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tomchr 30th October 2010 11:30 PM

Turning a 6.3 V secondary into a 5 V secondary on an Antek 2T350
 
5 Attachment(s)
Folks,

I figured I'd share... Transformers are not exactly rocket science, but some people seem to be a little intimidated by them.

I have an Antek 2T350 that I intend to use for a project involving 5AR4 rectifiers. These rectifiers have a 5 V filament. The trafo provides 6.3 V. What to do...

Option #1: Insert a 0.68 ohm resistor in series with the 5AR4 filament. This drops 1.3 V across the resistor and voila, 5.0 V across the filament. However, it also dissipates about 2.5 W in the resistor which will need to be gotten rid of.

Option #2: Unwind the secondary a little bit to change the turns ratio from 120:6.3 to 120:5.

I opted for the latter on this project. I counted 21 turns on the secondary. So a little back of the envelope math:

6.3/21 = 0.3 V per turn
5/0.3 = 16.7 turns

So to get a 5.0 V winding, I'd have to make a 16.7 turn secondary. 17 turns aught to be close enough... So if I remove four turns from the existing secondary, the voltage should drop to 5 V (maybe slightly higher). So I set out to remove four turns...

It's quite straight forward. The tape end of the plastic film wrapped around the transformer is found and the tape is removed. Then the plastic film is carefully unwound enough to expose the secondary connections. Taking care not to nick the windings, an exacto/utility knife is used to cut back the adhesive tape wrapped around the core. Note how the ends of the two secondary windings are interleaved between a piece of card stock to ensure that they don't short out. Pay attention to the sequence of the wires. I removed two turns from each end of each winding. The turns are actually bunched up slightly at the termination point, so I could do this and still have the secondary evenly spaced across the circumference of the core. With the now 17-turn secondary, I measured the secondary voltage. I loaded the secondary with a 5AR4 rectifier for this measurement. At 115 V input, I measured 5.0 V. At 120 V, I measured 5.25 V. That's within the +/-10 % normally required for heater windings, so I decided to stop here.

The secondary wires were cut, the laquer scraped off with a knife (followed by an emory cloth polish), and the old secondary output wires were soldered on. Then the windings were secured with tape and the plastic film was wound back on.

The whole process took me about 90 minutes including photography. With the transformer completely assembled, the output voltage on the secondary was 5.2 V for 120 V input.

Pictures from the various stages of the process below.

~Tom

aardvarkash10 31st October 2010 12:06 AM

easier than on an EI core! Nice work Tom.

taj 31st October 2010 12:39 AM

Thanks Tom. Good post.

..Todd

Ty_Bower 31st October 2010 12:51 AM

Yes, very nicely done. The photos are excellent too. The only pity is that Antek doesn't make it easy to specify either 6.3 & 6.3 or 6.3 & 5.0 at the time of order. The end user shouldn't have to make this modification at all if the factory would just sell him what he needed.

tubelab.com 31st October 2010 02:17 AM

1 Attachment(s)
I have been using a slightly different and easier technique. Since I wasn't sure that the two 6.3 volt windings could reliably handle 300 to 500 volts between them, I decided to add another winding. In normal use the rectifier winding operates at the B+ voltage and the 6.3 volt filament winding operates at or near ground potential. The two wires are in close proximity and are touching each other in some Anteks.

I use two ways to add a 5 volt winding. The ugly method, and the pretty method. Since toroids are already rather ugly, the ugly method is my first choice because it is easy. How? Simply take a length of hookup wire that is rated for 600 volts and wind it through the core and hook it up to your rectifier tube. Neatness does not count, or matter. The only critical factor is the number of turns. How many do you need? That depends on the transformer. Some need as few as 11 and some need up to 20. How do you know? Wind 15 or so turns and measure the voltage. Add or subtract as needed. I start with a long piece of wire (about 20 feet) loop half of it through the core and wind from both ends. At 10 turns or so, connect a rectifier tube (or suitable load) across the ends, Tape up the HV winding so you dont fry yourself, and make a measurement. Heep adding turns until you are just over 5 volts. The voltage will drop slightly when all the secondaries are hooked up.

The pretty method. Use the ugly method to find the number of turns needed. Wind the new secondary with #18 or 20 enameled wire right on top of the existing tape cover. Solder hookup wires to the ends and apply heat shrink. Make a new cover by wrapping Kapton tape on top of the new winding.

The picture shows an Antek 4T360 with two added 5 volt windings. The Antek powers 2 Simple P-P boards, each wired for mono PPP and I am cranking the EL84's at 435 volts! Result...50 WPC!

Mr. Zenith 31st October 2010 03:20 AM

I've been winding my own "custom" EI transformers years, using cores salvaged from discarded solid-state gear. Most of these were used in power supplies for reproduction vintage amateur radio gear.

I got my introduction to toroid-o-rama when I started building an Elecraft K2 with the internal antenna tuner. I thought I'd never finish winding them all. By the time it was done I had to buy some "Preparation T" to soothe the itching and burning of my inflamed toroidal tissues! :D

The one thing I did take away from all that was that when winding toroids, one pass through the core counts as one turn.

Seriously, this is really excellent stuff, fellows. I've been considering using toroidal transformers in the power supply of my next build, and I'll have to keep these methods in mind...

DualTriode 31st October 2010 06:56 AM

Quote:

Originally Posted by tomchr (Post 2348942)
Folks,

I figured I'd share... Transformers are not exactly rocket science, but some people seem to be a little intimidated by them.

I have an Antek 2T350 that I intend to use for a project involving 5AR4 rectifiers. These rectifiers have a 5 V filament. The trafo provides 6.3 V. What to do...

Option #1: Insert a 0.68 ohm resistor in series with the 5AR4 filament. This drops 1.3 V across the resistor and voila, 5.0 V across the filament. However, it also dissipates about 2.5 W in the resistor which will need to be gotten rid of.

Option #2: Unwind the secondary a little bit to change the turns ratio from 120:6.3 to 120:5.

I opted for the latter on this project. I counted 21 turns on the secondary. So a little back of the envelope math:

6.3/21 = 0.3 V per turn
5/0.3 = 16.7 turns

So to get a 5.0 V winding, I'd have to make a 16.7 turn secondary. 17 turns aught to be close enough... So if I remove four turns from the existing secondary, the voltage should drop to 5 V (maybe slightly higher). So I set out to remove four turns...

<snip>

~Tom

Hello,
If this was Diyrocket.com we would be discussing the ratio of sulfur to powered zinc not transformer turn ratios. If I had 100 million dollars to build a stem cell lab I would be sorting gram negative bacteria with a laser.
Transformers are about impedance matching for a given load. Adjusting the resistor for option #1 or the number of turns for option number 2 is more trial and error than back of the envelope calculation.
0.68 ohms was more of a street trap than a real number.
I am going to start my trial and error with 2 five watt 0.5R wire wound resistors in series.
DT
All just for fun!

tomchr 31st October 2010 07:38 AM

Quote:

Originally Posted by DualTriode (Post 2349223)
Hello,
Transformers are about impedance matching for a given load. Adjusting the resistor for option #1 or the number of turns for option number 2 is more trial and error than back of the envelope calculation.
0.68 ohms was more of a street trap than a real number.
I am going to start my trial and error with 2 five watt 0.5R wire wound resistors in series.

I'm not sure what your point is. I arrived at the 0.68 ohm by back of envelope calculation. The 5AR4 draws 1.9 A at 5.0 V.

(6.3-5)/1.9 = 0.684 ohm

0.68 is the closest standard value. Done. No trial and error. I hooked it up and got 5.0 V across the rectifier filament.

~Tom

tomchr 31st October 2010 07:49 AM

Quote:

Originally Posted by tubelab.com (Post 2349060)
I have been using a slightly different and easier technique. Since I wasn't sure that the two 6.3 volt windings could reliably handle 300 to 500 volts between them, I decided to add another winding. In normal use the rectifier winding operates at the B+ voltage and the 6.3 volt filament winding operates at or near ground potential. The two wires are in close proximity and are touching each other in some Anteks.

That's actually a really good point. The two 6.3 V windings look like they're wound as a pair. I.e. as two parallel strands of wire. They are touching most of the way, though, they are insulated by enamel.

It my case, it's OK as each winding will feed a separate rectifier so they'll both be at 400 V potential. Hence, the differential potential will be close to zero. I'll use a separate transformer for my 6.3 V and 5.0 V DC filament supplies. That was my original thought anyway. I'm thinking now to just add a pair of windings. Hmmm... :)

As far as the wire gauge, I'd probably go with a 3 A/mm^2 current density. That what I was taught in switchmode class in college anyway.

~Tom

Ian444 31st October 2010 11:40 AM

Great photos and write-up Tom. There is one other method too, just for info, I haven't tried it but it sounds feasible, wind some windings "backwards" on the outside of the tranny from the 6.3V taps to act as a bucking winding, about 4 turns by the sounds of it, to get the 6.3V back to 5V. It is a good thing though that a DIY'er can undo the tape, wind whatever he wants onto those trannies, tape it all back up, and it all looks stock standard. Or just lay the windings on the outside. All good!


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