[sdiy] AC Grounding

[John L Marshall]

It seems to me that you would want to make the heater positive relative to 
the cathode.  Current flow from the heater to the cathode would be limited 
thus reducing noise from the heater.

Take care,
John

----- Original Message ----- 
From: "Bob Weigel" <sounddoctorin at imt.net>
To: "klosmon" <klosmon at sbcglobal.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, May 21, 2005 3:01 PM
Subject: Re: [sdiy] AC Grounding


> 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
>>
>>
>>