[Jerwardemy]

The next time I get inside my Cornet I was wanting to address the hum that it has always had. The hum itself is not that bad, I've lived with it for a while now...but since I'm gonna upgrade the capacitors later, i figured I might as well try a couple things to lower the hum. 

I shorted the inputs and ran the outputs into Audacity and ran the Spectrum Analyzer...the hum shows up at 60hz @ -62db /// 120hz @ -63db  and the rest of the harmonics ( 180hz, 240hz, etc) are much lower...around -80db

I have searched the forum and tried all the tips and tricks except twisting the tranny wires.  I have my Cornet wired up for 120V...my question is, which wires do I twist together? I realize the point is to match wires with an opposite (ex 12v with a -12v) but I can not for the life of me figure out which ones to pair together.

Also, there was a good post from tubesforever where he explained using the grey wire from the transformer and hooking it to the star ground. He then explained that there was 4 wires running to the star ground...1) IEC Ground 2) PCB Ground 3) Transformer and 4) the Chassis

My question about this, is would you really need a wire to the chassis if the star ground is on the chassis itself anyways? Should the transformer be isolated from the top plate of the chassis if you are running the transformer ground wire to the star ground?

[poty]

I shorted the inputs and ran the outputs into Audacity and ran the Spectrum Analyzer...the hum shows up at 60hz @ -62db /// 120hz @ -63db  and the rest of the harmonics ( 180hz, 240hz, etc) are much lower...around -80db

And what "clear" (without anything connected) Audacity shows in the spots?

... which wires do I twist together?

I'd disconnect Blue\Yellow, Brown\Yellow, Black\Red, White/Black from the PCB, pull them above case (i.e. not go inside). The same thing I'd do with Green/Yellow and Violet wires as soon as they also not connect to the anything (the lattest wires you can cut short if you do not plan to use the transformer in different installation).
Then, twist together Blue - Brown pair, Black - White pair, 2 Reds - Red/Yellow (3 wires). 2 Yellows - Yellow/Black (3 wires), a pair of Greens. The most dangerous is the 2 Reds as soon as the max voltage between them is rather high (550*1.414 = 777V, in reality - even more, because you do not use full current capability of the transformer), so maybe it is better to use some addition heat-shrink tubes on them just in case.

Also, there was a good post from tubesforever where he explained using the grey wire from the transformer and hooking it to the star ground. He then explained that there was 4 wires running to the star ground...1) IEC Ground 2) PCB Ground 3) Transformer and 4) the Chassis
My question about this, is would you really need a wire to the chassis if the star ground is on the chassis itself anyways? Should the transformer be isolated from the top plate of the chassis if you are running the transformer ground wire to the star ground?

Good questions and not very easy to answer. If you follow JH layout - you have already had 1, 2, 4 connected to the single point (the Chassis), so you should only add the Grey wire from the transformer to the point. About isolation of the transformer - I don't think it is so nesessary, but it's better to try both ways and chose the better. I'd check also if all case parts have good electrical connection to the star-ground part of the chassis.
If you follow numerous discussions here - you may know, that Cornet 2 is very sensitive to placement. There are also problems with interconnects and tubes reported. Replacing (upgrading) capacitors in most cases aggravates the problems and need carefull wiring and placement of the new parts.

[hagtech]

I never twist the tranny wires.  It is enough to just tie wrap them together in a nice bundle.  That keeps loop area small to minimize self inductance (and the creation of larger magnetic fields) and also electric field cancellation (like poty mentioned with the red wires). 

Hum in a Cornet2 is not from this source.  You need to look elsewhere.  Ususally it is input wiring, magnetic coupling to power transformer, poor chassis connections (which can cause some pieces to be electrically isolated)...

jh

[Jerwardemy]

Ok..so I opened the Cornet up and took a few pics.  I also double checked that all of the case parts were getting a good connection to ground and they were. Here's a pic of the inside...hopefully someone will see a problem somewhere.




I believe the red wire running to the star ground is the grey wire from the transformer.


Spectral Analysis of Cornet hooked to turntable with Pickering XV-15 cartridge




Spectral Analysis of Cornet with nothing connected (open inputs)




Spectral Analysis of Cornet with shorted inputs



As for input wiring, it's all the standard pcb mount inputs...so I don't know what could be going on there. How would I check for magnetic coupling to the transformer? I'm not even sure what that is lol.

Poty, I think I understand what you are saying about the Blue/Yellow, Brown/Yellow, Black/Red, and White/Black wires as they are to the "0" lugs which are not connected to anything, correct?  Is that also the case with the Green/Yellow and Violet wires?  So this means I can unhook all of these wires and pull them up above the pcb, insulate the ends, and run the Cornet like normal?

[poty]

I also double checked that all of the case parts were getting a good connection to ground and they were.

Excellent!

I believe the red wire running to the star ground is the grey wire from the transformer.

Maybe. Often it is no grey wire (which has good connection inside the iron), but just a wire stuffed under a bolt.

Spectral Analysis of Cornet hooked to turntable with Pickering XV-15 cartridge
...
Spectral Analysis of Cornet with nothing connected (open inputs)
...
Spectral Analysis of Cornet with shorted inputs

Have you tested the open Audacity input? Maybe (part of) the 60 and 120 Hz harmonics amplitude is the Audacity noise?

As for input wiring, it's all the standard pcb mount inputs...so I don't know what could be going on there.

I think JH mentioned interstage wiring. It is possible that EMI influences the cartridge, wires to the cartridge, interstage cable...

How would I check for magnetic coupling to the transformer? I'm not even sure what that is lol.

The only way - to place the Cornet 2 differently (orientation, distance to nearest devices...).

Poty, I think I understand what you are saying about the Blue/Yellow, Brown/Yellow, Black/Red, and White/Black wires as they are to the "0" lugs which are not connected to anything, correct?

Yes! But they are not used (connect) only when you plug the Cornet to 120V. Cornet 2 is designed to "support" several voltages, so it is possible that they will be used in the future. That is why it is preferable just to pull them out of the case, but not cut!

Is that also the case with the Green/Yellow and Violet wires?

Yes, but in this case in the Cornet 2 they will never be used! If you don't plan to use the transformer in different projects you can cut them short.

So this means I can unhook all of these wires and pull them up above the pcb, insulate the ends, and run the Cornet like normal?

Yes!

[el34]

Ugh.  Don't leave transformers hanging above the case, that's a safety hazard.  They should have their ends covered in heatshrink (not tape) or connected to spare pads on the board.

Another thing to try is shields on the 12ax7 tubes.   The shield should be connected to case ground.  The first one especially can pick up some hum fields.

Try a spectral analysis of your sound card only, to eliminate that as a possible culprit.  -65 or -70db hum isn't way out of line for a tube phono.