signal ground and earth ground

[ttray321]

Dear brothers:

I am building a 300b tube power amp. I want to know that I should connect the signal ground directly to the earth ground or not? OR, use a R-100ohm // C-0.22uf to the earth. Which is better? Why?

Thanks a lot for advance!

ttray321

[anatech]

Hi ttray321,
Try just a 100R between signal ground and earth. This will not be subject as much from RFI and ground loops. Your cap will give a low RF impedance to earth, witch you may not want.

-Chris

[frank2395]

please explain why a resistor and not direct connect

[sanford1]

Nelson Pass has a unique idea of using a diode bridge on the safety ground. The diode drops will break up any ground loops and it will conduct if a fault exceeds a few volts or so.

Elegent.

[anatech]

Hi frank2395,
We use a resistance to reduce circulating ground currents. A direct connection would tend to encourage this.
The currents we are concerned about can flow through the sheilds of the connecting audio cords and back through the chassis and ground pin on the power cord back to the offending equipment. Could even be as simple as a different phase of power off the breaker (fuse) panel.
These currents can become very high (destructive). I'd rather burn out a resistor rather than PCB traces. This may possibly destroy components. Close lightning strikes are my favorite example of this. There is no real protection against this.

-Chris

[frank2395]

what if I do not have a conection between the two grounds re affect on sound and opperation, but not re safety (of this I am aware of).

what wattage and resistance value sugested

re valve amp

[ttray321]

Thanks for Chris and Sanford's reply,

Chris:

Would you tell me why so many circuit put the 0.22uF on it. Does 0.01uF value give less noise? What is the value of this cap more adequate?

Sanford:

If I use this circuit, I may try only one diode.

Again, Many Many Thanks!

Tray

[EC8010]

Originally posted by ttray321
If I use this circuit, I may try only one diode.

No, for safety you must use two diodes back to back in parallel, and high current ones at that. A cheap convenient source of high current diodes is a 35A bridge rectifier...

Incidentally, I believe the diagram in the earlier post has an error in it; the lower AC terminal of the bridge rectifier should connect to the same point as the upper AC terminal (ie, the negative side of the upper DC supply).

[anatech]

Hi ttray321,
The capacitor is for RF frequencies. Depending on what is earthed and what is at circuit ground. The value will be different depending on who designed and built the object in question. Try different values as these are ballpark anyhow.
I normally earth the chassis for safety and RF shielding, hence the lack of C for me. Then there is no safety issue when the resistor burns out.
-Chris

[sanford1]

Originally posted by EC8010


No, for safety you must use two diodes back to back in parallel, and high current ones at that. A cheap convenient source of high current diodes is a 35A bridge rectifier...

Incidentally, I believe the diagram in the earlier post has an error in it; the lower AC terminal of the bridge rectifier should connect to the same point as the upper AC terminal (ie, the negative side of the upper DC supply).

I truncated that diagragm, but maybe not enough. The only part that's really relavent is the bridge rectifier connected to the AC/Mains ground.
The bridge will not pass signal ground loop current ( no loop hum), but will conduct over a few volts and serve as a safety ground.

A 35A bridge rectifier is just fine.