HK Citation V Grounding Help Needed

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HK Citation V Grounding Help Needed

Postby dannyr » Mon Jun 16, 2014 1:25 pm

Hi, I have a Harmon Kardon Citation V that I rebuilt with McShane kits that I need some ideas/help on how to properly do the ground scheme to get things quiet. So far my efforts have resulted in different levels of hum. I've read and read theories on how to wire it but I get confused implementing the ideas as described and feel that i'm going in circles at this point. How should the RCA inputs be grounded with isolated jacks? What parts of the power supply should be grounded to each other and then to maybe a star or bus? Should the power supply join a star ground connecting to the rest of the amp and what parts of that should be joined before they go to the star?

Anyway, the parts of the amplifier that need to be grounded are the power, the two inputs, two driver boards, the power tubes which stock are separated into grounded pairs, bias supply, two 33ohm resistors going to the heaters, and a added circuit for DC heaters for the 12BY7's. I have the power chord grounded to a bolt of the power transformer. Any guidance, help, thoughts, encouragement are greatly appreciated.

Below is a Citation V's schematic. It looks like that it's based on a Mullard 5-20 type circuit.

Image
images

Dan
Last edited by dannyr on Tue Jun 17, 2014 5:07 pm, edited 1 time in total.
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Re: HK Citation V Grounding Help Needed

Postby Geek » Tue Jun 17, 2014 12:45 am

Single-point star ground is what I use no problem. Ground to the lowest impedance point (usually the transformer CT where it meets all the caps)

What grounding issues do you have?

Ian McShane is a good fellow. I'm sure he'd help if we can't.

Cheers!
-= Gregg =-
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Jun 17, 2014 12:47 pm

Hi Geek. Thank you for the reply/help. I greatly appreciate it. The issue that I'm having is a good amount of 60hz hum. It has disappeared when I have shorted the inputs. Also same when I pulled the small single tubes. My last attempt at grounding though resulted in the same amount of noise even shorted so I went down a bad road.

I believe that the amp's power transformer has no center tap. Or at least one that I can find. Jim recommends doing a bus bar on the amp's internal capacitor bracket and then grounding everything to it. I tried that route and had no luck. I'm also in contact with another fellow that is having the same issues as I am so I'm thinking that this amp can be sensitive to how it's grounded. We can't get our amps to not hum. Tried cheater plugs etc.

Okay. single star. What parts generally should be grounded to each other before they go to the star? Also how should I properly ground the input's? They are separated by over a foot on the rear of the amp not joined like a Dynaco. There is also two driver boards instead of a single. I'm concerned with them resulting in a ground loop if they go to the drivers.

Thanks again
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Re: HK Citation V Grounding Help Needed

Postby Geek » Tue Jun 17, 2014 1:18 pm

Hi,

Stops when the input is shorted? That's not a ground loop, that's an oscillation. Use a bigger grid snubber on that 12BY7 and careful of wire layout around that one ... high Gm tubes LOVE to make RF ;)
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Jun 17, 2014 1:28 pm

Interesting. I still have to get the grounding back to okay. It's all messed up now from my trails.

Okay. So on the schematic in the first post is the grid resistor 10k on the 12by7? What would you recommend it by changed to?

Update, I moved the 10k resistor from the driver board to the 12by7 tube socket and it helped. It had a few inches of wire between it and the socket orignally. Would going up in resistance help out?

Thanks again Geek.
Last edited by dannyr on Tue Jun 17, 2014 11:42 pm, edited 1 time in total.
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Jun 17, 2014 1:32 pm

Actually the 10k is a grid stopper. hmm. Is the snubber the 1k/430pf network? Also C5/C11 is paralleled with a .68uf cap resulting in about 1.1uf. Could this result in oscillation? Thanks

Dan
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Jun 17, 2014 5:09 pm

The schematic has been reuploaded in the first post. one side of C5/C11 was wrong.
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Re: HK Citation V Grounding Help Needed

Postby EWBrown » Tue Jun 17, 2014 7:13 pm

The power supply is a voltage doubler, no center tap, this design was frequently used in the "newer" tube amp designs of the 1960s - 1970s.
Mostly because it was less expensive to manufacture. Less HV secondary turns required, though the current requirements are higher,
3.6X the rectified DC current consumption. If your total B+ current adds up to 250 mA, then the 200VAC winding must deliver at least 900 mA @=
Copper was a lot cheaper per pound, 40-50 years ago $)

/ed B
Last edited by EWBrown on Thu Jun 19, 2014 3:47 pm, edited 1 time in total.
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Jun 17, 2014 7:44 pm

Okay. That makes sense. Explains why a lot of Fishers also don't have a center tap.

Is this on the right track for grounding?

Output tube grounds, output transformer grounds and the .01 capacitor C17 that is connected to the power diodes all connected at the voltage doubler cap ground lug then to either it's own star point or a full star ground?

On the rest the only part I may have figured out is the output jacks grounds going from each rca jack to it's corresponding driver board. Then taking one driver board's ground and running a wire to the other driver board's ground and that to the star to avoid a ground loop from happening at the source's output.

Please let me know I'm on the right track or went wrong somewhere.

Thanks
Dan
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Sat Jul 26, 2014 1:44 pm

Hi, I'm still at trying to fix this amp. I tried a bus bar grounding scheme and it didn't help. I found a picture that shows much of the original grounding of the unit and I see that the input grounds join at the center rear and connect to the chassis but its coax shield also goes to a ground of each drive board next to the input which then eventually goes to a filter capacitor's ground.

Image

How does this not result in a ground loop? I am thinking about just trying to mimic the original grounding scheme as close as possible if all else fails. I'm sure that the designers knew far more than I ever will but I'm trying to understand the reasoning.

I did get the below advice at Audiokarma from Palustris that I have yet to try though

"There are several ways to ground an amplifier properly and have a hum free unit. It this were my amp, I would connect the power supply cap grounds together and terminate them at the "transformer bolt power/cord ground." I would bus all the signal grounds together i.e., grid resistors, the cathode resistors, and RCA jacks and terminate that bus at the "transformer bolt power/cord ground." You might try connecting the output tube cathodes grounds to either the PS ground or the signal ground to see which is quieter. This would be more of a star grounding technique than a single bus with a "quiet" and "noisy" end."

Any feedback, help encouragement are greatly appreciated. Thanks
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Re: HK Citation V Grounding Help Needed

Postby johnmil » Sun Apr 26, 2015 4:03 am

Good morning,
I'm curious if there is any new information on your amp? Were you able to reduce
the hum????? What was the result? - no updated information in a year.

Regards to all,
John
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Sun Dec 27, 2015 3:26 pm

Hi John, I had no progress. It got shelfed the due to having to move. I'm getting the itch to drag the amp out of my closet and attempt to sort it out. If anyone has any ideas on things to try please let me know. I'd greatly appreciate it.
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Re: HK Citation V Grounding Help Needed

Postby DeathRex » Sun Dec 27, 2015 5:04 pm

Have an oscope?
Where do you live?
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Sun Dec 27, 2015 7:42 pm

Hi DeathRex, unfortunately I don't have a oscilloscope but I'd would be happy to purchase a used one maybe on ebay or something if someone can advise me on what to look for and walk me through on how to use it. I'd like to have and learn how to use one anyway.

I'm in the San Francisco bay area.
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Re: HK Citation V Grounding Help Needed

Postby dannyr » Tue Dec 29, 2015 1:27 am

Deathrex, would the below cheapo scope work okay with a x100 and x10 probe?

http://www.ebay.com/itm/121663892686
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