[HomeBrew] Full wave rectifying a 5v winding

[N2EY at aol.com]

In a message dated 1/13/06 11:43:32 PM Eastern Standard Time, 
n6kyr at netzero.net writes:


> I don't have a 6.3 VAC winding on a nice 310-0-310 VAC 150 mA transformer. 
> But I do have a 5 VAC 2A winding. I would like to "boost" this up to a level 
> acceptable to the filaments on a 6AG7 and 6L6. Will full wave rectification 
> do this? I think by doing so will boost the voltage up by 1.414 (or about 140% 
> or just about 7 V). 

You can try it, but if it were me I'd just get a filament transformer for the 
6AG7 and 6L6. That way, you could turn on the heaters of the rig without the 
B+.

The rectifier scheme may be possible, but you will need a heavy-duty bridge 
rectifier and some seriously huge filter capacitors. The 5V 2A winding will be 
working very hard. The two tubes need 6.3 V at 1.55 A - that's 9.765 watts. 
The transformer has to deliver that plus the rectifier drops, and the 
capacitor-filter load has a poor power factor. 

Unless you use Schottky rectifiers the resulting DC will be too low. Regular 
silicon rectifiers lose about 0.7 volts each, so that 5 x 1.414 = 7 will be 
reduced by 1.4 volts, to 5.6. 

A 6.3 V 3 A filament transformer will take up less space, probably cost less, 
allows separate switching and be a lot better in the long run. Plus you can 
use a tube rectifier!

73 de Jim, N2EY