OT: Tube Audio - test-drb@ccmp.vtda.org

20 Oct 2005

...

Subject: OT: Tube Audio
   From: lee davison <leeedavison at yahoo.co.uk>
   Date: Thu, 20 Oct 2005 17:02:33 +0100 (BST)
     To: cctalk <cctalk at classiccmp.org>

  I didn't say nonexistant, only rare.  Dynaco
was amoung the best
 then. The alternate was a filiment that was balanced to ground to
 minimize the amplitude of AC heater to cathode coupling, also only
 seen on quality gear. 
Another, rarer, alternate used with directly heated cathodes is to
have two cathodes heated by a Scott wound transformer that gives
two phase AC current with a 90 degree phase difference. This
reduced heater generated noise because the noise is a function of
the square of the heater voltage and sin^2 + cos^2 = 1, effectively
a DC component.

I have only ever seen this arrangement used on broadcast equipment.

Lee. 
The other technique is far simpler.  Balance, do the same thing only
in mirror image.  The reduction in hum and some types of noise is 
substantial and tube engineers understood it well.  The common
examples were 3A5s, 810s or maybe 813s in push pull pairs (both 
have directly heated cathodes). 

Allison