HV Secondary Faux CT?

[Cliff Schecht]

One of my amp designs uses a 230-240V CT secondary. After FW rectification I end up with about 190 V for the plates of the power stage. I'm trying to recreate my prototype but I just realized that the custom transformer that Mag Comp sent me, as well as any other unit in my collection, all don't have a center tap. After FW rectifier (four diode style only) I'm at a higher voltage and lower current than I want to be at. Is there any way to create a virtual center tap for the B+ windings?

[Phil_S]

Like this:
http://www.freewebs.com/valvewizard/Bridge3.jpg

[VacuumVoodoo]

That's not a virtual CT, that's a a hybrid silicon-tube bridge rectifier.
To answer original question: you can't make a virtual CT for the HT winding.
That hybrid bridge will drop some voltage depending on the tube type.

[burkeee]

You can do it the same as Phil's pic just replacing the tube with 2 diodes, and no need for the limiting resistors. Lots of old radios that used selenium bridge recs did it that way too, and I've built a couple amps with recycled radio PTs, diode bridge and virtual CT..

[Cliff Schecht]

Bridge rectifier multiply by 1.414, current goes down too. Two diode fw rectifiers multiply by .707. In my case I need 190-200v so I use a 240v CT transformer or a 135v non-center tap. The sample unit Mag Comp sent me doesn't have a HV or filaments CT but does have a dual primary. I'll have to revise that PT design before I can use it.

[burkeee]

Sorry, I see what your saying now. A bridge is unsuitable because it raises the voltage and drops the current, so you need to stick to a FW.

A virtual center tap like the 100R pair on a heater supply isn't going to work because it'll draw a whole lot of current at HV, unless you use values higher than would be allowable for max heater-cathode resistance.

[morcey2]

One way to do it, kind-of, is a choke-input bridge rectifier. It'll get you a factor of about 0.9. With a 230VAC, you'll have about 210VDC, and about 50% more current than with a capacitor-input bridge.

http://www.hammondmfg.com/pdf/5c007.pdf

The downside is that you need to buy a choke that can handle lots o' current. And it will change the dynamics of the amp with a much stiffer power supply.

Matt