Hum in Grounded Grid Pre

[g(f(e))]

I have some hum in my first DIY project and have a few questions.
I have been trying for over a week to eliminate it doing all of the obvious things. Making sure it wasn't the cords or outlets, double checking soldering, re-routing some wiring, etc.

I have a picture in the photo gallery of the wiring. It is post #1896 page 76.

The hum is constant and does not vary until the very last turn of the pot, where it then gets a little louder.

It makes the hum with the source on or off.

My first question is:

How do I check what frequency the hum is at? I have a scope but am not sure the best way to find the hum.

Should my pot be isolated from the chassis and then grounded at the star? Right now it is grounded directly to the chassis.

Also, I have done two things which I think could be causing the problem. My 12V DC heater ground is at the circuit isolated buss and so is the last B+ filter cap.

Any help would be appreciated.

Gary

[denny9167]

Originally posted by g(f(e))
I have some hum in my first DIY project and have a few questions.
I have been trying for over a week to eliminate it doing all of the obvious things. Making sure it wasn't the cords or outlets, double checking soldering, re-routing some wiring, etc.

I have a picture in the photo gallery of the wiring. It is post #1896 page 76.

The hum is constant and does not vary until the very last turn of the pot, where it then gets a little louder.

It makes the hum with the source on or off.

My first question is:

How do I check what frequency the hum is at? I have a scope but am not sure the best way to find the hum.

Should my pot be isolated from the chassis and then grounded at the star? Right now it is grounded directly to the chassis.

Also, I have done two things which I think could be causing the problem. My 12V DC heater ground is at the circuit isolated buss and so is the last B+ filter cap.

Any help would be appreciated.

Gary

It does sound like a grounding issue,is the first filter cap and center tap of the PT,grounded directly to the star point? and is the power supply ground isolated from the signal grounds,meaning does the PS ground go directly to the star point? And you may also ground the pot directly to the star point,the pot doesn't need to be isolated from the chassis,just don't connect the casing to the signal ground. Hope this helps.

[cerrem]

Here are some things to keep in mind about tube audio grounding and hum...
You have 3 current loops to deal with.... The DC current feeding the stage .... The AC ripple current..... The AC audio signal.....
The thing all 3 have in common is they all share the same return path, chassis GROUND.... this is really bad for ground loops and cross currents...
This poor design technique came about by trying to save $$$ and save on more copper line returns in the old days....it seems that many Hi-End builders use this technique as well...
I recommend seperating your AC and DC current loops in seperate return lines...
Keep all you filter cap returns off chassis DC return and on a seperate return line for each stage....also connect all your cathodes to this line as well for the AC audio loops to properly decouple..
STAR ground all this to a single point back at the transformer rectification ground...
The idea is to AC audio decouple each stage to it's own cathode via it's own return wire to filter cap...
Each cathode is also connected to a seperate DC return as well....
Each filter cap also has a seperate AC ripple return as well...
This will keep things quiet.....
This is common wireing in MIL grade electronics...

Chris

[g(f(e))]

Your replies will give me some things to try because I do have a few of those issues.

I have my heater ground ( 12 V DC ) sharing an isolated circuit ground with my last B+ filter also there. If I read your post correctly I need to get both of them off of there.

I will try that first.

The body of the pot is grounded to chassis but not at the star ground it is grounded where it is mounted by the little tab or the nut that bolts it to the chassis.


The wiper circuit of the pot is grounded to the the same isolated ground mentioned above.

Thanks

Gary

[g(f(e))]

I found the source of my hum so I felt obligated to report my results.

I had used a copper wire buss bar to connect a five lug solder tag that I used for my B+ power supply circuit. Two out of the five lugs did not have solder from the connector wire to the buss wire. I was reading 4 ohms plus with the meter. I had checked all of these connections before with the meter set on continuity ( which on my meter is 40 ohms ) . I re-soldered and the hum was gone.

Thanks