[sdiy] AC Grounding

[Bob Weigel]

As an example of AC legs going to ground...  heater filament windings in 
tube amps.  The quietest way to do it is to run regulated DC of course 
but older amps run AC direct from transformer to the tube's heater.  The 
12AX7 and similar tubes have a center tapped heater that can either be 
run on 12.6V or you can tie the outer legs together and run it on 6.3V.  
If you just leave the whole thing hanging anyway without any ground 
reference, the heater wiring all tends to act like an antennae and being 
in close proximity to the control grid of the tube, a lot of whatever is 
in the air can get transferred to your signal path that way.  Some very 
early tube amps in fact I've worked on (eg. Gibson EH-150) had no ground 
reference probably because back then, there wasn't nearly as much EM 
stray all around us creating noise problems!  Cuz it buzzes like crazy 
until you get it ground referenced these days :-)
     Anyway, so the ways that is done....Fender puts a 100 ohm resistor 
historically from each side of the 6.3V and they tie the preamp tube 
heaters as shown above.  Other amps include a 'hum balance' pot that 
allows you to vary the point at which the center of that field is to 
minimize it even further in some cases.  And then some use a center tap 
transformer.  BUT..the quick and dirty way to do it which some also do, 
is to ground one side of it.  It's not the quietest solution but it's 
way better than no ground at all.  You still have an antenna signal 
gradient across the heater from the ungrounded to the grounded.
       The other problem with not using resistors like Fender is, when a 
HV element happens to short agaisnt the heater it crobars the power 
supply to ground.  Whereas with the 100ohm 1/2W resistors they just blow 
up and make a little smoke and take the stress off the rectifier tube 
which often takes the hit in the direct ground case for instance. -Bob

klosmon wrote:

> Speaking of stupid questions, I have one...
>
> Over the last 20 years, I've been dealing mostly with DC circuitry -- 
> what happens after the step-down AC gets rectified & regulated (like,  
> for example, a synth power supply).
>
> However, I was recently helping some friends finalize the installation 
> of an intercom system in their beautiful Telegraph Hill home (near 
> where the wild parrots hang out), and noticed that the instructions 
> for the intercom amp specified that the two secondary legs from the 
> 18v transformer should connect to terminals on said amp, and that one 
> of the terminals should then be connected to earth ground.
>
> I've never seen an AC leg going straight to ground before -- could 
> someone who understands this explain it, so that even someone like me 
> could understand?
>
> Thanks.
>
> ~GMM
>
>
>