Grounding with wall-wart. Hmm....

[brett]

Hi
I'm building a tube-based overdrive, using a twin triode (kinda like a 12AX7).
The power is coming from a 12V wallwart.  It powers the tube heater (150mA) and an op-amp (the tube section doesn't generate much gain, so I need the op-amp). 

Trouble is, at max gain (only 11), there's a fair bit of hum.
I've smoothed (1000uF) and regulated (7812) the power supply.  The +12V line has about 5mV of triangular ripple.  Normally, I would assume that's ok.

But the power supply has no "true" earth.  If there's no true earth, is the 5mV ripple philosophically and practically the same as having a flat 12V, with 5mV ripple on the ground?

I know that when I hear the answer, it willl seem simple.
thanks for any help

[brett]

Maybe I should use the mid-point heater wire (pin 9) as an earth, like in the Real McTube?  I suppose then I'd have a +/- 6V power supply (which would still be ok for the TL071 which can swing +/- 5V with that).

[Paul Perry (Frostwave)]

Brett, when you say "triangular ripple", is that a continuous triangle, or is it flat, then a triangle notch, then flat, then a notch again & so on? If so, that's what you see when you run out of regulator headroom.
BUt the other problem might be, that the heater draws a fair ammount of current, so you can get a small (say a few mV) hum voltage developed along the current path going to and from the heater. So it is pretty crucial where you decide your "ground" is going to be.

[brett]

Thanks Paul
My triangular wave is a continuous sawtooth.  I have *just* enough incoming voltage to regulate down (ie about 2V over my regulated voltage).
You are right that I've used the same PCB tracks for the heater earth as the op-amp earth.  Although I made the tracks really wide (2mm) there's still likely to be some earthing/bias current issues for the op-amp.
Thanks again for the help