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Grounding filaments with 100 ohm resistors


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2/24/1999 8:15 AM
Steve Jones
Grounding filaments with 100 ohm resistors
I'm trying to solve the hum/hiss problem in my DIY amp and read in some of the archive posts about grounding each side of the 6.3V filament winding using 100 ohm resistors and have seen this on some schematics. I had planned to do this. But lately some other posts I've read seemed to indicate you only do this if the filament winding doesn't have a CT. Mine does.  
 
So would doing this to my amp be a bad idea?  
 
Thanks for any advice,  
 
Steve
 
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2/24/1999 8:34 AM
Bruce

Right, you don't have to do it, if the filament winding has a center tap.  
But, that implies your filament windings are perfectly balanced.  
For the extra 50 cents it cost to install a pair of 1% 100 ohm resistors, you can try it out and see if it makes any difference.  
My personal experience has been it does not,  
but, maybe I've been lucky.  
For an additional couple dollars, you can make a little voltage divider and filter cap from the B+ and reference that center tap wire to a positive voltage between two resistors (as referenced to ground from the B+) instead of zero volts.  
That does make a little difference in the filament supply induced hum.  
If you have a self biased power section , you can move the filament center tap up to the positive voltage of the cathode bias  
resistor(s) for a similar effect.  
Most of the time, noise and hums are grid, high impedence circuits, or input wire placement errors, not problems like grounded 100ohms vs grounded CT filament.  
 
Bruce
 
2/24/1999 9:08 AM
Steve Jones

Thank you Bruce!  
 
> If you have a self biased power section,  
> you can move the filament center tap up to  
> the positive voltage of the cathode bias  
> resistor(s) for a similar effect.  
 
Actually that's the way it is wired. So I guess I'm OK on that score and don't need the 100 ohm resistors.  
 
> Most of the time, noise and hums are grid,  
> high impedence circuits,or input wire  
> placement errors, not problems like grounded  
> 100ohms vs grounded CT filament.  
 
I was just trying to cover all the bases. Right now I've insulated the input jack from the chassis, installed a three conductor AC line (was two), and am now re-wiring to have a star ground. I will also re-do some of the filament wiring, make sure the signal wiring isn't near the filament wiring, and probably also put in shielded wire where it makes sense. If I still have a hum problem after that, then the fun will REALLY begin!  
 
Thanks,  
 
Steve
 
2/25/1999 8:20 PM
Steve A.


Steve:  
 
    If you do try the 100 ohm resistors, you would tape off the actual center tap lead; the junction between those resistors becomes the virtual center tap- to be connected either to ground or to a source of dc voltage as Bruce explained in his post.  
 
    If you have a bunch of 100 ohm resistors and can find two that are identical, I believe that you might get a slight reduction in hum (since the CT is probably not exact). For the cost of two resistors I'd say it would be worth a try.  
 
Steve Ahola  
 
P.S. It's been mentioned here many times by Bruce and others to keep the negative return from the big filter caps away from the grounds on the initial preamp stage(s). You'd typically run the big filter cap grounds to the PT mounting nut where you also run the CT from the PT secondary and the ac power cord ground. And then run a lead from that point to the main star ground point. That keeps most of the garbage filtered out in the power supply away from the high gain initial preamp stages.  
 
    Another tip from Bruce that I have found to be very helpful is to run all of the grounds from each preamp stage together to one point on the board, and then run a lead from that point to the star ground point. You may notice that these suggestions do not follow the strict definition of star grounding- but they do work! One last tip- when audio leads must cross power leads, try to keep them at right angles to minimimize any parasitic effect.
 
2/26/1999 7:56 AM
Steve Jones
Conflicting views on grounding?
> P.S. It's been mentioned here many times by  
> Bruce and others to keep the negative  
> return from the big filter caps away from  
> the grounds on the initial preamp stage(s).  
 
There seems to be conflicting viewpoints on this, so I'm in a quandry about how to wire my grounds. Other posts on this topic I found in the archive said (I thought--perhaps I misunderstood) to ground everything at the filter cap ground. This is what I'm doing. I've got solder lugs bolted to each of the metal filter can mount screws (the can is the common negative for all filter caps and is a twistlock type grounded to the chassis by the metal mounting washer). One lug goes to the PT CT, the other lug on the other bolt (actually several lugs on the same bolt) is where I'm grounding everything else. I'm tying grounds from the preamp and other stages together, and then running ground leads to this point.  
 
The only other grounding point is where the ground from the AC line is bolted to the chassis right next to where the AC line comes in. I don't have the AC line ground or anything else attached to the PT mounting nut. The AC line ground is not attached to the star ground.  
 
If this is not the best way to do it, please folks, straighten me out before I do too much more re-wiring!  
 
Thanks for all help and suggestions!  
 
Steve
 
2/26/1999 8:59 AM
Dave H

You should be OK. I think Steve A. and Bruce are referring to the connection between the PT CT and first filter cap -ve terminal. You donít want to make any other ground connections between the PT CT and first cap -ve as that wire has the cap charging current pulses flowing through it. First connect a wire from PT CT to first cap -ve then you can either make first cap -ve your star ground (as you have done) or you can take a wire from first cap -ve to the star ground point (as steve A. has done). For the second option the first cap -ve will have to be insulated from ground. For both options all jack sockets will also have to be isolated from chassis.  
 
Steve A. It sounds like you have two ground connections to chassis. One at the PT mounting nut and one at the star ground point. Is that correct ?  
 
Dave
 
2/26/1999 9:56 AM
Steve Jones

For both options all jack sockets will also have to be isolated from chassis.
 
 
Do you isolate all jacks, or just the input jacks? I.e. do the speaker output or line output jacks need to be isolated as well?  
 
Thanks  
 
Steve
 

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