater circuit. The larger capacitance on C2 would be a viable option which I have not tried. My problem seems tube related. 6922 tubes give me unacceptable levels of hum in the 11 mV AC range and 6N1P-EV tubes reduce hum to 2.0 mV AC at speakers. Decware's spec for an amp built in their facility is 1.5 mV AC max so I feel 2.0 mV is acceptable for my amp. For me, switching to the 6N1P solved the issue, the question of why still remains?

Steve
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