5 volt PT tap tied to 6.3 volt tap??

[bnwitt]

I've got a heavily modified 1976 Bassman Ten in my shop for a tune up.  It has an added 12AX7 effects loop, a presence control, Showman OT, gain switch in place of the deep switch, gain volume, EL34 tubes and  on and on.  It looks like Dumble was in this thing.  Silcone goup everywhere.  One thing that baffles me is the PT L0022798.  It has a non center tapped filament winding and a center tapped 5 volt winding.  The two windings are tied together on the filament circuit.  See the attached drawing.  Anyone have any ideas why?

[RicharD]

>It has a non center tapped filament winding and a center tapped 5 volt winding.  The two windings are tied together on the filament circuit.

Looks like the filaments are still tied to the non CT 6.3V so they are still seeing 6.3V.  The 5V is in series (a guess) creating 11.3V with a ground reference.  Check the lamp, I bet it's a 12V.  This is giving the filaments an elevated reference to ground.  That's fine until the lamp burns out and then there is no ground reference for the filaments and 60Hz hum kicks it up a notch.  Probably not the best idea in the world.

That's my guess.

-Richard

{edit: fixed broken font tag --PRR}

[bnwitt]

Now how weird is that?  You're right Buttery, the lamp is a 12 volter.  Now why the heck would anyone do something like that?  A couple of 100 ohm resistors for the artificial CT on the filament winding makes more sense.  That's got to go.

[Merlin]

Now how weird is that?   Now why the heck would anyone do something like that?  

It's just a way to power a 12V lamp, nothing weirder than that. The lamp does not affect the ground reference for the heaters. If you have no hum problems at the moment, then there's nothing wrong with this circuit arrangement.