Phantom grid noise?

[ioginy]

Good day all.
I threw together a power amp for a AudioBaton style preamp I build a few months ago but I have run into a very strange problem. The amp is beautifully quite from the PI through to the speaker, but I found I wanted a bit more gain. I wired in half of a 12ax7 with a master volume and everything was going fine until I hooked up the input. I was getting quite a bit of hum. I checked everything and started to shotgun the hum. The strange thing is that there is absolutely no hum coming from the 12ax7 until I touch something to the grid.
With everything connect I started removing components from the input to the grid.

Wire to 1M grid leak and 68K stopper. Removed with hum.
Grid leak removed. Hum.
Disconnected Grid stopper so it was flying free but still connected to the grid. Hum.
Removed the Grid stopper from the grid and the hum disappeared. I tried grounding the grid and the hum jumped to a new level. In fact touching anything to the grid, whether its grounded or free flying causes hum.
I rather confused. If anyone has any ideas I would love to get this hum taken care of.

Thanks a heap.
Cory.

[kheper]

Ground loop problem from the input source?

To test, try another source input, like the output from a computer soundcard, Ipad, tuner, etc.

Ground problem in the pre-amp itself?

Check to see that there are no dodgy solder-joints to ground. Best to tie all ground leads to one point in the pre-amp. Check the lugs (hot and ground) on input jack with a multi-meter for resistance and continuity.

AC hum?

Check to see that the filament wires are not looping over the grid lug. You could wire the filaments on the pre-amp tube for DC.

[Gingertube]

With a 12AX7 (and other hi mu tubes) it is very important to have balanced supply to the heaters and a low impedance path from the heaters to ground to stop noise coupling back to the grid.

Make sure that either:
1) If the heater winding is centre tapped then ground the centre tap.
OR
2) If no centre tap then create a psuedo centre tap using 100 Ohm from each side of the heater to ground.

Sometimes further sonic improvement can be had by taking that heater centre tap ground connection to a DC potential of say +30V instead of ground. This can be divided from the HV supply with a bypass cap across the bottom resistor of the voltage divider.

I had really bad hum from a 6SL7 front end until I added the resistors from heaters to ground.

Cheers,
Ian