[sdiy] AC Grounding

[Bob Weigel]

In typical guitar amp pre's the tubes don't have any electrical 
connection to the heater.  Typically the cathode is at 1-2V relative to 
the control grid.  And the control grid might be at ground, or..it might 
be at 100V or anything between if it is being used as a phase inverter 
for instance.  I know of no cases where the heater is actually placed at 
a DC voltage offset from the cathode as a whole for that purpose.  It 
might be a good idea though.  However, in the usual high gain positions, 
the voltage is only 1 volt or so like I say and you don't get much 
current near as the ear can tell.  Most high gain amps of that design 
when working properly are pretty quiet in terms of that type of hum 
contribution.  The guitar being an antenna is usually the major source 
of noise. -Bob

John L Marshall wrote:

> 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
>
>