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thebes

What the Hum Is Going On Here?

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7 hours ago, thebes said:

John, right now the center tap for the power trasnformer is grounded to a lug on the main power cap can.  Would running it over to the earth ground of the 3 prong wall cord be advisable?  

 

Leave it alone.

 

7 hours ago, thebes said:

 Also how would I go about testing DC balance.

 

Measure the Anode current of V4 and then on V5, compare.  The amp should be at idle, no signal at the inputs.  A close approximation of the current can be obtained by using a DVMM (FLUKE 117 for example) switched to mA DC with the common lead (BLK) clipped to the 380VDC B+ center-tap on the output transformer primary and the positive lead on PIN 7 of V4 (and then V5).  The readings should be close say 2mA within each other and measured at the same B+ voltage.  If they're off, say by 4mA or more then one tube is working harder than the other and push-pull symmetry lost.  When the two are working in symmetry the 120Hz HV ripple in the B+ line is cancelled out (common mode rejection).  Note that Anode current varies with wall voltage because B+ varies with wall voltage.   

 

There is risk of electrocution when making these measurements.  The measurement raises the DVMM to the B+ potential, in your case about 380VAC.  A good DVMM can perform this measurement with no issue.  Do not use probes, the safe way (the only way I would do it) is using Pomona mini-grabbers or probes with extendable clips.  On the meter side banna plugs that hold fast.  If you short the B+ to ground you'll explode the 380V supply.   Make the two connections with the amp off, set the meter to mA DC and preferable  bring the amp up to 117VAC with a Variac whilst monitoring current draw and B+ voltage.  The amp (or Variac) should have the proper rated fuse or one 1/2 amp lower.     

 

Then repeat for V8, V9.

 

 

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Thanks John. I got it. I've done this before just wasn't sure of what you referring too.  I've got the good clips, not the prongs, for the multimeter. Gonna have to wait a couple of days until I can report back. Work is going to keep me busy and I never play with these kind of voltages when I'm tired, had a drink, or am mad at the world. So my window to play with this stuff is very narrow.😊

 

Today, I did do the cheater plug thing, which was basically a waste of time, since I have already stripped everything out twice, put the old two prong cord back in, run it with and without the switch, the new fuse, etc. in the circuit etc.  No change in hum. 

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dummy plug in it simplest form is an RCA input plug with the center contact shorted to the shield. Since most unbalanced equip has a non zero output impedance a plug with a 1 K ohm between center and shield more closely simulates what the inputs of your integrated "see" when hooked up to a line level input source. So select a line level input(tuner or aux) and put dummy plugs on those input jacks. Is the hum still present?

You can start with the simple form, center contact shorted to shield.

BTW your integrated DOES NOT have a three prong power cord, correct?

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On 3/15/2020 at 12:22 PM, thebes said:

Still trying to finish up my rebuild of a Knight KN928 integrated.  The problem that won't go away is hum. I'm pretty sure its' 120hz hum because I tore apart the incoming wall supply piece by piece, several times with no change.  I've rebuilt the power supply, swamped all the tubes etc., tested every ground in the amp as best I can (no schematic anywhere on this unit).  The hum is very loud at idle with the volume pot at around 70db and goes up from there and is volume control dependent. The power switch is separate and has been tested, and replaced and tested again.

 

The weird thing is if I plug in a line level component (ie cd player, the hum immediately drops by about two-thirds. This is simply the act of attaching the rca cables, not actually turning it on. The phono section appears to be dead quite and switching between inputs does not change the hum.

 

I love this little baby. $58 new in 1965 and it's a challenger to the high-end Scott's and Fisher of that period. Pure lovely, detailed musical EL84 sound.

What to bring it all the way back.

 

So what the hum is going on here?

are you sure you did not cross any wires , or any polarity , in your amp rebuild , a loose wire , connector -----a loose solder joint ----

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12 hours ago, seti said:

 

Thank you for the link. Superb.

Yes Mr. Whitlock and Muncy know what the freak they are talking about. Physics and science not smoke and mirrors.

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4 hours ago, babadono said:

Yes Mr. Whitlock and Muncy know what the freak they are talking about. Physics and science not smoke and mirrors.

 

I can't say I'd worry about everything they write about but their discussion on ground is great.

 

 

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Got in too late to undertake the testing today, but should be able to do it tomorrow.  Again, I've converted this from a two plug to a three plug, but have cheater plugs and could simply snip the earth ground should it prove necessary. I've actually re-worked half the grounds in this thing trying to eliminate the hum. Trying to avoid a complete tear-down. The power switch was actually wrapped in a metal box  so they were trying to isolate hum from the getgo. It had hum before I began my work on it, and it's pretty much the same now.  More to follow, and thanks again for chipping in on this. I will also be building a dummy plug.

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OK so do we know if the hum is generated by the integrated all alone? You said in the first post I think that the phono has no hum. Do you mean with a turntable connected and the speakers of course and phono input selected that its all quiet and good(or good enough)?

Curious you say you converted from 2 prong plug to 3. Where did you connect the third prong?

Also have you done the DC balance test John suggested?

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19 hours ago, seti said:

 

I can't say I'd worry about everything they write about but their discussion on ground is great.

 

 

Yes I don't "worry" about hum and other nuisances I just prefer to use scientific principles to get rid of(or greatly reduce) them:)

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Step 1 completed. Measurements taken with only the loss of one fuse (positive probe touched ground) on my trusty Rat Shack multimeter. Fortunately I keep a few fuses on hand just such inevitable occurrences.

 

Measurements were taken with a cheater plug to a variac set at 117VAC.

 

The power tubes are russian versions of the 6BQ5.  Measurements:

 

V4: -22.89ma

V5: -21.90ma

 

V8:  -14.90ma

V9: -27.1ma

 

Aha!  We're onto something.  So I switched the last two tubes and the measurements followed.  Only had original 6BQ5's on hand so I swapped in a pair of them and I'm now getting -31.2 and -34.0 so pretty close, but higher (lower) voltage than the Russian tubes which have slightly different specs.

 

Sadly I still have hum.  It actually goes down when I attach the probe to Pin7 but comes right back up when I remove it.

 

That was fun! Thanks John.

 

What next?

 

 

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17 hours ago, babadono said:

OK so do we know if the hum is generated by the integrated all alone? You said in the first post I think that the phono has no hum. Do you mean with a turntable connected and the speakers of course and phono input selected that its all quiet and good(or good enough)?

Curious you say you converted from 2 prong plug to 3. Where did you connect the third prong?

Also have you done the DC balance test John suggested?

Yes it will hum with no hookup's. Louder with the phono selected but no phono cable or grounding wire attached.  I always have speakers attached when I fire up anything tubed.  I prefer it to a dummy load because you can hear if something goes wrong in a hurry.

 

Two prong to three prong because Maynard taught me that this should be done to all tube amp rebuilds or builds.

 

The hum cannot be heard while the music is playing but is loud enough to be irritating when there is no music on.

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3 hours ago, thebes said:

Yes it will hum with no hookup's. Louder with the phono selected but no phono cable or grounding wire attached.  I always have speakers attached when I fire up anything tubed.  I prefer it to a dummy load because you can hear if something goes wrong in a hurry.

Louder with phono selected makes sense to me(jump in and correct me  -those more knowledgeable) because phono inputs require more gain than line inputs. Speakers connected ........roger, check.

 

Two prong to three prong because Maynard taught me that this should be done to all tube amp rebuilds or builds.

Why? I'm not understanding this. Unbalanced equipment uses the chassis as signal return, adding an earth connection CAN cause problems.

 What did you connect the earth prong(the 3rd prong) to? Just any convenient point on the chassis? When you put a cheater plug on the 3 prong cord you added does it change anything?

 

The hum cannot be heard while the music is playing but is loud enough to be irritating when there is no music on.

Yea if it bothers you that's what matters, let's try to make it better:)

So if  John has other suggestions I say pursue them of course or Maynard. To go any further with my suggestions you need dummy plugs.

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On 3/23/2020 at 1:03 PM, babadono said:

Link to help:

https://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf

 A fancy dummy plug set up is available from Jensen Transformers for about 10 bucks. Their model TA-R1

Can't find them online, must be discontinued. Going to have to make up a pair this weekend.

 

The earth ground of the 3 point plug is attached to the ground lug of a small terminal strip. Cheater plug does not reduce hum, and has the same effect if I just desoldered the earth wire from the terminal strip.

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10 hours ago, babadono said:

Yes I don't "worry" about hum and other nuisances I just prefer to use scientific principles to get rid of(or greatly reduce) them:)

Not what meant... They go way way way down the rabbit hole 200pages and that is what I believe is the short version. I switched equipment around recently and all of a sudden had crazy hum. It took me hours of troubleshooting but now quiet as can be from every source.

 

 

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2 hours ago, seti said:

They go way way way down the rabbit hole 200pages

Yea but @seti each page is just a slide from a power point presentation not real heavy duty reading material:)

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3 hours ago, thebes said:

Can't find them online, must be discontinued

May have to call them. Page 107 of the document linked above shows the schematic.

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Hey Marty do you have an o-scope? This could help look around and actually see what everything is doing.

 

 

 

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I just tried calling Jensen Transformer. They are shut down following the mandatory order of Governor of CA and Mayor of LA until Monday 20th of April(at minimum).

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