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valve preamp transformers


 
2/4/2001 12:31 AM
Jaunt
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valve preamp transformers
after building ss stuff for years, i've been preparing to have a go at some valve stuff. the first project i want to build is the Hotbox.  
 
 
 
to get a xformer made for me, it's going to cost about US$60. i've found a cheap (US$7) xformer at Dick Smith's that i'm hoping will do the job, but i'd like someone to check my maths!  
 
 
 
it's rated 6.3-0-6.3 @ 500mA. if i run the filaments in series across the 12.6v, it looks ok to run two 12ax7's, they'd pull 300mA.  
 
if i put another xformer the same back-to-back, i'd end up with 240v (our mains voltage) again but no center tap. can i use the 0v from the filament system?  
 
 
 
then i can draw an extra 200mA @ 12.6V, which means i have about 10mA to use at 240V. from the voltage drops across the plate resistors, i've got each of the four stages idling between 0.5-1mA. it looks safe, but close.  
 
 
 
i'd also be running a B+ of about 170V instead of 250V. will this impact the sound a great deal - in other words, should i just spend the money on the custom xfmr?  
 
 
 
thanks for any help!  
 
 
 
:) darren
 
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