One 300B filament supply for stereo?

[dshortt9]

I would like to power both channels of a 300B amp and my transformer has one 5V ct winding. Is this possible and if so how? Will cathode bias work or will fixed be ok? Can I use 2 seperate bridges from the winding?

[rcavictim]

I see two possibilities here. The simplest is to run both tubes from a common C or CLC filter powered by the 5 volt winding. The more complicated, to use separate bridges and filter networks for each tube on the same common 5 volt winding. If you take either of these approaches your tubes will be AC coupled through the cathodes and stereo separation will be degraded unless the filament supply is at zero potential AC (signalwise) and the tubes are biased individually by a isolated negative grid bias supply for each tube. Connecting the CT of the 5 volt filament supply winding to chassis earth (and negative rail of B+ supply) will bring the cathodes to earth.

A second and somewhat novel approach would involve earthing the 5 volt winding CT and making a voltage doubler circuit for each separate 300B filament that can run on a 2.5 volt winding. This technique will give you more complete isolation of each cathode AC wise (you cannot avoid common coupling within the xfmer from one half of the 6.3 winding to the other but I doubt that will be a problem). You will still have to bias the tubes actively through G1`s.

If this were me, I`d get a stand alone filament xfmer for the second 300B. You will have to have a beefy current rating single 5 volt winding to support two voltage doublers. Most 5 volt windings on power xfmers are only 2 or 3 amps, being supplied only for a single rectifier tube filament.