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Virtual CT for Edcor Heater Wires - WOW! (Read 2084 times)
mboxler
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Virtual CT for Edcor Heater Wires - WOW!
11/14/12 at 21:51:36
 
I finally got around to testing my ZKIT1. †I tested with a spare Klipsch surround speaker rated at 97dB at 1 watt, 8 ohms. †Fired it up...noisy tubes, and a noticable hum out of the speaker. †I had tested the virtual CT on the heater wires before, and the tube noise almost vanished. †I powered down the amp, added the 100 ohm resistors between the heater wires and chassis ground on the board, powered up. †Not only did the tube hum go away, but the hum through the speaker also went away. †Music, as you would expect, sounds much better without the hum. †Couldn't stop smiling! ††Highly recommend this mod.

Mike †
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dank
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pair of dual 18
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #1 - 01/08/13 at 14:02:46
 
Could someone provide a link to where the Virtual CT mod is explained? †I've looked but can't find it. †I'm getting the impression its two 100 ohm resistors to chassis ground, wattage unknown, on the 6.3 vac heater wires and it is needed to reduce hum when using a transformer that does not have a center tap on the 6.3 vac. †On transformers with a center tap, just ground the center tap, the Virtual CT mod is not used/needed. †Is this correct? Anyone try just grounding one of the 6.3 vac wires? †Anyone try anything other than 100 ohms? †Old hum balance circuits I've seen had a 1k linear pot with center post tied to chassis ground and the 2 other posts tied to the 6.3 vac wires...makes me think a little tuning could make things even better.

Dan
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #2 - 01/08/13 at 15:40:28
 
Hi Dan...

Here's the link I found

http://www.valvewizard.co.uk/heater.html

You are correct.  If the heater wires are center tapped, solder the CT to chassis ground.  If not (as with the suggested Edcor transformer), use the resistors.  I used 1/2 watt resistors...1%.  I ordered 10 of them, and used two that were as perfectly matched as my DMM would detect.

As far as I can tell, a pot would allow you to adjust this CT to the point of least hum.  The two resistor method really silenced my tubes.  Now if I could reduce the hum I hear when I hook up my Klipschorns.

Mike

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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #3 - 01/22/13 at 03:20:15
 
I'm interested in this mod too. I have been trying to eliminate a hum in my amp that changes with the volume. I was wondering how to best trouble shoot this. So far all I have done is mount rubber washers under the Edcore power tranny and make sure the end bells are tight. I have grounded the PCB to chassis. What I have not done yet is ground the Alps Blue Velvet 100k pot to chassis. Any further suggestions about the tranny hum would be appreciated.
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #4 - 01/22/13 at 15:38:05
 
Does the hum go away when the input cables are disconnected?  Also, did you connect the ground wire from the IEC to the chassis as well?

Mike
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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #5 - 01/23/13 at 02:47:09
 
Hi Mike, No change when I remove the inputs. I used shielded wire for the inputs and the ground wire is connected on the PCB for both inputs but not connected at the RCA's. I notice other builders have used only single wires for the inputs. Maybe the ground/drain is inducing?

My chassis ground is connect to the IEC ground via a machine screw and nuts. On the same screw is the chassis ground wire from the PCB. I was going to add the volume pot onto this same screw. Is that cool or should I change the "star ground"?

Currently I'm using a 6922 driver but I also just picked up a 6n1p and a 6DJ8. Perhaps just swapping these?

Would the Wheatstone bridge be good to try anyways?
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #6 - 01/23/13 at 14:56:06
 
Wheatstone bridge?  Had to Google it...still not sure what that is.

Maybe this link will help...

http://www.oldstockaudio.com/DIY&Parts/basic_amplifier_design.htm#BasicLayou...

I noticed on my amp that the chassis itself was resonating.  Do you have rubber feet under the amp?

I'd start with the virtual heater CT first, then go from there.  Are you getting close to getting us some pictures?

Keep the faith!

Mike
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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #7 - 01/23/13 at 17:31:50
 
Hi Mike,

Ya I have rubber feet on my amp.
The wheastone bridge is was I read in the VCT article. I will go for the 2 100ohm resistors and try eliminating the ground wires on the inputs. It definitely sounds like 60Hz. I may have unclean power. I will post some pictures tonight.
Thanks for your enthusiasm and help.
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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #8 - 01/26/13 at 07:12:56
 
Hi Mike,

I have the 100ohm resistors in place, one on each heater and the other end on ground. Do I remove the center tap (black with red) wire from the PCB? I read somewhere that you don't use both. Can you you please let me know.

I will try to get some pictures going soon.
Thanks
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #9 - 01/26/13 at 16:18:36
 
Hi...

Nope, you must leave that on. †The B+ voltage is created using a full wave center-tapped rectifier. †The red/black wire is the 0 part of the 300-0-300 winding of the transformer, and must be wired to chassis ground.

Here's a link to a pretty good description, if you are interested. †See figure 3.8.

http://www.angelfire.com/electronic/funwithtubes/Amp-Power_Supply-More_Info.html...

The 100 ohm resistors are only needed because the Edcor Power Transformer doesn't have a center tap wire for the brown heater wires. †The 6.3 volt AC current is not rectified...that's why the wire is not needed. †The resistors, however, both reduce hum AND can act as a fuse if you accidently short out the B+ voltage with the heater voltage.

It's a win win!

Hope this helps!

Mike

P.S.  I'm curious where you read that you can't do both?  I'm learning too!
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« Last Edit: 01/26/13 at 16:24:25 by mboxler »  
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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #10 - 02/24/13 at 23:18:27
 
Anyone experiemnt with different diodes other than the IN4007. I was reading somewhere about hexfreds and was wondering about them as an alternative....
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #11 - 02/25/13 at 01:00:36
 
I did see this AFTER buyng the IN4007's...

http://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1203210651

Steve's comment is #5.

Mike
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faithintruth
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #12 - 02/25/13 at 01:47:59
 
To be clear I have the UF4007 in my amp right now and sure the sound is great but I think this may be even better. I just don't know anything about the appropriate configuration of the right hexfred. Mouser describes:
dual common cathode, dual parallel, single. Then do we install them right on the PCB in place of the UF4007's?

Hey Mike, I still owe you some pictures.
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mboxler
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Re: Virtual CT for Edcor Heater Wires - WOW!
Reply #13 - 02/25/13 at 17:54:54
 
We all love pictures!

Sorry about the diode confusion.  I read this from Nelson Pass...

"Rectifiers

Yeah, sure, rectifiers are important, after all, the AC has to get converted to DC, but I donít like the fast recovery types that some audiophiles have raved about. Fast recovery means that they withstand many amps and volts in a tenth of a few nano-seconds, something we donít see very often on the old 60 Hz AC line. They are essential element in switching power supplies, but for regular "linear" power supplies, I much prefer SLOW diodes, and we create them by placing small capacitor circuits across the diodes, which greatly reduces radiated noise."

Thought this was interesting.  Still, changing stuff is what makes the hobby fun!

Mike  

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