Voltage doubler for filament supply?

[N8CBX]

I need to get 6.3V @8A (SB-200 transformer) to 12.6 @ 3.6A (Russian GU-74B). Is it feasible to build a voltage doubler, much like the HV doubler to get the higher voltage? If not, how could I do it without adding a second transformer.
Jan N8CBX

[WX7G]

Yes it can be done with an AC voltage doubler. That is a transformer.

A transformer having an 8 amp or greater 12.6 volt center tapped winding can be configured as an autotransformer. 6.3 volts is applied across half the winding and 12.6 is present across the entire winding. The transformer can be a 120/12.6 volt center tapped filament transformer.

[AA4PB]

The type of capacitive voltage doubler used in the HV supply will only work to provide a DC voltage. While an AC voltage doubler could be designed, it wouldn't be very practical. Simply adding an additional transfomer would be much more practical.

[N8CBX]

Does the filament need to be AC? What difference does it make if AC or DC. I would think no difference.
Jan

[AA4PB]

Sure, you could use DC on the filiment. The size of capacitor you are going to need to handle that much current is probably pretty sizable. It seems a lot more practical to me to add an additional 6.3VAC filiment transformer in series with the existing 6.3VAC transformer winding.

[KM3F]

Both 6.3 volt transformers  in series need to be at or above the current needed of 3.6 amps.
If There is room or space, two 6.3 volt transformers at that current or above are not all that large.
If possible mount one on top of the other using long bolts and nuts.
Parallel the primaries and series the secondaries.
Remember only the voltage increases not the current in this type hookup. Both transformers must be at or above the required current rating of the tube.
Series secondaries does nothing for the current capacity.
If the transformers do not have the same current rating, the lower rating 'rules' and is the limit or it will overheat if higher current is drawn.
I would think this is far more preferrable than fooling with DC.
Good luck.

[WX7G]

Using LTSPICE I simulated a voltage doubler. The transformer is modeled as a 0.05 ohm source. As AA4PB says two large capacitors are needed - at least 50,000 uF. The transformer RMS current is 12 amps due to the waveform and this will double thetransformer copper losses and resistive power dissipation compared to 8 amps. Each of the two 25 amp Schottkey rectifiers dissipates 2 watts a small heatsink will be required.

So yes, a DC doubler can be used if the transformer has some headroom.

[K1ZJH]

Going to capacitive input filtering will derate the transformer somewhat. I hope there is a bit of headroom. Using a separate filament transformer is a more elegant solution.

Pete

[W8JX]

Does the filament need to be AC? What difference does it make if AC or DC. I would think no difference.
Jan

It actually does in that you may need to reduce voltage slightly since DC is constant current and potential. The amount of tweaking will depend on how clean DC is.

[G3RZP]

To what extent is metal migration a problem with DC filaments?

[N8CBX]

Okay, you guys have been a great help. I think I can find room for a small 6.3V 3A-4A transformer and connect it in series to the 6.3V 8A tap, I think that should work.
Jan N8CBX

[K8AXW]

Jan: Apparently you didn't read KM3F's post or didn't pay any attention to it.  You're solution will not work.  Please re-read KM3F's post!

[K6AER]

just use a 12 volt 4 amp switch mode power supply. They are very small and DC current on the tube will work just fine. AC input can be from 100 to 250 VAC.

[KE3WD]

While the original tranformer current rating is not matched to the addon xformer, that is not necessarily the kill that K8AXW points out. 

Why not just add a 4 to 5 Amp fuse in a holder on the secondary side of the circuit? 

That should blow in the event of any circumstance that would draw significantly more than the current that the smaller xformer can provide. 


And this is Ham Radio diy, not production run.  Over the years I've seen many a solid modification of this sort done by Hams who "run what they brung" -- meaning that they successfully utilized components available to the, stuff from the junkbox rather than stuff brand new off the shelf -- and have often done so myself as well.  Most all are successful, judging by the age of many of the modified rigs and such purchased at hamfests and the like over the years, most were still operating despite whatever modification some clever old ham had wrought. 


73

[N8CBX]

...just use a 12 volt 4 amp switch mode power supply...

Actually, that's a pretty good idea. I see that MCM Electronics has a 12VDC switcher rated @ 5A for $38.99 (on sale) and it's only about 1.4" height. So, it will fit under the chassis and save room on the deck. Its adjustable and wide range of line power input (85-264VAC).....Problem solved!
Thanks everyone.
Jan N8CBX