Jump to content
The Klipsch Audio Community
thebes

What the Hum Is Going On Here?

Recommended Posts

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?

Share this post


Link to post
Share on other sites

Sorta.  This one uses four 12ax7a and four 6bQ5's and a EX81, the other is the same only there is an extra 12ax7 on the 728, and the latter has things like a low pass filter built in.  The power supply is similar but with different cap values and the filter supply looks different. I've go the amp hooked up right now, I'll pull it and try to trace some of the circuits and let you know more.

Share this post


Link to post
Share on other sites

Since hum from the heaters is a common problem I've been looking over the heater setup and I've got a couple of questions.  First, the filament leads from the power transformer are split with one side going to the EZ81 heater and the other side going to one of the EL84 power tubes.  Both are soldered to pin 5.  The one on the EZ81 has the wires going to pin four of the heaters on all the rest of the tubes, and the one on the EL84 goes to pin five of the heater pins on all the other tubes.  What confuses me is shouldn't  the filament lead on EZ81 go to PIN 4 and the filament lead on the EL84 go to PIN5?

 

In other words isn't the signal being crossed with this setup.

 

Also I do believe Maynard taught me to put a one or two watt resistor on one side of the filaments as a means to reduce hum. Any suggestions for what size resistor I should use. ie 5k etc.

Share this post


Link to post
Share on other sites

Hi Marty-  the resistors are for controlling filament voltage, not hum.  I’ll try to help you out with this as soon as I can free up some time.  I’ve had to readjust some priorities in view of what’s going on.

 

 

Maynard

Share this post


Link to post
Share on other sites

Well slap me upside of the head, I should have remembered that.  Maybe I was thinking of a resistor across the outputs?  

 

No rush at all Maynard.  I haven't contacted you privately about this because I know you are always helping several someone else's off line at any given time and don't want to chew up any more of our fun time than I already have over the years. 

 

Besides, hum and it's ilk is a common occurrence in tube amps and I'm sure there are others in our learned community that can assist me with this .  Do you hear that my lovelies!  Help a Thebes today, or you deepen an already dire global situation.

 

 

atomicthebesorama.jpg

Share this post


Link to post
Share on other sites
On 3/15/2020 at 12:22 PM, thebes said:

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 center-tap at the 380VAC secondary section is a floating chassis ground.  The RCA phono plugs jacked into the amp from any source with a three-prong plug will tie the "floating" chassis ground of the amp to the Earth ground prong at the wall socket.  If hum goes down in volume, the signal has a low impedance path to ground and sinks to the wall prong.  

Share this post


Link to post
Share on other sites
On 3/15/2020 at 9:22 AM, thebes said:

Still trying to finish up my rebuild of a Knight KN928 integrated. 

 

You had an earlier thread where you mentioned the amplifier had low volume output. Did you solve that issue?

Did to the amplifier have hum problems before the rebuild?

 

 

Share this post


Link to post
Share on other sites

Thanks for weighing in guys. I'm guessing its  a grounding problem.  John, I appreciate your input which offers an explanation for why the volume of the hum is decreasing, but I'm not sure leads to me with my limited technical knowledge to where I should be looking for the grounding problem.

 

Mike, the volume is much better since I redid the power supply, still a bit high (75 db at 11 o'clock) but acceptable. I actually swapped out the volume pot, and did several other things with no discernible change in volume.  the power supply rebuild seemed to have the greatest impact on normalizing the volume.  For most of the testing before rebuild, w I had speakers and a cd player hooked up so it had for me to say with certainty that the hum got louder after the rebuild when, of course, for some strange reason, I decided to do the testing with no source hooked up. I did add a chassis ground lg for the ground cord of the phono section, and lifting that wire does introduce a lot of noise (not hum) into the sound.

 

There is a little something else going on with this amp.  It "snaps" when shutting it off.  I put a .02 cap across the power switch, which I also changed out with two other ones, and that seems to have eliminated it, but still leaves me wondering what is actually causing that.

 

I suspect that this amp needs some changing in groudn. I combined some of the grounding points at the power supply, ie a large resistor which formerly was grounded to a "leg" of the original power supply cap to being grounded at a newly installed terminal strip which also carries the grounding wires from a couple of other places.  I 'm going to separate all that stuff this weekend, and try again.  I have not tried  building a star ground for this amp, casue I'm not sure I can find all the grounds without a schematic, and don't want to muddy things any further without some guidance.

 

Hey Mike, recognize that picture?  I do believe that is your work.

 

 

Share this post


Link to post
Share on other sites

You can learn a lot about how your system is grounding by taking a long piece of wire and touching it between two pieces of equipment or ground leads. Use it as a sort of probe. Connect one end of the wire to a chassis and then probe ground points of other components or cables (GROUND LEADS OR CHASSIS ONLY). Use your amp as the initial grounding point.

Share this post


Link to post
Share on other sites

Does rebuilding the power supply include replacing the 5 caps and testing the 5 resistors, replacing anything out of tolerance?

Share this post


Link to post
Share on other sites

What’s the efficacy of a ground loop isolator in this type of  situation

Share this post


Link to post
Share on other sites
On 3/19/2020 at 3:05 PM, John Warren said:

 

The center-tap at the 380VAC secondary section is a floating chassis ground.  The RCA phono plugs jacked into the amp from any source with a three-prong plug will tie the "floating" chassis ground of the amp to the Earth ground prong at the wall socket.  If hum goes down in volume, the signal has a low impedance path to ground and sinks to the wall prong.  

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?  Also how would I go about testing DC balance.

Share this post


Link to post
Share on other sites
17 hours ago, Davecv41 said:

Does rebuilding the power supply include replacing the 5 caps and testing the 5 resistors, replacing anything out of tolerance?

Yes. It's actually six caps and 5 resistors, all of which were changed out.  The power supply values are different then those shown in the schematic Mike posted but the layout appears to be the same. 

 

Main cap values are 20/20/20/10 and the other can's values are 10/10.

Share this post


Link to post
Share on other sites
On 3/19/2020 at 5:11 PM, babadono said:

Got "dummy plugs"?

beep........ beep.......

Share this post


Link to post
Share on other sites

Whoops sorry.  Do you mean a cheater plug for the wall cord?  If not, probably not. Got a link on how to make some? Or is it just connecting the positive-to-positive, negative-to-negative wires from a set of rca plugs to each other.

Share this post


Link to post
Share on other sites
4 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?

 

4 minutes ago, billybob said:

Regular ac connector minus the ground.

 

Something he could try...

 

First, leave the power supply xformer CT grounded at the first capacitor ground lug. It's supposed to be that way.

 

Did the amplifier have a three pronged power cord, or a two pronged plug to begin with?

You mentioned 120Hz hum, which could be power supply related...I dunno...

 

Try lifting the ground of the three prong power cord, and see if the noise goes away, (make sure the polarity is right) or power the sources to the integrated amp from a different wall socket.

 

  • Thanks 1

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...