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.