5.5v filament transformer

[NR5P]

I'm needing a 5.5v (4.2A)X2 filament transformer  for 15E tubes.  I've thought that may be I can take a 6.3 and remove some windings but all the 6.3v filament transformers I have require drilling rivets to get to the insides.  I don't want to mess with this since I don't know if it's a good idea.  If it may work anyone have an idea of turns per volt to do for first check?  Maybe there is a better way to do this or maybe I should just check with a few people I know that might have one.  I haven't found even ONE for sale on ebay last time I checked.

[K1JJ]

Use a small Variac to control the 6.3V transformer. Mount it on the front panel.  It's good practice to ramp the filaments up on every turn-on     -    as well as the ability to set the fils at their precise requirements.

If you don't have a small Variac, there are often loads of them on eBay.

T

[NR5P]

Duh One of the chassis I'm going to use as a rack has a large variable resistor on it(about 3 inches across) I wonder if this could take the place of a variac.  I'm not sure how much power it could dissipate.  I guess it's better to get a real variac.  I don't even know what the resistance is since it's not hear right now to measure. 

[K1JJ]

"I'm needing a 5.5v (4.2A)X2 "


You need to drop 6.3V to 5.5V at 8.4A.   That's 0.8V  X  8.4A = 6.72 watts.   I'd say you could use either method and it'd be a wash.

A variable 30 ohms at 20 watts in the 120V primary would do it.

But, I would still opt for a Variac.


Use this wheel calculator for easy work:

http://www.diyalarmforum.com/ohms-law-calculator/

T

[W7TFO]

Broadcast rigs always used a rheostat in the filament line, rather than a Variac.

It works more like a soft-start than a big surge when hitting them cold.

Life is too short to ramp filaments....

73DG

[K1JJ]

Roger on the built-in soft start.

Being hams, we tend to try to match up whatever fil transformer we have available. I have run high current 12V transformers into 7.5V  4-1000As and even 3CX-2500s. The power waste would have been tremendous using a resistor. The Variac solved the problem with virtually no loss.

It all depends what we have to work with. In his case, it would make little difference in power waste, so maybe the resistor wud be an easier solution to keep from having to turn up and down the Variac each time.

BTW, in my own pair of 4X1s plate modulated rig (Fabio) I use a single fixed 50 watt 1 ohm resistor in the fil xfmr primary in both finals and modulators.. The 7.5v transformer was very close and needed only about 15 watts drop (a few tenths of a volt) to deliver 7.50 volts perfectly.  Much more compact solution than using Variacs since I was out of space on the front panel anyway....

T

[W3RSW]

A 115 to 5.0 vac (old days) fil. Xformer of sufficient over current rating will yield close to 5.5 volts when fed from modern 120 vac mains.  Also As you know, With small current drawn, all normal xformers feed way over nominal voltage.  This just enhances the effect.

If that doesn't work a small bucking transformer in the primary for an add'l five volts or so will make it.

And c'mon, we're hams.  For non-potted xformers, One more turn of the secondary stuffed through the corner of the core might make it.  If it's less, stuff it the other way. If it's too much, use the resistor in the primary trick.

[pa0ast]

 On a used 50VA toroid  at 3 windings per volt You need 5.5 x 3 is 17 windings of 1.5 mm square copper wire. Normal for a trafo is 3 Amp per square mm. Due to the core size the max inrush  current is also limited. Unwind the secundairy,  count the turns and with a small calculation You can see how many windings per volt it is. Easy to  make 17 windings on a toroid.. piece of cake..

[K1JJ]

That's a good way to make a transformer, indeed.


Many times I didn't have a proper filament transformer and made one from an old Variac core.  If you have a Variac that has a bad wiper, then it's the perfect choice.

Just wind on ten or how many turns are needed thru the donut using heavy wire that will handle the fil current. The old Variac wire is now the primary and the new wire is the secondary.    Adjust a turn or two to find the sweet spot under full filament load.  Lastly, add a center tap to the secondary if used for a directly heated cathode tube.

T